2690 lines
109 KiB
XML
2690 lines
109 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<chapter id="using_postgis_dbmanagement">
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<title>Data Management</title>
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<sect1 id="RefObject">
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<title>Spatial Data Model</title>
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||
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<sect2 id="OGC_Geometry">
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<title>OGC Geometry</title>
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<para>The Open Geospatial Consortium (OGC) developed the
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<ulink url="https://www.ogc.org/standards/sfa"><emphasis>Simple Features Access</emphasis></ulink>
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standard (SFA) to provide a model for geospatial data.
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It defines the fundamental spatial type of <emphasis role="bold">Geometry</emphasis>,
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along with operations which manipulate and transform geometry values
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to perform spatial analysis tasks.
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PostGIS implements the OGC Geometry model as the PostgreSQL data types
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<link linkend="PostGIS_Geometry">geometry</link> and
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<link linkend="PostGIS_Geography">geography</link>.
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</para>
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<para>
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Geometry is an <emphasis>abstract</emphasis> type.
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Geometry values belong to one of its <emphasis>concrete</emphasis> subtypes
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which represent various kinds and dimensions of geometric shapes.
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These include the <emphasis role="bold">atomic</emphasis> types
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<link linkend="Point">Point</link>,
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<link linkend="LineString">LineString</link>,
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<link linkend="LinearRing">LinearRing</link> and
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<link linkend="Polygon">Polygon</link>,
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||
and the <emphasis role="bold">collection</emphasis> types
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<link linkend="MultiPoint">MultiPoint</link>,
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<link linkend="MultiLineString">MultiLineString</link>,
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<link linkend="MultiPolygon">MultiPolygon</link> and
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<link linkend="GeometryCollection">GeometryCollection</link>.
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The <ulink url="https://portal.ogc.org/files/?artifact_id=25355"><emphasis>Simple Features Access - Part 1: Common architecture v1.2.1</emphasis></ulink>
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adds subtypes for the structures
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<link linkend="PolyhedralSurface">PolyhedralSurface</link>,
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<link linkend="Triangle">Triangle</link> and
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<link linkend="TIN">TIN</link>.
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</para>
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<para>Geometry models shapes in the 2-dimensional Cartesian plane.
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The PolyhedralSurface, Triangle, and TIN types can also represent shapes in 3-dimensional space.
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The size and location of shapes are specified by their <emphasis role="bold">coordinates</emphasis>.
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Each coordinate has a X and Y <emphasis role="bold">ordinate</emphasis> value determining its location in the plane.
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Shapes are constructed from points or line segments, with points specified by a single coordinate,
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and line segments by two coordinates.
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</para>
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<para>Coordinates may contain optional Z and M ordinate values.
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The Z ordinate is often used to represent elevation.
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The M ordinate contains a measure value, which may represent time or distance.
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If Z or M values are present in a geometry value, they must be defined for each point in the geometry.
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If a geometry has Z or M ordinates the <emphasis role="bold">coordinate dimension</emphasis> is 3D;
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if it has both Z and M the coordinate dimension is 4D.
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</para>
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<para>Geometry values are associated with a
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<emphasis role="bold">spatial reference system</emphasis>
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indicating the coordinate system in which it is embedded.
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The spatial reference system is identified by the geometry SRID number.
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The units of the X and Y axes are determined by the spatial reference system.
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In <emphasis role="bold">planar</emphasis> reference systems the X and Y coordinates typically
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represent easting and northing,
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while in <emphasis role="bold">geodetic</emphasis> systems
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they represent longitude and latitude.
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SRID 0 represents an infinite Cartesian plane with no units assigned to its axes.
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See <xref linkend="spatial_ref_sys" />.
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</para>
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<para>The geometry <emphasis role="bold">dimension</emphasis> is a property of geometry types.
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Point types have dimension 0, linear types have dimension 1,
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and polygonal types have dimension 2.
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Collections have the dimension of the maximum element dimension.
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</para>
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<para>A geometry value may be <emphasis role="bold">empty</emphasis>.
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Empty values contain no vertices (for atomic geometry types)
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or no elements (for collections).
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</para>
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<para>An important property of geometry values is their spatial
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<emphasis role="bold">extent</emphasis> or <emphasis role="bold">bounding box</emphasis>,
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which the OGC model calls <emphasis role="bold">envelope</emphasis>.
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This is the 2 or 3-dimensional box which encloses the coordinates of a geometry.
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It is an efficient way to represent a geometry's
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extent in coordinate space and to check whether two geometries interact.
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</para>
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<para>The geometry model allows evaluating topological spatial relationships as
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described in <xref linkend="DE-9IM" />.
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To support this the concepts of
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<emphasis role="bold">interior</emphasis>,
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<emphasis role="bold">boundary</emphasis> and
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<emphasis role="bold">exterior</emphasis>
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are defined for each geometry type.
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Geometries are topologically closed, so they always contain their boundary.
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The boundary is a geometry of dimension one less than that of the geometry itself.
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</para>
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<para>The OGC geometry model defines validity rules for each geometry type.
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These rules ensure that geometry values represents realistic
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situations (e.g. it is possible to specify a polygon
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with a hole lying outside the shell, but this makes no sense geometrically
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and is thus invalid).
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PostGIS also allows storing and manipulating invalid geometry values.
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This allows detecting and fixing them if needed.
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See <xref linkend="OGC_Validity" />
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</para>
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<sect3 id="Point">
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<title>Point</title>
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<para>A Point is a 0-dimensional geometry that represents a single location in coordinate space.</para>
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<programlisting>POINT (1 2)
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POINT Z (1 2 3)
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POINT ZM (1 2 3 4)
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</programlisting>
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</sect3>
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<sect3 id="LineString">
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<title>LineString</title>
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<para>A LineString is a 1-dimensional line formed by a contiguous sequence of line segments.
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Each line segment is defined by two points, with the end point of one segment
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forming the start point of the next segment.
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An OGC-valid LineString has either zero or two or more points,
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but PostGIS also allows single-point LineStrings.
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LineStrings may cross themselves (self-intersect).
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A LineString is <emphasis role="bold">closed</emphasis> if the start and end points are the same.
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A LineString is <emphasis role="bold">simple</emphasis> if it does not self-intersect.
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</para>
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<programlisting>LINESTRING (1 2, 3 4, 5 6)</programlisting>
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</sect3>
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<sect3 id="LinearRing">
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<title>LinearRing</title>
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<para>A LinearRing is a LineString which is both closed and simple.
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The first and last points must be equal, and the line must not self-intersect.</para>
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<programlisting>LINEARRING (0 0 0, 4 0 0, 4 4 0, 0 4 0, 0 0 0)</programlisting>
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</sect3>
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<sect3 id="Polygon">
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<title>Polygon</title>
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<para>A Polygon is a 2-dimensional planar region,
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delimited by an exterior boundary (the shell)
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and zero or more interior boundaries (holes).
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Each boundary is a <link linkend="LinearRing">LinearRing</link>.
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</para>
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<programlisting>POLYGON ((0 0 0,4 0 0,4 4 0,0 4 0,0 0 0),(1 1 0,2 1 0,2 2 0,1 2 0,1 1 0))</programlisting>
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</sect3>
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<sect3 id="MultiPoint">
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<title>MultiPoint</title>
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<para>A MultiPoint is a collection of Points.</para>
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<programlisting>MULTIPOINT ( (0 0), (1 2) )</programlisting>
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</sect3>
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<sect3 id="MultiLineString">
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<title>MultiLineString</title>
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<para>A MultiLineString is a collection of LineStrings.
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A MultiLineString is closed if each of its elements is closed.
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</para>
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<programlisting>MULTILINESTRING ( (0 0,1 1,1 2), (2 3,3 2,5 4) )</programlisting>
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</sect3>
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<sect3 id="MultiPolygon">
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<title>MultiPolygon</title>
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<para>A MultiPolygon is a collection of non-overlapping, non-adjacent Polygons.
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Polygons in the collection may touch only at a finite number of points.
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</para>
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<programlisting>MULTIPOLYGON (((1 5, 5 5, 5 1, 1 1, 1 5)), ((6 5, 9 1, 6 1, 6 5)))</programlisting>
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</sect3>
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<sect3 id="GeometryCollection">
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<title>GeometryCollection</title>
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<para>A GeometryCollection is a heterogeneous (mixed) collection of geometries.</para>
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<programlisting>GEOMETRYCOLLECTION ( POINT(2 3), LINESTRING(2 3, 3 4))</programlisting>
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</sect3>
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<sect3 id="PolyhedralSurface">
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<title>PolyhedralSurface</title>
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<para>A PolyhedralSurface is a contiguous collection of patches or facets which share some edges.
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Each patch is a planar Polygon.
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If the Polygon coordinates have Z ordinates then the surface is 3-dimensional.</para>
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<programlisting>POLYHEDRALSURFACE Z (
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((0 0 0, 0 0 1, 0 1 1, 0 1 0, 0 0 0)),
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((0 0 0, 0 1 0, 1 1 0, 1 0 0, 0 0 0)),
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((0 0 0, 1 0 0, 1 0 1, 0 0 1, 0 0 0)),
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((1 1 0, 1 1 1, 1 0 1, 1 0 0, 1 1 0)),
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((0 1 0, 0 1 1, 1 1 1, 1 1 0, 0 1 0)),
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((0 0 1, 1 0 1, 1 1 1, 0 1 1, 0 0 1)) )</programlisting>
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</sect3>
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<sect3 id="Triangle">
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<title>Triangle</title>
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<para>A Triangle is a polygon defined by three distinct non-collinear vertices.
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Because a Triangle is a polygon it is specified by four coordinates,
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with the first and fourth being equal.
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</para>
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<programlisting>TRIANGLE ((0 0, 0 9, 9 0, 0 0))</programlisting>
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</sect3>
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<sect3 id="TIN">
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<title>TIN</title>
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<para>A TIN is a collection of non-overlapping
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<link linkend="Triangle">Triangle</link>s representing a
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<ulink url="https://en.wikipedia.org/wiki/Triangulated_irregular_network">Triangulated Irregular Network</ulink>.
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</para>
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<programlisting>TIN Z ( ((0 0 0, 0 0 1, 0 1 0, 0 0 0)), ((0 0 0, 0 1 0, 1 1 0, 0 0 0)) )</programlisting>
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</sect3>
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</sect2>
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<sect2 id="SQL_MM_Part3">
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<title>SQL/MM Part 3 - Curves</title>
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<para>The
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<ulink url="https://www.iso.org/obp/ui/#iso:std:iso-iec:13249:-3:ed-5:v1:en"><emphasis>ISO/IEC 13249-3 SQL Multimedia - Spatial</emphasis></ulink>
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standard (SQL/MM) extends the
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OGC SFA to define Geometry subtypes containing curves with circular arcs.
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The SQL/MM types support 3DM, 3DZ and 4D coordinates.
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</para>
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<note>
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<para>All floating point comparisons within the SQL-MM implementation
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are performed to a specified tolerance, currently 1E-8.</para>
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</note>
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<sect3 id="CircularString">
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<title>CircularString</title>
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<para>CircularString is the basic curve type, similar to a
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LineString in the linear world. A single arc segment is specified by three
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points: the start and end points (first and third) and some other
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point on the arc.
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To specify a closed circle the start and end points are the same
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and the middle point is the opposite point on the circle diameter
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(which is the center of the arc).
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||
In a sequence of arcs the end point of the previous
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arc is the start point of the next arc, just like the segments of a LineString.
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This means that a CircularString must have an
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odd number of points greater than 1.</para>
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||
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<programlisting>CIRCULARSTRING(0 0, 1 1, 1 0)
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CIRCULARSTRING(0 0, 4 0, 4 4, 0 4, 0 0)</programlisting>
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</sect3>
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<sect3 id="CompoundCurve">
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<title>CompoundCurve</title>
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<para>A CompoundCurve is a single continuous curve that may contain both
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||
circular arc segments and linear segments. That means that
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||
in addition to having well-formed components, the end point of
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||
every component (except the last) must be coincident with the
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||
start point of the following component.</para>
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||
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<programlisting>COMPOUNDCURVE( CIRCULARSTRING(0 0, 1 1, 1 0),(1 0, 0 1))</programlisting>
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</sect3>
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<sect3 id="CurvePolygon">
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<title>CurvePolygon</title>
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||
<para>A CurvePolygon is like a polygon, with an outer ring
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||
and zero or more inner rings. The difference is that a ring can be a
|
||
CircularString or CompoundCurve as well as a LineString.
|
||
</para>
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||
<para>As of PostGIS 1.4 PostGIS supports compound curves in a curve polygon.</para>
|
||
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||
<programlisting>CURVEPOLYGON(
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||
CIRCULARSTRING(0 0, 4 0, 4 4, 0 4, 0 0),
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||
(1 1, 3 3, 3 1, 1 1) )</programlisting>
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||
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<para>Example: A CurvePolygon with the shell defined by a CompoundCurve
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||
containing a CircularString and a LineString,
|
||
and a hole defined by a CircularString</para>
|
||
<programlisting>CURVEPOLYGON(
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||
COMPOUNDCURVE( CIRCULARSTRING(0 0,2 0, 2 1, 2 3, 4 3),
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||
(4 3, 4 5, 1 4, 0 0)),
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CIRCULARSTRING(1.7 1, 1.4 0.4, 1.6 0.4, 1.6 0.5, 1.7 1) )</programlisting>
|
||
|
||
</sect3>
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<sect3 id="MultiCurve">
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<title>MultiCurve</title>
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<para>A MultiCurve is a collection of curves which can include
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LineStrings, CircularStrings or CompoundCurves.</para>
|
||
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<programlisting>MULTICURVE( (0 0, 5 5), CIRCULARSTRING(4 0, 4 4, 8 4))</programlisting>
|
||
</sect3>
|
||
<sect3 id="MultiSurface">
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<title>MultiSurface</title>
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<para>A MultiSurface is a collection of surfaces, which can be (linear)
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Polygons or CurvePolygons.</para>
|
||
|
||
<programlisting>MULTISURFACE(
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CURVEPOLYGON(
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CIRCULARSTRING( 0 0, 4 0, 4 4, 0 4, 0 0),
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||
(1 1, 3 3, 3 1, 1 1)),
|
||
((10 10, 14 12, 11 10, 10 10), (11 11, 11.5 11, 11 11.5, 11 11)))</programlisting>
|
||
|
||
</sect3>
|
||
|
||
</sect2>
|
||
|
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<sect2 id="OpenGISWKBWKT">
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<title>WKT and WKB</title>
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||
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||
<para>The OGC SFA specification defines two formats for representing
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||
geometry values for external use: Well-Known Text (WKT) and Well-Known
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||
Binary (WKB). Both WKT and WKB include information about the type
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||
of the object and the coordinates which define it.</para>
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||
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<para>Well-Known Text (WKT) provides a standard textual representation of spatial data.
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||
Examples of WKT representations of spatial objects are:</para>
|
||
|
||
<itemizedlist>
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||
<listitem>
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<para>POINT(0 0)</para>
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||
</listitem>
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||
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<listitem>
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<para>POINT Z (0 0 0)</para>
|
||
</listitem>
|
||
|
||
<listitem>
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<para>POINT ZM (0 0 0 0)</para>
|
||
</listitem>
|
||
|
||
<listitem>
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<para>POINT EMPTY</para>
|
||
</listitem>
|
||
|
||
<listitem>
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||
<para>LINESTRING(0 0,1 1,1 2)</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>LINESTRING EMPTY</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>POLYGON((0 0,4 0,4 4,0 4,0 0),(1 1, 2 1, 2 2, 1 2,1 1))</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>MULTIPOINT((0 0),(1 2))</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>MULTIPOINT Z ((0 0 0),(1 2 3))</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>MULTIPOINT EMPTY</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>MULTILINESTRING((0 0,1 1,1 2),(2 3,3 2,5 4))</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>MULTIPOLYGON(((0 0,4 0,4 4,0 4,0 0),(1 1,2 1,2 2,1 2,1 1)),
|
||
((-1 -1,-1 -2,-2 -2,-2 -1,-1 -1)))</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>GEOMETRYCOLLECTION(POINT(2 3),LINESTRING(2 3,3 4))</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>GEOMETRYCOLLECTION EMPTY</para>
|
||
</listitem>
|
||
</itemizedlist>
|
||
|
||
<para>Input and output of WKT is provided by the functions
|
||
<xref linkend="ST_AsText" /> and <xref linkend="ST_GeomFromText" />:</para>
|
||
|
||
<programlisting>text WKT = ST_AsText(geometry);
|
||
geometry = ST_GeomFromText(text WKT, SRID);</programlisting>
|
||
|
||
<para>For example, a statement to create and insert a spatial object from WKT and a SRID is:</para>
|
||
|
||
<programlisting>INSERT INTO geotable ( geom, name )
|
||
VALUES ( ST_GeomFromText('POINT(-126.4 45.32)', 312), 'A Place');</programlisting>
|
||
|
||
<para>Well-Known Binary (WKB) provides a portable, full-precision representation
|
||
of spatial data as binary data (arrays of bytes).
|
||
Examples of the WKB representations of spatial objects are:</para>
|
||
|
||
<itemizedlist>
|
||
<listitem>
|
||
<para>WKT: POINT(1 1)</para>
|
||
<para>WKB: 0101000000000000000000F03F000000000000F03</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>WKT: LINESTRING (2 2, 9 9)</para>
|
||
<para>WKB: 0102000000020000000000000000000040000000000000004000000000000022400000000000002240</para>
|
||
</listitem>
|
||
</itemizedlist>
|
||
|
||
<para>Input and output of WKB is provided by the functions
|
||
<xref linkend="ST_AsBinary" /> and <xref linkend="ST_GeomFromWKB" />:</para>
|
||
|
||
<programlisting>
|
||
bytea WKB = ST_AsBinary(geometry);
|
||
geometry = ST_GeomFromWKB(bytea WKB, SRID);
|
||
</programlisting>
|
||
|
||
<para>For example, a statement to create and insert a
|
||
spatial object from WKB is:</para>
|
||
|
||
<programlisting>INSERT INTO geotable ( geom, name )
|
||
VALUES ( ST_GeomFromWKB('\x0101000000000000000000f03f000000000000f03f', 312), 'A Place');</programlisting>
|
||
</sect2>
|
||
|
||
</sect1>
|
||
|
||
<sect1 id="PostGIS_Geometry">
|
||
<title>Geometry Data Type</title>
|
||
|
||
<para>PostGIS implements the OGC Simple Features model
|
||
by defining a PostgreSQL data type called <varname>geometry</varname>.
|
||
It represents all of the geometry subtypes by using an internal type code
|
||
(see <xref linkend="GeometryType" /> and <xref linkend="ST_GeometryType" />).
|
||
This allows modelling spatial features as rows of tables defined
|
||
with a column of type <varname>geometry</varname>.
|
||
</para>
|
||
|
||
<para>The <varname>geometry</varname> data type is <emphasis>opaque</emphasis>,
|
||
which means that all access is done via invoking functions on geometry values.
|
||
Functions allow creating geometry objects,
|
||
accessing or updating all internal fields,
|
||
and compute new geometry values.
|
||
PostGIS supports all the functions specified in the OGC
|
||
<ulink url="https://portal.ogc.org/files/?artifact_id=25354"><emphasis>Simple feature access - Part 2: SQL option</emphasis></ulink>
|
||
(SFS) specification, as well many others.
|
||
See <xref linkend="reference" /> for the full list of functions.</para>
|
||
|
||
<note>
|
||
<para>PostGIS follows the SFA standard by prefixing spatial functions with "ST_".
|
||
This was intended to stand for "Spatial and Temporal",
|
||
but the temporal part of the standard was never developed.
|
||
Instead it can be interpreted as "Spatial Type".
|
||
</para>
|
||
</note>
|
||
|
||
<para>The SFA standard specifies that spatial objects include a Spatial Reference System
|
||
identifier (SRID). The SRID is required when creating spatial objects
|
||
for insertion into the database (it may be defaulted to 0).
|
||
See <xref linkend="ST_SRID" /> and <xref linkend="spatial_ref_sys" /></para>
|
||
|
||
<para>To make querying geometry efficient PostGIS defines
|
||
various kinds of spatial indexes, and spatial operators to use them.
|
||
See <xref linkend="build-indexes" /> and <xref linkend="using-query-indexes" /> for details.
|
||
</para>
|
||
|
||
<sect2 id="EWKB_EWKT">
|
||
<title>PostGIS EWKB and EWKT</title>
|
||
|
||
<para>OGC SFA specifications initially supported only 2D geometries,
|
||
and the geometry SRID is not included in the input/output representations.
|
||
The OGC SFA specification 1.2.1 (which aligns with the ISO 19125 standard)
|
||
adds support for 3D (ZYZ) and measured (XYM and XYZM) coordinates,
|
||
but still does not include the SRID value.</para>
|
||
|
||
<para>Because of these limitations PostGIS defined extended EWKB and EWKT formats.
|
||
They provide 3D (XYZ and XYM) and 4D (XYZM) coordinate support and include SRID information.
|
||
Including all geometry information allows PostGIS to use EWKB as the format of record
|
||
(e.g. in DUMP files).
|
||
</para>
|
||
|
||
<para>EWKB and EWKT are used for the "canonical forms" of PostGIS data objects.
|
||
For input, the canonical form for binary data is EWKB,
|
||
and for text data either EWKB or EWKT is accepted.
|
||
This allows geometry values to be created by casting
|
||
a text value in either HEXEWKB or EWKT to a geometry value using <varname>::geometry</varname>.
|
||
For output, the canonical form for binary is EWKB, and for text
|
||
it is HEXEWKB (hex-encoded EWKB).
|
||
</para>
|
||
|
||
<para>For example this statement creates a geometry by casting from an EWKT text value,
|
||
and outputs it using the canonical form of HEXEWKB:</para>
|
||
|
||
<programlisting>SELECT 'SRID=4;POINT(0 0)'::geometry;
|
||
geometry
|
||
----------------------------------------------------
|
||
01010000200400000000000000000000000000000000000000
|
||
</programlisting>
|
||
|
||
<para>PostGIS EWKT output has a few differences to OGC WKT:</para>
|
||
|
||
<itemizedlist>
|
||
<listitem>
|
||
<para>For 3DZ geometries the Z qualifier is omitted:</para>
|
||
<para>OGC: POINT Z (1 2 3)</para>
|
||
<para>EWKT: POINT (1 2 3)</para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>For 3DM geometries the M qualifier is included:</para>
|
||
<para>OGC: POINT M (1 2 3)</para>
|
||
<para>EWKT: POINTM (1 2 3)</para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>For 4D geometries the ZM qualifier is omitted:</para>
|
||
<para>OGC: POINT ZM (1 2 3 4)</para>
|
||
<para>EWKT: POINT (1 2 3 4)</para>
|
||
</listitem>
|
||
</itemizedlist>
|
||
|
||
<para>EWKT avoids over-specifying dimensionality
|
||
and the inconsistencies that can occur with the OGC/ISO format, such as:
|
||
</para>
|
||
|
||
<itemizedlist>
|
||
<listitem>
|
||
<para>POINT ZM (1 1)</para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>POINT ZM (1 1 1)</para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>POINT (1 1 1 1)</para>
|
||
</listitem>
|
||
</itemizedlist>
|
||
|
||
<caution>
|
||
<para>PostGIS extended formats are currently a superset of the OGC ones,
|
||
so that every valid OGC WKB/WKT is also valid EWKB/EWKT.
|
||
However, this might vary in the future,
|
||
if the OGC extends a format in a way that conflicts with the PosGIS definition.
|
||
Thus you SHOULD NOT rely on this compatibility!</para>
|
||
</caution>
|
||
|
||
<para>Examples of the EWKT text representation of spatial objects are:</para>
|
||
|
||
<itemizedlist>
|
||
<listitem>
|
||
<para>POINT(0 0 0) -- XYZ</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>SRID=32632;POINT(0 0) -- XY with SRID</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>POINTM(0 0 0) -- XYM</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>POINT(0 0 0 0) -- XYZM</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>SRID=4326;MULTIPOINTM(0 0 0,1 2 1) -- XYM with SRID</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>MULTILINESTRING((0 0 0,1 1 0,1 2 1),(2 3 1,3 2 1,5 4
|
||
1))</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>POLYGON((0 0 0,4 0 0,4 4 0,0 4 0,0 0 0),(1 1 0,2 1 0,2 2 0,1 2
|
||
0,1 1 0))</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>MULTIPOLYGON(((0 0 0,4 0 0,4 4 0,0 4 0,0 0 0),(1 1 0,2 1 0,2 2
|
||
0,1 2 0,1 1 0)),((-1 -1 0,-1 -2 0,-2 -2 0,-2 -1 0,-1 -1 0)))</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>GEOMETRYCOLLECTIONM( POINTM(2 3 9), LINESTRINGM(2 3 4, 3 4 5) )</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>MULTICURVE( (0 0, 5 5), CIRCULARSTRING(4 0, 4 4, 8 4) )</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>POLYHEDRALSURFACE( ((0 0 0, 0 0 1, 0 1 1, 0 1 0, 0 0 0)),
|
||
((0 0 0, 0 1 0, 1 1 0, 1 0 0, 0 0 0)), ((0 0 0, 1 0 0, 1 0 1, 0 0 1, 0 0 0)), ((1 1 0, 1 1 1, 1 0 1, 1 0 0, 1 1 0)),
|
||
((0 1 0, 0 1 1, 1 1 1, 1 1 0, 0 1 0)), ((0 0 1, 1 0 1, 1 1 1, 0 1 1, 0 0 1)) )</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>TRIANGLE ((0 0, 0 10, 10 0, 0 0))</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>TIN( ((0 0 0, 0 0 1, 0 1 0, 0 0 0)),
|
||
((0 0 0, 0 1 0, 1 1 0, 0 0 0)) )</para>
|
||
</listitem>
|
||
</itemizedlist>
|
||
|
||
<para>Input and output using these formats is available using the following functions:</para>
|
||
|
||
<programlisting>bytea EWKB = ST_AsEWKB(geometry);
|
||
text EWKT = ST_AsEWKT(geometry);
|
||
geometry = ST_GeomFromEWKB(bytea EWKB);
|
||
geometry = ST_GeomFromEWKT(text EWKT);</programlisting>
|
||
|
||
<para>For example, a statement to create and insert a PostGIS spatial object using EWKT is:</para>
|
||
|
||
<programlisting>INSERT INTO geotable ( geom, name )
|
||
VALUES ( ST_GeomFromEWKT('SRID=312;POINTM(-126.4 45.32 15)'), 'A Place' )</programlisting>
|
||
|
||
</sect2>
|
||
|
||
</sect1>
|
||
|
||
<sect1 id="PostGIS_Geography">
|
||
<title>Geography Data Type</title>
|
||
|
||
<para>The PostGIS <varname>geography</varname> data type provides native support for spatial features represented on "geographic" coordinates (sometimes called "geodetic" coordinates, or "lat/lon", or "lon/lat"). Geographic coordinates are spherical coordinates expressed in angular units (degrees). </para>
|
||
|
||
<para>The basis for the PostGIS geometry data type is a plane.
|
||
The shortest path between two points on the plane is a straight line.
|
||
That means functions on geometries (areas, distances, lengths, intersections, etc)
|
||
are calculated using straight line vectors and cartesian mathematics.
|
||
This makes them simpler to implement and faster to execute,
|
||
but also makes them inaccurate for data on the spheroidal surface of the earth.
|
||
</para>
|
||
|
||
<para>The PostGIS geography data type is based on a spherical model.
|
||
The shortest path between two points on the sphere is a great circle arc.
|
||
Functions on geographies (areas, distances, lengths, intersections, etc) are calculated using arcs on the sphere.
|
||
By taking the spheroidal shape of the world into account, the functions provide more accurate results.</para>
|
||
|
||
<para>Because the underlying mathematics is more complicated, there are fewer functions defined for the geography type than for the geometry type.
|
||
Over time, as new algorithms are added the capabilities of the geography type will expand.
|
||
As a workaround one can convert back and forth between geometry and geography types.</para>
|
||
|
||
<para>Like the geometry data type, geography data is associated
|
||
with a spatial reference system via a spatial reference system identifier (SRID).
|
||
Any geodetic (long/lat based) spatial reference system defined in the <varname>spatial_ref_sys</varname> table can be used.
|
||
(Prior to PostGIS 2.2, the geography type supported only WGS 84 geodetic (SRID:4326)).
|
||
You can add your own custom geodetic spatial reference system as described in <xref linkend="user-spatial-ref-sys" />.</para>
|
||
|
||
<para>For all spatial reference systems the units returned by measurement functions
|
||
(e.g. <xref linkend="ST_Distance" />, <xref linkend="ST_Length" />, <xref linkend="ST_Perimeter" />, <xref linkend="ST_Area" />)
|
||
and for the distance argument of <xref linkend="ST_DWithin" /> are in meters.</para>
|
||
|
||
<sect2 id="Create_Geography_Tables">
|
||
<title>Creating Geography Tables</title>
|
||
|
||
<para>You can create a table to store geography data using the
|
||
<ulink url="https://www.postgresql.org/docs/current/sql-createtable.html">CREATE TABLE</ulink>
|
||
SQL statement with a column of type <varname>geography</varname>.
|
||
The following example creates a table with a geography column storing 2D LineStrings
|
||
in the WGS84 geodetic coordinate system (SRID 4326):</para>
|
||
|
||
<programlisting>CREATE TABLE global_points (
|
||
id SERIAL PRIMARY KEY,
|
||
name VARCHAR(64),
|
||
location geography(POINT,4326)
|
||
);</programlisting>
|
||
|
||
<para>The geography type supports two optional type modifiers:</para>
|
||
|
||
<itemizedlist>
|
||
<listitem>
|
||
<para>the spatial type modifier restricts the kind of shapes and dimensions allowed in the column.
|
||
Values allowed for the spatial type are: POINT, LINESTRING, POLYGON, MULTIPOINT, MULTILINESTRING, MULTIPOLYGON, GEOMETRYCOLLECTION.
|
||
The geography type does not support curves, TINS, or POLYHEDRALSURFACEs.
|
||
The modifier supports coordinate dimensionality restrictions by adding suffixes: Z, M and ZM.
|
||
For example, a modifier of 'LINESTRINGM' only allows linestrings with three dimensions, and treats the third dimension as a measure.
|
||
Similarly, 'POINTZM' requires four dimensional (XYZM) data.
|
||
</para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>the SRID modifier restricts the spatial reference system SRID to a particular number.
|
||
If omitted, the SRID defaults to 4326 (WGS84 geodetic), and all calculations are performed using WGS84.
|
||
</para>
|
||
</listitem>
|
||
</itemizedlist>
|
||
|
||
<para></para>
|
||
|
||
<para>Examples of creating tables with geography columns:</para>
|
||
<itemizedlist>
|
||
<listitem>
|
||
<para>Create a table with 2D POINT geography with the default SRID 4326 (WGS84 long/lat):</para>
|
||
<para><programlisting>CREATE TABLE ptgeogwgs(gid serial PRIMARY KEY, geog geography(POINT) );</programlisting></para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>Create a table with 2D POINT geography in NAD83 longlat:</para>
|
||
<para><programlisting>CREATE TABLE ptgeognad83(gid serial PRIMARY KEY, geog geography(POINT,4269) );</programlisting></para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>Create a table with 3D (XYZ) POINTs and an explicit SRID of 4326:</para>
|
||
<para><programlisting>CREATE TABLE ptzgeogwgs84(gid serial PRIMARY KEY, geog geography(POINTZ,4326) );</programlisting></para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>Create a table with 2D LINESTRING geography with the default SRID 4326:</para>
|
||
<para><programlisting>CREATE TABLE lgeog(gid serial PRIMARY KEY, geog geography(LINESTRING) );</programlisting></para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>Create a table with 2D POLYGON geography with the SRID 4267 (NAD 1927 long lat):</para>
|
||
<para><programlisting>CREATE TABLE lgeognad27(gid serial PRIMARY KEY, geog geography(POLYGON,4267) );</programlisting></para>
|
||
</listitem>
|
||
<!-- TODO: Add other examples -->
|
||
<!--
|
||
<listitem>
|
||
<para>MULTIPOINT</para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>MULTILINESTRING</para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>MULTIPOLYGON</para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>GEOMETRYCOLLECTION</para>
|
||
</listitem>
|
||
-->
|
||
</itemizedlist>
|
||
<para>Geography fields are registered in the <varname>geography_columns</varname> system view.
|
||
You can query the <varname>geography_columns</varname> view and see that the table is listed:</para>
|
||
|
||
<programlisting>
|
||
SELECT * FROM geography_columns;</programlisting>
|
||
|
||
<para>Creating a spatial index works the same as for geometry columns.
|
||
PostGIS will note that the column type is GEOGRAPHY and create an appropriate sphere-based index instead of the usual planar index used for GEOMETRY.</para>
|
||
|
||
<programlisting>-- Index the test table with a spherical index
|
||
CREATE INDEX global_points_gix ON global_points USING GIST ( location );</programlisting>
|
||
|
||
</sect2>
|
||
|
||
<sect2 id="Use_Geography_Tables">
|
||
<title>Using Geography Tables</title>
|
||
|
||
<para>You can insert data into geography tables in the same way as geometry.
|
||
Geometry data will autocast to the geography type if it has SRID 4326.
|
||
The <link linkend="EWKB_EWKT">EWKT and EWKB</link> formats can also be used
|
||
to specify geography values.</para>
|
||
|
||
<programlisting>-- Add some data into the test table
|
||
INSERT INTO global_points (name, location) VALUES ('Town', 'SRID=4326;POINT(-110 30)');
|
||
INSERT INTO global_points (name, location) VALUES ('Forest', 'SRID=4326;POINT(-109 29)');
|
||
INSERT INTO global_points (name, location) VALUES ('London', 'SRID=4326;POINT(0 49)');
|
||
</programlisting>
|
||
|
||
<para>Any geodetic (long/lat) spatial reference system listed in
|
||
<varname>spatial_ref_sys</varname> table may be specified as a geography SRID.
|
||
Non-geodetic coordinate systems raise an error if used.
|
||
</para>
|
||
|
||
<programlisting>-- NAD 83 lon/lat
|
||
SELECT 'SRID=4269;POINT(-123 34)'::geography;
|
||
geography
|
||
----------------------------------------------------
|
||
0101000020AD1000000000000000C05EC00000000000004140
|
||
</programlisting>
|
||
|
||
<programlisting>-- NAD27 lon/lat
|
||
SELECT 'SRID=4267;POINT(-123 34)'::geography;
|
||
geography
|
||
----------------------------------------------------
|
||
0101000020AB1000000000000000C05EC00000000000004140
|
||
</programlisting>
|
||
|
||
<programlisting>-- NAD83 UTM zone meters - gives an error since it is a meter-based planar projection
|
||
SELECT 'SRID=26910;POINT(-123 34)'::geography;
|
||
|
||
ERROR: Only lon/lat coordinate systems are supported in geography.
|
||
</programlisting>
|
||
|
||
<para>Query and measurement functions use units of meters. So distance parameters should be expressed in meters, and return values should be expected in meters (or square meters for areas).</para>
|
||
|
||
<programlisting>-- A distance query using a 1000km tolerance
|
||
SELECT name FROM global_points WHERE ST_DWithin(location, 'SRID=4326;POINT(-110 29)'::geography, 1000000);</programlisting>
|
||
|
||
|
||
<para>You can see the power of geography in action by calculating how close a plane flying
|
||
a great circle route from Seattle to London (LINESTRING(-122.33 47.606, 0.0 51.5))
|
||
comes to Reykjavik (POINT(-21.96 64.15))
|
||
(<ulink url="http://gc.kls2.com/cgi-bin/gc?PATH=SEA-LHR">map the route</ulink>).
|
||
</para>
|
||
|
||
<para>The geography type calculates the true shortest distance of 122.235 km over the sphere
|
||
between Reykjavik and the great circle flight path between Seattle and London.</para>
|
||
|
||
<programlisting>-- Distance calculation using GEOGRAPHY
|
||
SELECT ST_Distance('LINESTRING(-122.33 47.606, 0.0 51.5)'::geography, 'POINT(-21.96 64.15)'::geography);
|
||
st_distance
|
||
-----------------
|
||
122235.23815667</programlisting>
|
||
|
||
<para>
|
||
The geometry type calculates a meaningless cartesian distance between Reykjavik
|
||
and the straight line path from Seattle to London plotted on a flat map of the world.
|
||
The nominal units of the result is "degrees",
|
||
but the result doesn't correspond to any true angular difference between the points,
|
||
so even calling them "degrees" is inaccurate.</para>
|
||
|
||
<programlisting>-- Distance calculation using GEOMETRY
|
||
SELECT ST_Distance('LINESTRING(-122.33 47.606, 0.0 51.5)'::geometry, 'POINT(-21.96 64.15)'::geometry);
|
||
st_distance
|
||
--------------------
|
||
13.342271221453624
|
||
</programlisting>
|
||
|
||
</sect2>
|
||
|
||
<sect2 id="PostGIS_GeographyVSGeometry">
|
||
<title>When to use the Geography data type</title>
|
||
<para>The geography data type allows you to store data in longitude/latitude coordinates, but at a cost: there are fewer functions defined on GEOGRAPHY than there are on GEOMETRY; those functions that are defined take more CPU time to execute.</para>
|
||
<para>The data type you choose should be determined by the expected working area of the application you are building. Will your data span the globe or a large continental area, or is it local to a state, county or municipality? </para>
|
||
<itemizedlist>
|
||
<listitem><para>If your data is contained in a small area, you might find that choosing an appropriate projection and using GEOMETRY is the best solution, in terms of performance and functionality available.</para></listitem>
|
||
<listitem><para>If your data is global or covers a continental region, you may find that GEOGRAPHY allows you to build a system without having to worry about projection details.
|
||
You store your data in longitude/latitude, and use the functions that have been defined on GEOGRAPHY.</para></listitem>
|
||
<listitem><para>If you don't understand projections, and you don't want to learn about them, and you're prepared to accept the limitations in functionality available in GEOGRAPHY, then it might be easier for you to use GEOGRAPHY than GEOMETRY.
|
||
Simply load your data up as longitude/latitude and go from there.</para></listitem>
|
||
</itemizedlist>
|
||
<para>Refer to <xref linkend="PostGIS_TypeFunctionMatrix" /> for compare between
|
||
what is supported for Geography vs. Geometry. For a brief listing and description of Geography functions, refer to
|
||
<xref linkend="PostGIS_GeographyFunctions" />
|
||
</para>
|
||
</sect2>
|
||
<sect2 id="PostGIS_Geography_AdvancedFAQ">
|
||
<title>Geography Advanced FAQ</title>
|
||
<qandaset>
|
||
<qandaentry>
|
||
<question>
|
||
<para>Do you calculate on the sphere or the spheroid?</para>
|
||
</question>
|
||
|
||
<answer>
|
||
<para> By default, all distance and area calculations are done on the spheroid. You should find that the results of calculations in local areas match up will with local planar results in good local projections.
|
||
Over larger areas, the spheroidal calculations will be more accurate than any calculation done on a projected plane.
|
||
</para>
|
||
<para>All the geography functions have the option of using a sphere calculation, by setting a final boolean parameter to 'FALSE'. This will somewhat speed up calculations, particularly for cases where the geometries are very simple.</para>
|
||
</answer>
|
||
</qandaentry>
|
||
|
||
<qandaentry>
|
||
<question>
|
||
<para>What about the date-line and the poles?</para>
|
||
</question>
|
||
|
||
<answer>
|
||
<para> All the calculations have no conception of date-line or poles, the coordinates are spherical (longitude/latitude)
|
||
so a shape that crosses the dateline is, from a calculation point of view, no different from any other shape.
|
||
</para>
|
||
</answer>
|
||
</qandaentry>
|
||
|
||
<qandaentry>
|
||
<question>
|
||
<para>What is the longest arc you can process?</para>
|
||
</question>
|
||
|
||
<answer>
|
||
<para>We use great circle arcs as the "interpolation line" between two points. That means any two points are actually joined up two ways, depending on which direction you travel along the great circle. All our code assumes that the points are joined by the *shorter* of the two paths along the great circle.
|
||
As a consequence, shapes that have arcs of more than 180 degrees will not be correctly modelled.</para>
|
||
</answer>
|
||
</qandaentry>
|
||
|
||
<qandaentry>
|
||
<question>
|
||
<para>Why is it so slow to calculate the area of Europe / Russia / insert big geographic region here ?</para>
|
||
</question>
|
||
|
||
<answer>
|
||
<para>Because the polygon is so darned huge! Big areas are bad for two reasons: their bounds are huge,
|
||
so the index tends to pull the feature no matter what query you run; the number of vertices is huge,
|
||
and tests (distance, containment) have to traverse the vertex list at least once and sometimes N times
|
||
(with N being the number of vertices in the other candidate feature).
|
||
</para>
|
||
<para>As with GEOMETRY, we recommend that when you have very large polygons, but are doing queries in small areas, you "denormalize" your geometric data into smaller chunks so that the index can effectively subquery parts of the object and so queries don't have to pull out the whole object every time. Please consult <xref linkend="ST_Subdivide" /> function documentation.
|
||
Just because you *can* store all of Europe in one polygon doesn't mean you *should*.</para>
|
||
</answer>
|
||
</qandaentry>
|
||
</qandaset>
|
||
</sect2>
|
||
</sect1>
|
||
<!-- ============================================================== -->
|
||
<sect1 id="OGC_Validity">
|
||
<title>Geometry Validation</title>
|
||
|
||
<para>PostGIS is compliant with the Open Geospatial Consortium’s (OGC)
|
||
Simple Features specification.
|
||
That standard defines the concepts of geometry being
|
||
<emphasis>simple</emphasis> and <emphasis>valid</emphasis>.
|
||
These definitions allow the Simple Features geometry model
|
||
to represent spatial objects in a consistent and unambiguous way
|
||
that supports efficient computation.
|
||
(Note: the OGC SF and SQL/MM have the same definitions for simple and valid.)
|
||
</para>
|
||
|
||
<sect2 id="Simple_Geometry">
|
||
<title>Simple Geometry</title>
|
||
|
||
<para>A <emphasis>simple</emphasis>
|
||
geometry is one that has no anomalous geometric points, such as self
|
||
intersection or self tangency.
|
||
</para>
|
||
|
||
<para>A <varname>POINT</varname> is inherently <emphasis>simple</emphasis>
|
||
as a 0-dimensional geometry object.</para>
|
||
|
||
<para><varname>MULTIPOINT</varname>s are <emphasis>simple</emphasis> if
|
||
no two coordinates (<varname>POINT</varname>s) are equal (have identical
|
||
coordinate values).</para>
|
||
|
||
<para>A <varname>LINESTRING</varname> is <emphasis>simple</emphasis> if
|
||
it does not pass through the same point twice, except for the endpoints.
|
||
If the endpoints of a simple LineString are identical it is called <emphasis>closed</emphasis>
|
||
and referred to as a Linear Ring.</para>
|
||
|
||
<informaltable border="0" frame="none">
|
||
<tgroup cols="1">
|
||
<tbody>
|
||
<row>
|
||
<entry><para><emphasis>
|
||
<emphasis role="bold">(a)</emphasis> and
|
||
<emphasis role="bold">(c)</emphasis> are simple <varname>LINESTRING</varname>s.
|
||
<emphasis role="bold">(b)</emphasis> and <emphasis role="bold">(d)</emphasis> are not simple.
|
||
<emphasis role="bold">(c)</emphasis> is a closed Linear Ring.
|
||
</emphasis></para></entry>
|
||
</row>
|
||
</tbody>
|
||
</tgroup>
|
||
<tgroup cols="2" align="center">
|
||
<tbody>
|
||
<row>
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_issimple01.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(a)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_issimple02.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(b)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
</row>
|
||
|
||
<row>
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_issimple03.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(c)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_issimple04.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(d)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
</row>
|
||
</tbody>
|
||
</tgroup>
|
||
|
||
</informaltable>
|
||
|
||
<para>A <varname>MULTILINESTRING</varname> is <emphasis>simple</emphasis>
|
||
only if all of its elements are simple and the only intersection between
|
||
any two elements occurs at points that are on the
|
||
boundaries of both elements. </para>
|
||
|
||
<informaltable border="0" frame="none">
|
||
<tgroup cols="1">
|
||
<tbody>
|
||
<row>
|
||
<entry><para><emphasis>
|
||
<emphasis role="bold">(e)</emphasis> and
|
||
<emphasis role="bold">(f)</emphasis> are simple
|
||
<varname>MULTILINESTRING</varname>s.
|
||
<emphasis role="bold">(g)</emphasis> is not simple.
|
||
</emphasis></para></entry>
|
||
</row>
|
||
</tbody>
|
||
</tgroup>
|
||
<tgroup cols="3" align="center">
|
||
<tbody>
|
||
<row>
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_issimple05.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(e)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_issimple06.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(f)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_issimple07.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(g)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
</row>
|
||
</tbody>
|
||
</tgroup>
|
||
</informaltable>
|
||
|
||
<para><varname>POLYGON</varname>s are formed from linear rings, so
|
||
valid polygonal geometry is always <emphasis>simple</emphasis>.</para>
|
||
|
||
<para>To test if a geometry is simple
|
||
use the <xref linkend="ST_IsSimple" /> function:</para>
|
||
|
||
<programlisting>
|
||
SELECT
|
||
ST_IsSimple('LINESTRING(0 0, 100 100)') AS straight,
|
||
ST_IsSimple('LINESTRING(0 0, 100 100, 100 0, 0 100)') AS crossing;
|
||
|
||
straight | crossing
|
||
----------+----------
|
||
t | f
|
||
</programlisting>
|
||
|
||
<para>Generally, PostGIS functions do not require geometric arguments to be simple.
|
||
Simplicity is primarily used as a basis for defining geometric validity.
|
||
It is also a requirement for some kinds of spatial data models
|
||
(for example, linear networks often disallow lines that cross).
|
||
Multipoint and linear geometry can be made simple using <xref linkend="ST_UnaryUnion" />.
|
||
</para>
|
||
</sect2>
|
||
|
||
<sect2 id="Valid_Geometry">
|
||
<title>Valid Geometry</title>
|
||
|
||
<para>Geometry validity primarily applies to 2-dimensional
|
||
geometries (<varname>POLYGON</varname>s and <varname>MULTIPOLYGON</varname>s) .
|
||
Validity is defined by rules that allow polygonal geometry
|
||
to model planar areas unambiguously.
|
||
</para>
|
||
|
||
<para>A <varname>POLYGON</varname> is <emphasis>valid</emphasis> if:
|
||
</para>
|
||
|
||
<orderedlist>
|
||
<listitem><para>
|
||
the polygon boundary rings (the exterior shell ring and interior hole rings)
|
||
are <emphasis>simple</emphasis> (do not cross or self-touch).
|
||
Because of this a polygon cannnot have cut lines, spikes or loops.
|
||
This implies that polygon holes must be represented as interior rings,
|
||
rather than by the exterior ring self-touching (a so-called "inverted hole").
|
||
</para></listitem>
|
||
<listitem><para>
|
||
boundary rings do not cross
|
||
</para></listitem>
|
||
<listitem><para>
|
||
boundary rings may touch at points but only as a tangent (i.e. not in a line)
|
||
</para></listitem>
|
||
<listitem><para>
|
||
interior rings are contained in the exterior ring
|
||
</para></listitem>
|
||
<listitem><para>
|
||
the polygon interior is simply connected
|
||
(i.e. the rings must not touch in a way that splits the polygon into more than one part)
|
||
</para></listitem>
|
||
</orderedlist>
|
||
|
||
<informaltable border="0" frame="none">
|
||
<tgroup cols="1">
|
||
<tbody>
|
||
<row>
|
||
<entry><para><emphasis>
|
||
<emphasis role="bold">(h)</emphasis> and
|
||
<emphasis role="bold">(i)</emphasis> are valid <varname>POLYGON</varname>s.
|
||
<emphasis role="bold">(j-m)</emphasis> are invalid.
|
||
<emphasis role="bold">(j)</emphasis>
|
||
can be represented as a valid <varname>MULTIPOLYGON</varname>.
|
||
</emphasis></para></entry>
|
||
</row>
|
||
</tbody>
|
||
</tgroup>
|
||
<tgroup cols="3" align="center">
|
||
<tbody>
|
||
<row>
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid01.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(h)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid02.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(i)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid03.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(j)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
</row>
|
||
<row>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid04.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(k)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid05.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(l)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid06.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(m)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
</row>
|
||
</tbody>
|
||
</tgroup>
|
||
</informaltable>
|
||
|
||
<para>A <varname>MULTIPOLYGON</varname> is <emphasis>valid</emphasis> if:
|
||
</para>
|
||
|
||
<orderedlist>
|
||
<listitem><para>
|
||
its element <varname>POLYGON</varname>s are valid
|
||
</para></listitem>
|
||
<listitem><para>
|
||
elements do not overlap (i.e. their interiors must not intersect)
|
||
</para></listitem>
|
||
<listitem><para>
|
||
elements touch only at points (i.e. not along a line)
|
||
</para></listitem>
|
||
</orderedlist>
|
||
|
||
<informaltable border="0" frame="none">
|
||
<tgroup cols="1">
|
||
<tbody>
|
||
<row>
|
||
<entry><para><emphasis>
|
||
<emphasis role="bold">(n)</emphasis> is a valid <varname>MULTIPOLYGON</varname>.
|
||
<emphasis role="bold">(o)</emphasis> and <emphasis role="bold">(p)</emphasis> are invalid.
|
||
</emphasis></para></entry>
|
||
</row>
|
||
</tbody>
|
||
</tgroup>
|
||
<tgroup cols="3" align="center">
|
||
<tbody>
|
||
<row>
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid09.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(n)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid07.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(o)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
|
||
<entry><para><informalfigure>
|
||
<mediaobject>
|
||
<imageobject>
|
||
<imagedata fileref="images/st_isvalid08.png" />
|
||
</imageobject>
|
||
|
||
<caption><para><emphasis role="bold">(p)</emphasis></para></caption>
|
||
</mediaobject>
|
||
</informalfigure></para></entry>
|
||
</row>
|
||
</tbody>
|
||
</tgroup>
|
||
</informaltable>
|
||
|
||
<para>These rules mean that valid polygonal geometry is also <emphasis>simple</emphasis>.
|
||
</para>
|
||
|
||
<para>For linear geometry the only validity rule is that <varname>LINESTRING</varname>s must
|
||
have at least two points and have non-zero length
|
||
(or equivalently, have at least two distinct points.)
|
||
Note that non-simple (self-intersecting) lines are valid.
|
||
</para>
|
||
|
||
<programlisting>
|
||
SELECT
|
||
ST_IsValid('LINESTRING(0 0, 1 1)') AS len_nonzero,
|
||
ST_IsValid('LINESTRING(0 0, 0 0, 0 0)') AS len_zero,
|
||
ST_IsValid('LINESTRING(10 10, 150 150, 180 50, 20 130)') AS self_int;
|
||
|
||
len_nonzero | len_zero | self_int
|
||
-------------+----------+----------
|
||
t | f | t
|
||
</programlisting>
|
||
|
||
<para><varname>POINT</varname> and <varname>MULTIPOINT</varname> geometries
|
||
have no validity rules.
|
||
</para>
|
||
</sect2>
|
||
|
||
<sect2 id="Managing_Validity">
|
||
<title>Managing Validity</title>
|
||
|
||
<para>PostGIS allows creating and storing both valid and invalid Geometry.
|
||
This allows invalid geometry to be detected and flagged or fixed.
|
||
There are also situations where the OGC validity rules are stricter than desired
|
||
(examples of this are zero-length linestrings and polygons with inverted holes.)
|
||
</para>
|
||
|
||
<para>Many of the functions provided by PostGIS rely on the
|
||
assumption that geometry arguments are valid.
|
||
For example, it does not make sense to calculate the area of
|
||
a polygon that has a hole defined outside of the polygon, or to construct
|
||
a polygon from a non-simple boundary line.
|
||
Assuming valid geometric inputs allows functions to operate more efficiently,
|
||
since they do not need to check for topological correctness.
|
||
(Notable exceptions are that zero-length lines
|
||
and polygons with inversions are generally handled correctly.)
|
||
Also, most PostGIS functions produce valid geometry output if the inputs are valid.
|
||
This allows PostGIS functions to be chained together safely.
|
||
</para>
|
||
|
||
<para>If you encounter unexpected error messages when calling PostGIS functions
|
||
(such as "GEOS Intersection() threw an error!"),
|
||
you should first confirm that the function arguments are valid.
|
||
If they are not, then consider using one of the techniques below to ensure
|
||
the data you are processing is valid.
|
||
</para>
|
||
|
||
<note><para>
|
||
If a function reports an error with valid inputs,
|
||
then you may have found an error in either PostGIS or one of
|
||
the libraries it uses, and you should report this to the PostGIS project.
|
||
The same is true if a PostGIS function returns an invalid geometry for
|
||
valid input.</para></note>
|
||
|
||
<para>To test if a geometry is valid use the
|
||
<xref linkend="ST_IsValid" /> function:
|
||
</para>
|
||
<programlisting>
|
||
SELECT ST_IsValid('POLYGON ((20 180, 180 180, 180 20, 20 20, 20 180))');
|
||
-----------------
|
||
t
|
||
</programlisting>
|
||
<para>Information about the nature and location of an geometry invalidity are provided by
|
||
the <xref linkend="ST_IsValidDetail" /> function:
|
||
</para>
|
||
<programlisting>
|
||
SELECT valid, reason, ST_AsText(location) AS location
|
||
FROM ST_IsValidDetail('POLYGON ((20 20, 120 190, 50 190, 170 50, 20 20))') AS t;
|
||
|
||
valid | reason | location
|
||
-------+-------------------+---------------------------------------------
|
||
f | Self-intersection | POINT(91.51162790697674 141.56976744186045)
|
||
</programlisting>
|
||
|
||
<para>In some situations it is desirable to correct invalid geometry automatically.
|
||
Use the <xref linkend="ST_MakeValid" /> function to do this.
|
||
(<code>ST_MakeValid</code> is a case of a spatial function that <emphasis>does</emphasis> allow invalid input!)
|
||
</para>
|
||
|
||
<para>By default, PostGIS does not check for validity when loading geometry,
|
||
because validity testing can take a lot of CPU time for complex
|
||
geometries. If you do not trust your data sources,
|
||
you can enforce a validity check on your tables by adding a check
|
||
constraint:</para>
|
||
|
||
<programlisting>ALTER TABLE mytable
|
||
ADD CONSTRAINT geometry_valid_check
|
||
CHECK (ST_IsValid(geom));</programlisting>
|
||
|
||
</sect2>
|
||
</sect1>
|
||
<!-- ============================================================== -->
|
||
<sect1 id="spatial_ref_sys">
|
||
<title>Spatial Reference Systems</title>
|
||
|
||
<para>A <ulink url="https://en.wikipedia.org/wiki/Spatial_reference_system">Spatial Reference System</ulink> (SRS)
|
||
(also called a Coordinate Reference System (CRS))
|
||
defines how geometry is referenced to locations on the Earth's surface.
|
||
There are three types of SRS:
|
||
</para>
|
||
<itemizedlist>
|
||
<listitem><para>A <emphasis role="bold">geodetic</emphasis> SRS uses angular coordinates
|
||
(longitude and latitude) which map directly to the surface of the earth.
|
||
</para></listitem>
|
||
<listitem><para>A <emphasis role="bold">projected</emphasis> SRS
|
||
uses a mathematical projection transformation
|
||
to "flatten" the surface of the spheroidal earth onto a plane.
|
||
It assigns location coordinates in a way that allows direct measurement
|
||
of quantities such as distance, area, and angle.
|
||
The coordinate system is Cartesian, which means it has a defined origin point
|
||
and two perpendicular axes (usually oriented North and East).
|
||
Each projected SRS uses a stated length unit (usually metres or feet).
|
||
A projected SRS may be limited in its area of applicability to avoid distortion
|
||
and fit within the defined coordinate bounds.
|
||
</para></listitem>
|
||
<listitem><para>A <emphasis role="bold">local</emphasis> SRS
|
||
is a Cartesian coordinate system which is not referenced to the earth's surface.
|
||
In PostGIS this is specified by a SRID value of 0.
|
||
</para></listitem>
|
||
</itemizedlist>
|
||
<para>
|
||
There are many different spatial reference systems in use.
|
||
Common SRSes are standardized in the
|
||
European Petroleum Survey Group
|
||
<ulink url="http://www.epsg.org/">EPSG database</ulink>.
|
||
For convenience PostGIS (and many other spatial systems) refers to SRS
|
||
definitions using an integer identifier called a SRID.
|
||
</para>
|
||
|
||
<para>A geometry is associated with a Spatial Reference System by its SRID value,
|
||
which is accessed by <xref linkend="ST_SRID" />.
|
||
The SRID for a geometry can be assigned using <xref linkend="ST_SetSRID" />.
|
||
Some geometry constructor functions allow supplying a SRID
|
||
(such as <xref linkend="ST_Point" /> and <xref linkend="ST_MakeEnvelope" />).
|
||
The <link linkend="EWKB_EWKT">EWKT</link> format supports SRIDs with the <code>SRID=n;</code> prefix.
|
||
</para>
|
||
|
||
<para>
|
||
Spatial functions processing pairs of geometries
|
||
(such as <link linkend="Overlay_Functions">overlay</link> and
|
||
<link linkend="Spatial_Relationships">relationship</link> functions)
|
||
require that the input geometries are in the same spatial reference system (have the same SRID).
|
||
Geometry data can be transformed into a different spatial reference system using
|
||
<xref linkend="ST_Transform" /> and <xref linkend="ST_TransformPipeline" />.
|
||
Geometry returned from functions has the same SRS as the input geometries.
|
||
</para>
|
||
|
||
<sect2 id="spatial_ref_sys_table">
|
||
<title>SPATIAL_REF_SYS Table</title>
|
||
|
||
<para>The <varname>SPATIAL_REF_SYS</varname> table used by PostGIS
|
||
is an OGC-compliant database table that defines the available
|
||
spatial reference systems.
|
||
It holds the numeric SRIDs and textual descriptions of the coordinate systems.
|
||
</para>
|
||
|
||
<para>The <varname>spatial_ref_sys</varname> table definition is:</para>
|
||
|
||
<programlisting>CREATE TABLE spatial_ref_sys (
|
||
srid INTEGER NOT NULL PRIMARY KEY,
|
||
auth_name VARCHAR(256),
|
||
auth_srid INTEGER,
|
||
srtext VARCHAR(2048),
|
||
proj4text VARCHAR(2048)
|
||
)</programlisting>
|
||
|
||
<para>The columns are:</para>
|
||
|
||
<variablelist>
|
||
<varlistentry>
|
||
<term>srid</term>
|
||
|
||
<listitem>
|
||
<para>An integer code that uniquely identifies the <ulink url="http://en.wikipedia.org/wiki/SRID">Spatial
|
||
Reference System</ulink> (SRS) within the database.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>auth_name</term>
|
||
|
||
<listitem>
|
||
<para>The name of the standard or standards body that is being
|
||
cited for this reference system. For example, "EPSG" is a
|
||
valid <varname>auth_name</varname>.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>auth_srid</term>
|
||
|
||
<listitem>
|
||
<para>The ID of the Spatial Reference System as defined by the
|
||
Authority cited in the <varname>auth_name</varname>. In the case
|
||
of EPSG, this is the EPSG code.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>srtext</term>
|
||
|
||
<listitem>
|
||
<para>The Well-Known Text representation of the Spatial Reference
|
||
System. An example of a WKT SRS representation is:</para>
|
||
|
||
<programlisting>PROJCS["NAD83 / UTM Zone 10N",
|
||
GEOGCS["NAD83",
|
||
DATUM["North_American_Datum_1983",
|
||
SPHEROID["GRS 1980",6378137,298.257222101]
|
||
],
|
||
PRIMEM["Greenwich",0],
|
||
UNIT["degree",0.0174532925199433]
|
||
],
|
||
PROJECTION["Transverse_Mercator"],
|
||
PARAMETER["latitude_of_origin",0],
|
||
PARAMETER["central_meridian",-123],
|
||
PARAMETER["scale_factor",0.9996],
|
||
PARAMETER["false_easting",500000],
|
||
PARAMETER["false_northing",0],
|
||
UNIT["metre",1]
|
||
]</programlisting>
|
||
|
||
<para>For a discussion of SRS WKT, see the OGC standard <ulink
|
||
url="http://docs.opengeospatial.org/is/12-063r5/12-063r5.html">Well-known text representation of coordinate reference systems</ulink>.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>proj4text</term>
|
||
|
||
<listitem>
|
||
<para>PostGIS uses the PROJ library to provide coordinate
|
||
transformation capabilities. The <varname>proj4text</varname>
|
||
column contains the PROJ coordinate definition string for a
|
||
particular SRID. For example:</para>
|
||
|
||
<programlisting>+proj=utm +zone=10 +ellps=clrk66 +datum=NAD27 +units=m</programlisting>
|
||
|
||
<para>For more information see the
|
||
<ulink url="https://proj.org/">PROJ web site</ulink>.
|
||
The <filename>spatial_ref_sys.sql</filename> file contains both
|
||
<varname>srtext</varname> and <varname>proj4text</varname>
|
||
definitions for all EPSG projections.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
</variablelist>
|
||
|
||
<para>When retrieving spatial reference system definitions for use in transformations,
|
||
PostGIS uses fhe following strategy:
|
||
</para>
|
||
<itemizedlist>
|
||
<listitem><para>If <varname>auth_name</varname> and <varname>auth_srid</varname>
|
||
are present (non-NULL)
|
||
use the PROJ SRS based on those entries (if one exists).
|
||
</para></listitem>
|
||
<listitem><para>If <varname>srtext</varname> is present
|
||
create a SRS using it, if possible.
|
||
</para></listitem>
|
||
<listitem><para>If <varname>proj4text</varname> is present
|
||
create a SRS using it, if possible.
|
||
</para></listitem>
|
||
</itemizedlist>
|
||
</sect2>
|
||
<sect2 id="user-spatial-ref-sys">
|
||
<title>User-Defined Spatial Reference Systems</title>
|
||
|
||
<para>The PostGIS <varname>spatial_ref_sys</varname> table contains over 3000 of
|
||
the most common spatial reference system definitions that are handled by the
|
||
<ulink url="https://proj.org">PROJ</ulink> projection library.
|
||
But there are many coordinate systems that it does not contain.
|
||
You can add SRS definitions to the table if you have
|
||
the required information about the spatial reference system.
|
||
Or, you can define your own custom spatial reference system if you are familiar with PROJ constructs.
|
||
Keep in mind that most spatial reference systems are regional
|
||
and have no meaning when used outside of the bounds they were intended for.</para>
|
||
|
||
<para>A resource for finding spatial reference systems not defined in the core set is <ulink url="http://spatialreference.org/">http://spatialreference.org/</ulink></para>
|
||
|
||
<para>Some commonly used spatial reference systems are:
|
||
<ulink url="http://spatialreference.org/ref/epsg/4326/">4326 - WGS 84 Long Lat</ulink>,
|
||
<ulink url="http://spatialreference.org/ref/epsg/4269/">4269 - NAD 83 Long Lat</ulink>,
|
||
<ulink url="http://spatialreference.org/ref/epsg/3395/">3395 - WGS 84 World Mercator</ulink>,
|
||
<ulink url="http://spatialreference.org/ref/epsg/2163/">2163 - US National Atlas Equal Area</ulink>,
|
||
and the 60 WGS84 UTM zones.
|
||
UTM zones are one of the most ideal for measurement, but only cover 6-degree regions.
|
||
(To determine which UTM zone to use for your area of interest, see the <ulink url="http://trac.osgeo.org/postgis/wiki/UsersWikiplpgsqlfunctionsDistance">utmzone PostGIS plpgsql helper function</ulink>.)
|
||
</para>
|
||
<para>
|
||
US states use State Plane spatial reference systems (meter or feet based) - usually one or 2 exists per state.
|
||
Most of the meter-based ones are in the core set, but many of the
|
||
feet-based ones or ESRI-created ones will need to be copied from <ulink url="http://spatialreference.org">spatialreference.org</ulink>.
|
||
</para>
|
||
|
||
<para>You can even define non-Earth-based coordinate systems,
|
||
such as <ulink url="http://spatialreference.org/ref/iau2000/mars-2000/">Mars 2000</ulink>
|
||
This Mars coordinate system is non-planar (it's in degrees spheroidal),
|
||
but you can use it with the <varname>geography</varname> type
|
||
to obtain length and proximity measurements in meters instead of degrees.</para>
|
||
|
||
<para>Here is an example of loading a custom coordinate system using
|
||
an unassigned SRID and the PROJ definition for a US-centric Lambert Conformal projection:</para>
|
||
|
||
<programlisting>
|
||
INSERT INTO spatial_ref_sys (srid, proj4text)
|
||
VALUES ( 990000,
|
||
'+proj=lcc +lon_0=-95 +lat_0=25 +lat_1=25 +lat_2=25 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs'
|
||
);
|
||
</programlisting>
|
||
|
||
</sect2>
|
||
|
||
</sect1>
|
||
<!-- ============================================================== -->
|
||
<sect1>
|
||
<title>Spatial Tables</title>
|
||
|
||
<sect2 id="Create_Spatial_Table">
|
||
<title>Creating a Spatial Table</title>
|
||
|
||
<para>You can create a table to store geometry data using the
|
||
<ulink url="https://www.postgresql.org/docs/current/sql-createtable.html">CREATE TABLE</ulink>
|
||
SQL statement with a column of type <varname>geometry</varname>.
|
||
The following example creates a table with a geometry column storing 2D (XY) LineStrings
|
||
in the BC-Albers coordinate system (SRID 3005):</para>
|
||
|
||
<programlisting>CREATE TABLE roads (
|
||
id SERIAL PRIMARY KEY,
|
||
name VARCHAR(64),
|
||
geom geometry(LINESTRING,3005)
|
||
);</programlisting>
|
||
|
||
<para>The <varname>geometry</varname> type supports two optional <emphasis role="bold">type modifiers</emphasis>:</para>
|
||
|
||
<itemizedlist>
|
||
<listitem>
|
||
<para>the <emphasis role="bold">spatial type modifier</emphasis>
|
||
restricts the kind of shapes and dimensions allowed in the column.
|
||
The value can be any of the supported
|
||
<link linkend="RefObject">geometry subtypes</link>
|
||
(e.g. POINT, LINESTRING, POLYGON, MULTIPOINT, MULTILINESTRING, MULTIPOLYGON, GEOMETRYCOLLECTION, etc).
|
||
The modifier supports coordinate dimensionality restrictions by adding suffixes: Z, M and ZM.
|
||
For example, a modifier of 'LINESTRINGM' allows only linestrings with three dimensions, and treats the third dimension as a measure.
|
||
Similarly, 'POINTZM' requires four dimensional (XYZM) data.
|
||
</para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>the <emphasis role="bold">SRID modifier</emphasis> restricts the
|
||
<link linkend="spatial_ref_sys">spatial reference system</link> SRID to a particular number.
|
||
If omitted, the SRID defaults to 0.
|
||
</para>
|
||
</listitem>
|
||
</itemizedlist>
|
||
|
||
<para></para>
|
||
|
||
<para>Examples of creating tables with geometry columns:</para>
|
||
<itemizedlist>
|
||
<listitem>
|
||
<para>Create a table holding any kind of geometry with the default SRID:</para>
|
||
<para><programlisting>CREATE TABLE geoms(gid serial PRIMARY KEY, geom geometry );</programlisting></para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>Create a table with 2D POINT geometry with the default SRID:</para>
|
||
<para><programlisting>CREATE TABLE pts(gid serial PRIMARY KEY, geom geometry(POINT) );</programlisting></para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>Create a table with 3D (XYZ) POINTs and an explicit SRID of 3005:</para>
|
||
<para><programlisting>CREATE TABLE pts(gid serial PRIMARY KEY, geom geometry(POINTZ,3005) );</programlisting></para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>Create a table with 4D (XYZM) LINESTRING geometry with the default SRID:</para>
|
||
<para><programlisting>CREATE TABLE lines(gid serial PRIMARY KEY, geom geometry(LINESTRINGZM) );</programlisting></para>
|
||
</listitem>
|
||
<listitem>
|
||
<para>Create a table with 2D POLYGON geometry with the SRID 4267 (NAD 1927 long lat):</para>
|
||
<para><programlisting>CREATE TABLE polys(gid serial PRIMARY KEY, geom geometry(POLYGON,4267) );</programlisting></para>
|
||
</listitem>
|
||
</itemizedlist>
|
||
|
||
<para>It is possible to have more than one geometry column in a table.
|
||
This can be specified when the table is created, or a column can be added using the
|
||
<ulink url="https://www.postgresql.org/docs/current/sql-altertable.html">ALTER TABLE</ulink>
|
||
SQL statement.
|
||
This example adds a column that can hold 3D LineStrings:</para>
|
||
|
||
<programlisting>ALTER TABLE roads ADD COLUMN geom2 geometry(LINESTRINGZ,4326);</programlisting>
|
||
</sect2>
|
||
|
||
<sect2 id="geometry_columns">
|
||
<title>GEOMETRY_COLUMNS View</title>
|
||
|
||
<para>The OGC <emphasis>Simple Features Specification for SQL</emphasis> defines
|
||
the <varname>GEOMETRY_COLUMNS</varname> metadata table to describe geometry table structure.
|
||
In PostGIS <varname>geometry_columns</varname> is a view reading from database system catalog tables.
|
||
This ensures that the spatial metadata information is always consistent with the currently defined tables and views.
|
||
The view structure is:</para>
|
||
|
||
<programlisting>\d geometry_columns</programlisting>
|
||
<screen> View "public.geometry_columns"
|
||
Column | Type | Modifiers
|
||
-------------------+------------------------+-----------
|
||
f_table_catalog | character varying(256) |
|
||
f_table_schema | character varying(256) |
|
||
f_table_name | character varying(256) |
|
||
f_geometry_column | character varying(256) |
|
||
coord_dimension | integer |
|
||
srid | integer |
|
||
type | character varying(30) |</screen>
|
||
|
||
<para>The columns are:</para>
|
||
|
||
<variablelist>
|
||
<varlistentry>
|
||
<term>f_table_catalog, f_table_schema, f_table_name</term>
|
||
|
||
<listitem>
|
||
<para>The fully qualified name of the feature table containing the
|
||
geometry column. There is no PostgreSQL analogue of "catalog" so that
|
||
column is left blank. For "schema" the PostgreSQL schema name is
|
||
used (<varname>public</varname> is the default).</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>f_geometry_column</term>
|
||
|
||
<listitem>
|
||
<para>The name of the geometry column in the feature table.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>coord_dimension</term>
|
||
|
||
<listitem>
|
||
<para>The coordinate dimension (2, 3 or 4) of the column.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>srid</term>
|
||
|
||
<listitem>
|
||
<para>The ID of the spatial reference system used for the
|
||
coordinate geometry in this table. It is a foreign key reference
|
||
to the <varname>spatial_ref_sys</varname> table
|
||
(see <xref linkend="spatial_ref_sys_table" />).</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>type</term>
|
||
|
||
<listitem>
|
||
<para>The type of the spatial object. To restrict the spatial
|
||
column to a single type, use one of: POINT, LINESTRING, POLYGON,
|
||
MULTIPOINT, MULTILINESTRING, MULTIPOLYGON, GEOMETRYCOLLECTION or
|
||
corresponding XYM versions POINTM, LINESTRINGM, POLYGONM,
|
||
MULTIPOINTM, MULTILINESTRINGM, MULTIPOLYGONM, GEOMETRYCOLLECTIONM.
|
||
For heterogeneous (mixed-type) collections, you can use "GEOMETRY"
|
||
as the type.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
</variablelist>
|
||
</sect2>
|
||
|
||
<sect2 id="Manual_Register_Spatial_Column">
|
||
<title>Manually Registering Geometry Columns</title>
|
||
|
||
<para>Two of the cases where you may need this are the case of SQL Views and bulk inserts. For bulk insert case, you can correct the registration in the geometry_columns table
|
||
by constraining the column or doing an alter table. For views, you could expose using a CAST operation.
|
||
Note, if your column is typmod based, the creation process would register it correctly, so no need to do anything.
|
||
Also views that have no spatial function applied to the geometry will register the same as the underlying table geometry column.</para>
|
||
|
||
<programlisting>-- Lets say you have a view created like this
|
||
CREATE VIEW public.vwmytablemercator AS
|
||
SELECT gid, ST_Transform(geom, 3395) As geom, f_name
|
||
FROM public.mytable;
|
||
|
||
-- For it to register correctly
|
||
-- You need to cast the geometry
|
||
--
|
||
DROP VIEW public.vwmytablemercator;
|
||
CREATE VIEW public.vwmytablemercator AS
|
||
SELECT gid, ST_Transform(geom, 3395)::geometry(Geometry, 3395) As geom, f_name
|
||
FROM public.mytable;
|
||
|
||
-- If you know the geometry type for sure is a 2D POLYGON then you could do
|
||
DROP VIEW public.vwmytablemercator;
|
||
CREATE VIEW public.vwmytablemercator AS
|
||
SELECT gid, ST_Transform(geom,3395)::geometry(Polygon, 3395) As geom, f_name
|
||
FROM public.mytable;</programlisting>
|
||
<programlisting>--Lets say you created a derivative table by doing a bulk insert
|
||
SELECT poi.gid, poi.geom, citybounds.city_name
|
||
INTO myschema.my_special_pois
|
||
FROM poi INNER JOIN citybounds ON ST_Intersects(citybounds.geom, poi.geom);
|
||
|
||
-- Create 2D index on new table
|
||
CREATE INDEX idx_myschema_myspecialpois_geom_gist
|
||
ON myschema.my_special_pois USING gist(geom);
|
||
|
||
-- If your points are 3D points or 3M points,
|
||
-- then you might want to create an nd index instead of a 2D index
|
||
CREATE INDEX my_special_pois_geom_gist_nd
|
||
ON my_special_pois USING gist(geom gist_geometry_ops_nd);
|
||
|
||
-- To manually register this new table's geometry column in geometry_columns.
|
||
-- Note it will also change the underlying structure of the table to
|
||
-- to make the column typmod based.
|
||
SELECT populate_geometry_columns('myschema.my_special_pois'::regclass);
|
||
|
||
-- If you are using PostGIS 2.0 and for whatever reason, you
|
||
-- you need the constraint based definition behavior
|
||
-- (such as case of inherited tables where all children do not have the same type and srid)
|
||
-- set optional use_typmod argument to false
|
||
SELECT populate_geometry_columns('myschema.my_special_pois'::regclass, false); </programlisting>
|
||
|
||
<para>Although the old-constraint based method is still supported, a constraint-based geometry column used directly
|
||
in a view, will not register correctly in geometry_columns, as will a typmod one.
|
||
In this example we define a column using typmod and another using constraints.</para>
|
||
<programlisting>CREATE TABLE pois_ny(gid SERIAL PRIMARY KEY, poi_name text, cat text, geom geometry(POINT,4326));
|
||
SELECT AddGeometryColumn('pois_ny', 'geom_2160', 2160, 'POINT', 2, false);</programlisting>
|
||
<para>If we run in psql</para>
|
||
<programlisting>\d pois_ny;</programlisting>
|
||
<para>We observe they are defined differently -- one is typmod, one is constraint</para>
|
||
<screen> Table "public.pois_ny"
|
||
Column | Type | Modifiers
|
||
|
||
-----------+-----------------------+------------------------------------------------------
|
||
gid | integer | not null default nextval('pois_ny_gid_seq'::regclass)
|
||
poi_name | text |
|
||
cat | character varying(20) |
|
||
geom | geometry(Point,4326) |
|
||
geom_2160 | geometry |
|
||
Indexes:
|
||
"pois_ny_pkey" PRIMARY KEY, btree (gid)
|
||
Check constraints:
|
||
"enforce_dims_geom_2160" CHECK (st_ndims(geom_2160) = 2)
|
||
"enforce_geotype_geom_2160" CHECK (geometrytype(geom_2160) = 'POINT'::text
|
||
OR geom_2160 IS NULL)
|
||
"enforce_srid_geom_2160" CHECK (st_srid(geom_2160) = 2160)</screen>
|
||
<para>In geometry_columns, they both register correctly</para>
|
||
<programlisting>SELECT f_table_name, f_geometry_column, srid, type
|
||
FROM geometry_columns
|
||
WHERE f_table_name = 'pois_ny';</programlisting>
|
||
<screen>f_table_name | f_geometry_column | srid | type
|
||
-------------+-------------------+------+-------
|
||
pois_ny | geom | 4326 | POINT
|
||
pois_ny | geom_2160 | 2160 | POINT</screen>
|
||
<para>However -- if we were to create a view like this</para>
|
||
<programlisting>CREATE VIEW vw_pois_ny_parks AS
|
||
SELECT *
|
||
FROM pois_ny
|
||
WHERE cat='park';
|
||
|
||
SELECT f_table_name, f_geometry_column, srid, type
|
||
FROM geometry_columns
|
||
WHERE f_table_name = 'vw_pois_ny_parks';</programlisting>
|
||
<para>The typmod based geom view column registers correctly,
|
||
but the constraint based one does not.</para>
|
||
<screen> f_table_name | f_geometry_column | srid | type
|
||
------------------+-------------------+------+----------
|
||
vw_pois_ny_parks | geom | 4326 | POINT
|
||
vw_pois_ny_parks | geom_2160 | 0 | GEOMETRY</screen>
|
||
|
||
<para>This may change in future versions of PostGIS, but for now
|
||
to force the constraint-based view column to register correctly, you need to do this:</para>
|
||
<programlisting>DROP VIEW vw_pois_ny_parks;
|
||
CREATE VIEW vw_pois_ny_parks AS
|
||
SELECT gid, poi_name, cat,
|
||
geom,
|
||
geom_2160::geometry(POINT,2160) As geom_2160
|
||
FROM pois_ny
|
||
WHERE cat = 'park';
|
||
SELECT f_table_name, f_geometry_column, srid, type
|
||
FROM geometry_columns
|
||
WHERE f_table_name = 'vw_pois_ny_parks';</programlisting>
|
||
<screen> f_table_name | f_geometry_column | srid | type
|
||
------------------+-------------------+------+-------
|
||
vw_pois_ny_parks | geom | 4326 | POINT
|
||
vw_pois_ny_parks | geom_2160 | 2160 | POINT</screen>
|
||
</sect2>
|
||
</sect1>
|
||
|
||
<!-- ============================================================== -->
|
||
|
||
<sect1 id="loading-data">
|
||
<title>Loading Spatial Data</title>
|
||
|
||
<para>Once you have created a spatial table, you are ready to upload spatial
|
||
data to the database. There are two built-in ways to get spatial data into a
|
||
PostGIS/PostgreSQL database: using formatted SQL statements or using the
|
||
Shapefile loader.</para>
|
||
|
||
<sect2 id="load-data-sql">
|
||
<title>Using SQL to Load Data</title>
|
||
|
||
<para>If spatial data can be converted to a text representation (as either WKT or WKB), then using
|
||
SQL might be the easiest way to get data into PostGIS.
|
||
Data can be bulk-loaded into PostGIS/PostgreSQL by loading a
|
||
text file of SQL <code>INSERT</code> statements using the <code>psql</code> SQL utility.</para>
|
||
|
||
<para>A SQL load file (<filename>roads.sql</filename> for example)
|
||
might look like this:</para>
|
||
|
||
<programlisting>BEGIN;
|
||
INSERT INTO roads (road_id, roads_geom, road_name)
|
||
VALUES (1,'LINESTRING(191232 243118,191108 243242)','Jeff Rd');
|
||
INSERT INTO roads (road_id, roads_geom, road_name)
|
||
VALUES (2,'LINESTRING(189141 244158,189265 244817)','Geordie Rd');
|
||
INSERT INTO roads (road_id, roads_geom, road_name)
|
||
VALUES (3,'LINESTRING(192783 228138,192612 229814)','Paul St');
|
||
INSERT INTO roads (road_id, roads_geom, road_name)
|
||
VALUES (4,'LINESTRING(189412 252431,189631 259122)','Graeme Ave');
|
||
INSERT INTO roads (road_id, roads_geom, road_name)
|
||
VALUES (5,'LINESTRING(190131 224148,190871 228134)','Phil Tce');
|
||
INSERT INTO roads (road_id, roads_geom, road_name)
|
||
VALUES (6,'LINESTRING(198231 263418,198213 268322)','Dave Cres');
|
||
COMMIT;</programlisting>
|
||
|
||
<para>The SQL file can be loaded into PostgreSQL using <code>psql</code>:</para>
|
||
|
||
<programlisting>psql -d [database] -f roads.sql</programlisting>
|
||
</sect2>
|
||
|
||
<sect2 id="shp2pgsql_usage">
|
||
<title>Using the Shapefile Loader</title>
|
||
|
||
<para>
|
||
The <filename>shp2pgsql</filename> data loader converts Shapefiles into SQL suitable for
|
||
insertion into a PostGIS/PostgreSQL database either in geometry or geography format.
|
||
The loader has several operating modes selected by command line flags.
|
||
</para>
|
||
<para>There is also a <filename>shp2pgsql-gui</filename> graphical interface with most
|
||
of the options as the command-line loader.
|
||
This may be easier to use for one-off non-scripted loading or if you are new to PostGIS.
|
||
It can also be configured as a plugin to PgAdminIII.
|
||
</para>
|
||
|
||
<variablelist>
|
||
<varlistentry>
|
||
<term>(c|a|d|p) These are mutually exclusive options:</term>
|
||
<listitem>
|
||
<para>
|
||
<variablelist>
|
||
<varlistentry>
|
||
<term>-c</term>
|
||
<listitem>
|
||
<para>
|
||
Creates a new table and populates it from the Shapefile. <emphasis>This is the
|
||
default mode.</emphasis>
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-a</term>
|
||
<listitem>
|
||
<para>
|
||
Appends data from the Shapefile into the database table. Note that to use this
|
||
option to load multiple files, the files must have the same attributes and same
|
||
data types.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-d</term>
|
||
<listitem>
|
||
<para>
|
||
Drops the database table before creating a new table with the data in the Shapefile.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-p</term>
|
||
<listitem>
|
||
<para>
|
||
Only produces the table creation SQL code, without adding any actual data. This
|
||
can be used if you need to completely separate the table creation and data loading
|
||
steps.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
</variablelist>
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-?</term>
|
||
<listitem>
|
||
<para>
|
||
Display help screen.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-D</term>
|
||
<listitem>
|
||
<para>
|
||
Use the PostgreSQL "dump" format for the output data. This can be combined with -a, -c and
|
||
-d. It is much faster to load than the default "insert" SQL format. Use this for very
|
||
large data sets.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-s [<FROM_SRID>:]<SRID></term>
|
||
<listitem>
|
||
<para>
|
||
Creates and populates the geometry tables with the specified SRID.
|
||
Optionally specifies that the input shapefile uses the given
|
||
FROM_SRID, in which case the geometries will be reprojected to the
|
||
target SRID.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-k</term>
|
||
<listitem>
|
||
<para>
|
||
Keep identifiers' case (column, schema and attributes). Note that attributes in Shapefile
|
||
are all UPPERCASE.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-i</term>
|
||
<listitem>
|
||
<para>
|
||
Coerce all integers to standard 32-bit integers, do not create 64-bit bigints, even if the
|
||
DBF header signature appears to warrant it.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-I</term>
|
||
<listitem>
|
||
<para>
|
||
Create a GiST index on the geometry column.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-m</term>
|
||
<listitem>
|
||
<para>
|
||
-m <filename>a_file_name</filename> Specify a file containing a set of mappings of (long) column
|
||
names to 10 character DBF column names. The content of the file is one or
|
||
more lines of two names separated by white space and no trailing or
|
||
leading space. For example:
|
||
<programlisting>COLUMNNAME DBFFIELD1
|
||
AVERYLONGCOLUMNNAME DBFFIELD2</programlisting>
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-S </term>
|
||
<listitem>
|
||
<para>
|
||
Generate simple geometries instead of MULTI geometries. Will only succeed if
|
||
all the geometries are actually single (I.E. a MULTIPOLYGON with a single shell, or
|
||
or a MULTIPOINT with a single vertex).
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-t <dimensionality></term>
|
||
<listitem>
|
||
<para>
|
||
Force the output geometry to have the specified dimensionality. Use the following
|
||
strings to indicate the dimensionality: 2D, 3DZ, 3DM, 4D.
|
||
</para>
|
||
<para>
|
||
If the input has fewer dimensions that specified, the output will have those dimensions filled
|
||
in with zeroes. If the input has more dimensions that specified, the unwanted dimensions will
|
||
be stripped.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-w</term>
|
||
<listitem>
|
||
<para>
|
||
Output WKT format, instead of WKB. Note that this can
|
||
introduce coordinate drifts due to loss of precision.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-e</term>
|
||
<listitem>
|
||
<para>
|
||
Execute each statement on its own, without using a transaction.
|
||
This allows loading of the majority of good data when there are some bad
|
||
geometries that generate errors. Note that this cannot be used with the
|
||
-D flag as the "dump" format always uses a transaction.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-W <encoding></term>
|
||
<listitem>
|
||
<para>
|
||
Specify encoding of the input data (dbf file). When used, all attributes of the dbf are
|
||
converted from the specified encoding to UTF8. The resulting SQL output will contain a
|
||
<code>SET CLIENT_ENCODING to UTF8</code> command, so that the backend will be able to
|
||
reconvert from UTF8 to whatever encoding the database is configured to use internally.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-N <policy></term>
|
||
<listitem>
|
||
<para>
|
||
NULL geometries handling policy (insert*,skip,abort)
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
<varlistentry>
|
||
<term>-n</term>
|
||
<listitem>
|
||
<para>
|
||
-n Only import DBF file. If your data has no corresponding shapefile, it will automatically switch to this mode
|
||
and load just the dbf. So setting this flag is only needed if you have a full shapefile set, and you only want the attribute data and no geometry.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-G</term>
|
||
<listitem>
|
||
<para>
|
||
Use geography type instead of geometry (requires lon/lat data) in WGS84 long lat (SRID=4326)
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
<varlistentry>
|
||
<term>-T <tablespace></term>
|
||
<listitem>
|
||
<para>
|
||
Specify the tablespace for the new table. Indexes will still use the
|
||
default tablespace unless the -X parameter is also used. The PostgreSQL
|
||
documentation has a good description on when to use custom tablespaces.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
<varlistentry>
|
||
<term>-X <tablespace></term>
|
||
<listitem>
|
||
<para>
|
||
Specify the tablespace for the new table's indexes. This applies to
|
||
the primary key index, and the GIST spatial index if -I is also used.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
<varlistentry>
|
||
<term>-Z</term>
|
||
<listitem>
|
||
<para>
|
||
When used, this flag will prevent the generation of <code>ANALYZE</code> statements.
|
||
Without the -Z flag (default behavior), the <code>ANALYZE</code> statements will
|
||
be generated.
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
</variablelist>
|
||
|
||
<para>
|
||
An example session using the loader to create an input file and loading it might look like
|
||
this:
|
||
</para>
|
||
|
||
<programlisting># shp2pgsql -c -D -s 4269 -i -I shaperoads.shp myschema.roadstable > roads.sql
|
||
# psql -d roadsdb -f roads.sql</programlisting>
|
||
|
||
<para>
|
||
A conversion and load can be done in one step using UNIX pipes:
|
||
</para>
|
||
|
||
<programlisting># shp2pgsql shaperoads.shp myschema.roadstable | psql -d roadsdb</programlisting>
|
||
</sect2>
|
||
</sect1>
|
||
|
||
<sect1 id="extracting-data">
|
||
<title>Extracting Spatial Data</title>
|
||
|
||
<para>Spatial data can be extracted from the database using either SQL or the
|
||
Shapefile dumper. The section on SQL presents some of
|
||
the functions available to do comparisons and queries on spatial tables.
|
||
</para>
|
||
|
||
<sect2 id="extract-data-sql">
|
||
<title>Using SQL to Extract Data</title>
|
||
|
||
<para>The most straightforward way of extracting spatial data out of the
|
||
database is to use a SQL <code>SELECT</code> query
|
||
to define the data set to be extracted
|
||
and dump the resulting columns into a parsable text file:</para>
|
||
|
||
<programlisting>db=# SELECT road_id, ST_AsText(road_geom) AS geom, road_name FROM roads;
|
||
|
||
road_id | geom | road_name
|
||
--------+-----------------------------------------+-----------
|
||
1 | LINESTRING(191232 243118,191108 243242) | Jeff Rd
|
||
2 | LINESTRING(189141 244158,189265 244817) | Geordie Rd
|
||
3 | LINESTRING(192783 228138,192612 229814) | Paul St
|
||
4 | LINESTRING(189412 252431,189631 259122) | Graeme Ave
|
||
5 | LINESTRING(190131 224148,190871 228134) | Phil Tce
|
||
6 | LINESTRING(198231 263418,198213 268322) | Dave Cres
|
||
7 | LINESTRING(218421 284121,224123 241231) | Chris Way
|
||
(6 rows)</programlisting>
|
||
|
||
<para>There will be times when some kind of restriction is
|
||
necessary to cut down the number of records returned. In the case of
|
||
attribute-based restrictions, use the same SQL syntax as used
|
||
with a non-spatial table. In the case of spatial restrictions, the
|
||
following functions are useful:</para>
|
||
|
||
<variablelist>
|
||
<varlistentry>
|
||
<term>ST_Intersects</term>
|
||
|
||
<listitem>
|
||
<para>This function tells whether two geometries share any space.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>=</term>
|
||
|
||
<listitem>
|
||
<para>This tests whether two geometries are
|
||
geometrically identical. For example, if 'POLYGON((0 0,1 1,1 0,0
|
||
0))' is the same as 'POLYGON((0 0,1 1,1 0,0 0))' (it is).
|
||
</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
</variablelist>
|
||
|
||
<para>Next, you can use these operators in queries. Note that when
|
||
specifying geometries and boxes on the SQL command line, you must
|
||
explicitly turn the string representations into geometries function.
|
||
The 312 is a fictitious spatial reference system that matches our data.
|
||
So, for example:</para>
|
||
|
||
<programlisting>SELECT road_id, road_name
|
||
FROM roads
|
||
WHERE roads_geom='SRID=312;LINESTRING(191232 243118,191108 243242)'::geometry;</programlisting>
|
||
|
||
<para>The above query would return the single record from the
|
||
"ROADS_GEOM" table in which the geometry was equal to that value.</para>
|
||
|
||
<para>To check whether some of the roads passes in the area defined by a polygon:</para>
|
||
|
||
<programlisting>SELECT road_id, road_name
|
||
FROM roads
|
||
WHERE ST_Intersects(roads_geom, 'SRID=312;POLYGON((...))');</programlisting>
|
||
|
||
|
||
|
||
<para>The most common spatial query will probably be a "frame-based"
|
||
query, used by client software, like data browsers and web mappers, to
|
||
grab a "map frame" worth of data for display. </para>
|
||
<para>When using the "&&" operator, you can specify either a
|
||
BOX3D as the comparison feature or a GEOMETRY. When you specify a
|
||
GEOMETRY, however, its bounding box will be used for the
|
||
comparison.</para>
|
||
<para>Using a "BOX3D" object for the frame, such a query looks like this:</para>
|
||
|
||
<programlisting>SELECT ST_AsText(roads_geom) AS geom
|
||
FROM roads
|
||
WHERE
|
||
roads_geom && ST_MakeEnvelope(191232, 243117,191232, 243119,312);</programlisting>
|
||
|
||
<para>Note the use of the SRID 312, to specify the projection of the envelope.</para>
|
||
|
||
|
||
</sect2>
|
||
|
||
<sect2 id="pgsql2shp-usage">
|
||
<title>Using the Shapefile Dumper</title>
|
||
|
||
<para>The <filename>pgsql2shp</filename> table dumper connects
|
||
to the database and converts a table (possibly defined by a query) into
|
||
a shape file. The basic syntax is:</para>
|
||
|
||
<programlisting>pgsql2shp [<options>] <database> [<schema>.]<table></programlisting>
|
||
|
||
<programlisting>pgsql2shp [<options>] <database> <query></programlisting>
|
||
|
||
<para>The commandline options are:</para>
|
||
|
||
<variablelist>
|
||
<varlistentry>
|
||
<term>-f <filename></term>
|
||
|
||
<listitem>
|
||
<para>Write the output to a particular filename.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-h <host></term>
|
||
|
||
<listitem>
|
||
<para>The database host to connect to.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-p <port></term>
|
||
|
||
<listitem>
|
||
<para>The port to connect to on the database host.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-P <password></term>
|
||
|
||
<listitem>
|
||
<para>The password to use when connecting to the database.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-u <user></term>
|
||
|
||
<listitem>
|
||
<para>The username to use when connecting to the database.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-g <geometry column></term>
|
||
|
||
<listitem>
|
||
<para>In the case of tables with multiple geometry columns, the
|
||
geometry column to use when writing the shape file.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-b</term>
|
||
|
||
<listitem>
|
||
<para>Use a binary cursor. This will make the operation faster,
|
||
but will not work if any NON-geometry attribute in the table lacks
|
||
a cast to text.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-r</term>
|
||
|
||
<listitem>
|
||
<para>Raw mode. Do not drop the <varname>gid</varname> field, or
|
||
escape column names.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
|
||
<varlistentry>
|
||
<term>-m <varname>filename</varname></term>
|
||
<listitem>
|
||
<para> Remap identifiers to ten character names.
|
||
The content of the file is lines of two symbols separated by
|
||
a single white space and no trailing or leading space:
|
||
VERYLONGSYMBOL SHORTONE
|
||
ANOTHERVERYLONGSYMBOL SHORTER
|
||
etc.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
</variablelist>
|
||
</sect2>
|
||
</sect1>
|
||
|
||
<sect1 id="build-indexes">
|
||
<title>Spatial Indexes</title>
|
||
|
||
<para>Spatial indexes make using a spatial database for large data sets
|
||
possible. Without indexing, a search for features requires a
|
||
sequential scan of every record in the database. Indexing speeds up
|
||
searching by organizing the data into a structure which can be quickly
|
||
traversed to find matching records.
|
||
</para>
|
||
<para>The B-tree index method commonly used for attribute data
|
||
is not very useful for spatial data, since it only supports storing and querying
|
||
data in a single dimension.
|
||
Data such as geometry (which has 2 or more dimensions)
|
||
requires an index method that supports range query across all the data dimensions.
|
||
One of the key advantages of PostgreSQL for spatial data handling is that it offers several kinds of
|
||
index methods which work well for multi-dimensional data: GiST, BRIN and SP-GiST indexes.</para>
|
||
|
||
<itemizedlist>
|
||
<listitem>
|
||
<para><emphasis role="bold">GiST (Generalized Search Tree)</emphasis> indexes break up data into
|
||
"things to one side", "things which overlap", "things which are
|
||
inside" and can be used on a wide range of data-types, including GIS
|
||
data. PostGIS uses an R-Tree index implemented on top of GiST to index
|
||
spatial data. GiST is the most commonly-used and versatile spatial index method,
|
||
and offers very good query performance.
|
||
</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para><emphasis role="bold">BRIN (Block Range Index)</emphasis> indexes operate by summarizing
|
||
the spatial extent of ranges of table records.
|
||
Search is done via a scan of the ranges.
|
||
BRIN is only appropriate for use for some kinds of data
|
||
(spatially sorted, with infrequent or no update).
|
||
But it provides much faster index create time, and much smaller index size.
|
||
</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para><emphasis role="bold">SP-GiST (Space-Partitioned Generalized Search Tree)</emphasis>
|
||
is a generic index method that supports partitioned search trees
|
||
such as quad-trees, k-d trees, and radix trees (tries).
|
||
</para>
|
||
</listitem>
|
||
</itemizedlist>
|
||
|
||
<para>Spatial indexes store only the bounding box of geometries.
|
||
Spatial queries use the index as a <emphasis role="bold">primary filter</emphasis>
|
||
to quickly determine a set of geometries potentially matching the query condition.
|
||
Most spatial queries require a <emphasis role="bold">secondary filter</emphasis>
|
||
that uses a spatial predicate function to test a more specific spatial condition.
|
||
For more information on queying with spatial predicates see <xref linkend="using-query-indexes" />.
|
||
</para>
|
||
|
||
<para>See also the
|
||
<ulink url="https://postgis.net/workshops/postgis-intro/indexing.html">PostGIS Workshop section on spatial indexes</ulink>,
|
||
and the <ulink url="https://www.postgresql.org/docs/current/indexes.html">PostgreSQL manual</ulink>.
|
||
</para>
|
||
|
||
<sect2 id="gist_indexes">
|
||
<title>GiST Indexes</title>
|
||
|
||
<para>GiST stands for "Generalized Search Tree" and is a generic form of
|
||
indexing for multi-dimensional data.
|
||
PostGIS uses an R-Tree index implemented on top of GiST to index spatial data.
|
||
GiST is the most commonly-used and versatile spatial index method, and offers very good query performance.
|
||
Other implementations of GiST are used to speed up searches
|
||
on all kinds of irregular data structures (integer arrays, spectral
|
||
data, etc) which are not amenable to normal B-Tree indexing.
|
||
For more information see the <ulink url="https://www.postgresql.org/docs/current/gist.html">PostgreSQL manual</ulink>.
|
||
</para>
|
||
|
||
<para>Once a spatial data table exceeds a few thousand rows, you will want
|
||
to build an index to speed up spatial searches of the data (unless all
|
||
your searches are based on attributes, in which case you'll want to
|
||
build a normal index on the attribute fields).</para>
|
||
|
||
<para>The syntax for building a GiST index on a "geometry" column is as
|
||
follows:</para>
|
||
|
||
<para><programlisting>CREATE INDEX [indexname] ON [tablename] USING GIST ( [geometryfield] ); </programlisting></para>
|
||
|
||
<para>The above syntax will always build a 2D-index. To get the an n-dimensional index for the geometry type, you can create one using this syntax:</para>
|
||
<programlisting>CREATE INDEX [indexname] ON [tablename] USING GIST ([geometryfield] gist_geometry_ops_nd);</programlisting>
|
||
|
||
<para>Building a spatial index is a computationally intensive exercise. It also blocks write access to your table for the time it creates, so on a production system you may want to do in in a slower CONCURRENTLY-aware way:</para>
|
||
<para><programlisting>CREATE INDEX CONCURRENTLY [indexname] ON [tablename] USING GIST ( [geometryfield] ); </programlisting></para>
|
||
|
||
<para>After building an index, it is sometimes helpful to force PostgreSQL to collect
|
||
table statistics, which are used to optimize query plans:</para>
|
||
|
||
<para><programlisting>VACUUM ANALYZE [table_name] [(column_name)];</programlisting></para>
|
||
|
||
</sect2>
|
||
|
||
<sect2 id="brin_indexes">
|
||
<title>BRIN Indexes</title>
|
||
|
||
<para>BRIN stands for "Block Range Index". It is a general-purpose
|
||
index method introduced in PostgreSQL 9.5.
|
||
BRIN is a <emphasis>lossy</emphasis>
|
||
index method, meaning that a secondary check is required to confirm
|
||
that a record matches a given search condition
|
||
(which is the case for all provided spatial indexes).
|
||
It provides much faster index creation and much smaller index size,
|
||
with reasonable read performance.
|
||
Its primary purpose is to support indexing very large tables
|
||
on columns which have a correlation with their
|
||
physical location within the table. In addition to spatial indexing,
|
||
BRIN can speed up searches on various kinds of attribute data
|
||
structures (integer, arrays etc).
|
||
For more information see the <ulink url="https://www.postgresql.org/docs/current/brin.html">PostgreSQL manual</ulink>.
|
||
</para>
|
||
|
||
<para>Once a spatial table exceeds a few thousand rows, you will want
|
||
to build an index to speed up spatial searches of the data.
|
||
GiST indexes are very performant as long as their size doesn't exceed the amount of RAM
|
||
available for the database, and as long as you can afford the index storage
|
||
size, and the cost of index update on write. Otherwise, for very large tables BRIN index can be
|
||
considered as an alternative.</para>
|
||
|
||
<para>A BRIN index stores the bounding box enclosing
|
||
all the geometries contained in the rows in a contiguous set of table blocks,
|
||
called a <emphasis>block range</emphasis>.
|
||
When executing a query using the index the block ranges are scanned to
|
||
find the ones that intersect the query extent.
|
||
This is efficient only if the data is physically ordered so that the bounding
|
||
boxes for block ranges have minimal overlap (and ideally are mutually exclusive).
|
||
The resulting index is very small in size,
|
||
but is typically less performant for read than a GiST index over the same data.</para>
|
||
|
||
<para>Building a BRIN index is much less CPU-intensive than building a GiST index.
|
||
It's common to find that a BRIN index is ten times faster to build
|
||
than a GiST index over the same data. And because a BRIN index stores only one
|
||
bounding box for each range of table blocks, it's common to use
|
||
up to a thousand times less disk space than a GiST index.</para>
|
||
|
||
<para>You can choose the number of blocks to summarize in a range. If you
|
||
decrease this number, the index will be bigger but will probably provide
|
||
better performance.</para>
|
||
|
||
<para>For BRIN to be effective, the table data should be stored in
|
||
a physical order which minimizes the amount of block extent overlap.
|
||
It may be that the data is already sorted appropriately
|
||
(for instance, if it is loaded from another dataset that is already sorted in spatial order).
|
||
Otherwise, this can be accomplished by sorting the data by a one-dimensional spatial key.
|
||
One way to do this is to create a new table sorted by the geometry values
|
||
(which in recent PostGIS versions uses an efficient Hilbert curve ordering):
|
||
</para>
|
||
|
||
<para><programlisting>
|
||
CREATE TABLE table_sorted AS
|
||
SELECT * FROM table ORDER BY geom;
|
||
</programlisting></para>
|
||
|
||
<para>Alternatively, data can be sorted in-place by using a GeoHash as a (temporary) index,
|
||
and clustering on that index:
|
||
</para>
|
||
|
||
<para><programlisting>
|
||
CREATE INDEX idx_temp_geohash ON table
|
||
USING btree (ST_GeoHash( ST_Transform( geom, 4326 ), 20));
|
||
CLUSTER table USING idx_temp_geohash;
|
||
</programlisting></para>
|
||
|
||
|
||
<para>The syntax for building a BRIN index on a <code>geometry</code> column is:</para>
|
||
|
||
<para><programlisting>CREATE INDEX [indexname] ON [tablename] USING BRIN ( [geome_col] ); </programlisting></para>
|
||
|
||
<para>The above syntax builds a 2D index. To build a 3D-dimensional index, use this syntax:</para>
|
||
|
||
<programlisting>
|
||
CREATE INDEX [indexname] ON [tablename]
|
||
USING BRIN ([geome_col] brin_geometry_inclusion_ops_3d);</programlisting>
|
||
|
||
<para>You can also get a 4D-dimensional index using the 4D operator class:</para>
|
||
|
||
<programlisting>
|
||
CREATE INDEX [indexname] ON [tablename]
|
||
USING BRIN ([geome_col] brin_geometry_inclusion_ops_4d);</programlisting>
|
||
|
||
<para>The above commands use the default number of blocks in a range, which is 128.
|
||
To specify the number of blocks to summarise in a range, use this syntax</para>
|
||
|
||
<para><programlisting>
|
||
CREATE INDEX [indexname] ON [tablename]
|
||
USING BRIN ( [geome_col] ) WITH (pages_per_range = [number]); </programlisting></para>
|
||
|
||
<para>Keep in mind that a BRIN index only stores one index
|
||
entry for a large number of rows. If your table stores geometries with
|
||
a mixed number of dimensions, it's likely that the resulting index will
|
||
have poor performance. You can avoid this performance penalty by
|
||
choosing the operator class with the least number of dimensions of the
|
||
stored geometries
|
||
</para>
|
||
|
||
<para>The <code>geography</code> datatype is supported for BRIN indexing. The
|
||
syntax for building a BRIN index on a geography column is:</para>
|
||
|
||
<para><programlisting>CREATE INDEX [indexname] ON [tablename] USING BRIN ( [geog_col] ); </programlisting></para>
|
||
|
||
<para>The above syntax builds a 2D-index for geospatial objects on the spheroid. </para>
|
||
|
||
<para>Currently, only "inclusion support" is provided, meaning
|
||
that just the <varname>&&</varname>, <varname>~</varname> and
|
||
<varname>@</varname> operators can be used for the 2D cases (for both
|
||
<code>geometry</code> and <code>geography</code>), and just the <varname>&&&</varname>
|
||
operator for 3D geometries.
|
||
There is currently no support for kNN searches.</para>
|
||
|
||
<para>An important difference between BRIN and other index types is that the database does not
|
||
maintain the index dynamically. Changes to spatial data in the table
|
||
are simply appended to the end of the index. This will cause index search performance to
|
||
degrade over time. The index can be updated by performing a <code>VACUUM</code>,
|
||
or by using a special function <code>brin_summarize_new_values(regclass)</code>.
|
||
For this reason BRIN may be most appropriate for use with data that is read-only,
|
||
or only rarely changing. For more information refer to the
|
||
<ulink url="https://www.postgresql.org/docs/current/brin-intro.html#BRIN-OPERATION">manual</ulink>.
|
||
</para>
|
||
|
||
<para>To summarize using BRIN for spatial data:
|
||
</para>
|
||
|
||
<itemizedlist>
|
||
<listitem><para>Index build time is very fast, and index size is very small.</para></listitem>
|
||
<listitem><para>Index query time is slower than GiST, but can still be very acceptable.</para></listitem>
|
||
<listitem><para>Requires table data to be sorted in a spatial ordering.</para></listitem>
|
||
<listitem><para>Requires manual index maintenance.</para></listitem>
|
||
<listitem><para>Most appropriate for very large tables,
|
||
with low or no overlap (e.g. points),
|
||
which are static or change infrequently.</para></listitem>
|
||
<listitem><para>More effective for queries which return relatively large numbers of data records.</para></listitem>
|
||
</itemizedlist>
|
||
|
||
</sect2>
|
||
|
||
<sect2 id="spgist_indexes">
|
||
<title>SP-GiST Indexes</title>
|
||
|
||
<para>SP-GiST stands for "Space-Partitioned Generalized Search Tree" and is
|
||
a generic form of indexing for multi-dimensional data types
|
||
that supports partitioned search trees, such as
|
||
quad-trees, k-d trees, and radix trees (tries).
|
||
The common feature of these
|
||
data structures is that they repeatedly divide the search space into
|
||
partitions that need not be of equal size. In addition to spatial indexing,
|
||
SP-GiST is used to speed up searches on many kinds of data, such as phone
|
||
routing, ip routing, substring search, etc.
|
||
For more information see the <ulink url="https://www.postgresql.org/docs/current/spgist.html">PostgreSQL manual</ulink>.
|
||
</para>
|
||
|
||
<para>As it is the case for GiST indexes, SP-GiST indexes are lossy, in the
|
||
sense that they store the bounding box enclosing spatial objects.
|
||
SP-GiST indexes can be considered as an alternative to GiST indexes.</para>
|
||
|
||
<para>Once a GIS data table exceeds a few thousand rows, an SP-GiST index
|
||
may be used to speed up spatial searches of the data. The syntax for
|
||
building an SP-GiST index on a "geometry" column is as follows:</para>
|
||
|
||
<para><programlisting>CREATE INDEX [indexname] ON [tablename] USING SPGIST ( [geometryfield] ); </programlisting></para>
|
||
|
||
<para>The above syntax will build a 2-dimensional index. A 3-dimensional
|
||
index for the geometry type can be created using the 3D operator class:</para>
|
||
|
||
<para><programlisting>CREATE INDEX [indexname] ON [tablename] USING SPGIST ([geometryfield] spgist_geometry_ops_3d);</programlisting></para>
|
||
|
||
<para>Building a spatial index is a computationally intensive operation.
|
||
It also blocks write access to your table for the time it creates, so on a
|
||
production system you may want to do in in a slower CONCURRENTLY-aware way:</para>
|
||
|
||
<para><programlisting>CREATE INDEX CONCURRENTLY [indexname] ON [tablename] USING SPGIST ( [geometryfield] ); </programlisting></para>
|
||
|
||
<para>After building an index, it is sometimes helpful to force PostgreSQL to
|
||
collect table statistics, which are used to optimize query plans:</para>
|
||
|
||
<para><programlisting>VACUUM ANALYZE [table_name] [(column_name)];</programlisting></para>
|
||
|
||
<para>An SP-GiST index can accelerate queries involving the following operators:</para>
|
||
<itemizedlist>
|
||
<listitem><para><<, &<, &>, >>, <<|, &<|, |&>, |>>, &&, @>, <@, and ~=, for 2-dimensional indexes,</para></listitem>
|
||
<listitem><para> &/&, ~==, @>>, and <<@, for 3-dimensional indexes.</para></listitem>
|
||
</itemizedlist>
|
||
<para>There is no support for kNN searches at the moment.</para>
|
||
</sect2>
|
||
<sect2 id="tuning-index-usage">
|
||
<title>Tuning Index Usage</title>
|
||
|
||
<para>Ordinarily, indexes invisibly speed up data access: once an index
|
||
is built, the PostgreSQL query planner automatically decides when to use it
|
||
to improve query performance. But there are some situations
|
||
where the planner does not choose to use existing indexes,
|
||
so queries end up using slow sequential scans instead of a spatial index.</para>
|
||
|
||
<para>If you find your spatial indexes are not being used,
|
||
there are a few things you can do:</para>
|
||
|
||
<itemizedlist>
|
||
<listitem>
|
||
<para>Examine the query plan and check your query actually computes the
|
||
thing you need. An erroneous JOIN, either forgotten or to the wrong table,
|
||
can unexpectedly retrieve table records multiple times.
|
||
To get the query plan, execute with <code>EXPLAIN</code> in front of the query.</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>Make sure statistics are gathered about the number
|
||
and distributions of values in a table, to provide the query planner
|
||
with better information to make decisions around index usage.
|
||
<command>VACUUM ANALYZE</command> will compute both.</para>
|
||
|
||
<para>You should regularly vacuum your databases anyways. Many PostgreSQL DBAs run
|
||
<command>VACUUM</command> as an off-peak cron job on a regular basis.</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>If vacuuming does not help, you can temporarily force the planner to use
|
||
the index information by using the command <command>SET ENABLE_SEQSCAN TO OFF;</command>.
|
||
This way you can check whether the planner is at all able to generate
|
||
an index-accelerated query plan for your query.
|
||
You should only use this command for debugging; generally
|
||
speaking, the planner knows better than you do about when to use
|
||
indexes. Once you have run your query, do not forget to run
|
||
<command>SET ENABLE_SEQSCAN TO ON;</command> so that the planner
|
||
will operate normally for other queries.</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>If <command>SET ENABLE_SEQSCAN TO OFF;</command> helps your query to run faster,
|
||
your Postgres is likely not tuned for your hardware.
|
||
If you find the planner wrong about the cost of sequential versus
|
||
index scans try reducing the value of <varname>RANDOM_PAGE_COST</varname> in
|
||
<code>postgresql.conf</code>, or use <command>SET RANDOM_PAGE_COST TO 1.1;</command>.
|
||
The default value for <varname>RANDOM_PAGE_COST</varname> is 4.0.
|
||
Try setting it to 1.1 (for SSD) or 2.0 (for fast magnetic disks).
|
||
Decreasing the value makes the planner more likely to use index scans.</para>
|
||
</listitem>
|
||
|
||
<listitem>
|
||
<para>If <command>SET ENABLE_SEQSCAN TO OFF;</command> does not help your query,
|
||
the query may be using a SQL construct that the Postgres planner is not yet able to optimize.
|
||
It may be possible to rewrite the query in a way that the planner is able to handle.
|
||
For example, a subquery with an inline SELECT may not produce an efficient plan,
|
||
but could possibly be rewritten using a LATERAL JOIN.</para>
|
||
</listitem>
|
||
|
||
</itemizedlist>
|
||
|
||
<para>
|
||
For more information see the Postgres manual section on
|
||
<ulink url="https://www.postgresql.org/docs/current/runtime-config-query.html">Query Planning</ulink>.
|
||
</para>
|
||
|
||
</sect2>
|
||
</sect1>
|
||
|
||
</chapter>
|