postgis/doc/using_postgis_app.xml

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<chapter>
  <title id="using_postgis_applications">Building Applications</title>
  <sect1 id="Using_MapServer">
    <title>Using MapServer</title>

    <para>The Minnesota MapServer is an internet web-mapping server which
    conforms to the OpenGIS Web Map Service specification.</para>

    <itemizedlist>
      <listitem>
        <para>The MapServer homepage is at <ulink
        url="http://mapserver.org">http://mapserver.org</ulink>.</para>
      </listitem>

      <listitem>
        <para>The OpenGIS Web Map Service specification is at <ulink
        url="http://www.opengeospatial.org/standards/wms">http://www.opengeospatial.org/standards/wms</ulink>.</para>
      </listitem>
    </itemizedlist>

    <sect2>
      <title>Basic Usage</title>

      <para>To use PostGIS with MapServer, you need to know how to
      configure MapServer, which is beyond the scope of this documentation.
      This section covers specific PostGIS issues and configuration
      details.</para>

      <para>To use PostGIS with MapServer, you will need:</para>

      <itemizedlist>
        <listitem>
          <para>Version 0.6 or newer of PostGIS.</para>
        </listitem>

        <listitem>
          <para>Version 3.5 or newer of MapServer.</para>
        </listitem>
      </itemizedlist>

      <para>MapServer accesses PostGIS/PostgreSQL data like any other
      PostgreSQL client, using the <filename>libpq</filename> interface. This means that
      MapServer can be installed on any machine with network access to the
      PostGIS server, and use PostGIS as a source of data. The faster the connection
      between the systems, the better.</para>

      <orderedlist>
        <listitem>
          <para>Compile and install MapServer, with whatever options you
          desire, including the "--with-postgis" configuration option.</para>
        </listitem>

        <listitem>
          <para>In your MapServer map file, add a PostGIS layer. For
          example:</para>

          <programlisting>LAYER
  CONNECTIONTYPE postgis
  NAME "widehighways"
  # Connect to a remote spatial database
  CONNECTION "user=dbuser dbname=gisdatabase host=bigserver"
  PROCESSING "CLOSE_CONNECTION=DEFER"
  # Get the lines from the 'geom' column of the 'roads' table
  DATA "geom from roads using srid=4326 using unique gid"
  STATUS ON
  TYPE LINE
  # Of the lines in the extents, only render the wide highways
  FILTER "type = 'highway' and numlanes &gt;= 4"
  CLASS
    # Make the superhighways brighter and 2 pixels wide
    EXPRESSION ([numlanes] &gt;= 6)
    STYLE
      COLOR 255 22 22
      WIDTH 2
    END
  END
  CLASS
    # All the rest are darker and only 1 pixel wide
    EXPRESSION ([numlanes] &lt; 6)
    STYLE
      COLOR 205 92 82
    END
  END
END</programlisting>

          <para>In the example above, the PostGIS-specific directives are as
          follows:</para>

          <variablelist>
            <varlistentry>
              <term>CONNECTIONTYPE</term>

              <listitem>
                <para>For PostGIS layers, this is always "postgis".</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term>CONNECTION</term>

              <listitem>
                <para>The database connection is governed by the a 'connection
                string' which is a standard set of keys and values like this
                (with the default values in &lt;&gt;):</para>

                <para>user=&lt;username&gt; password=&lt;password&gt;
                dbname=&lt;username&gt; hostname=&lt;server&gt;
                port=&lt;5432&gt;</para>

                <para>An empty connection string is still valid, and any of
                the key/value pairs can be omitted. At a minimum you will
                generally supply the database name and username to connect
                with.</para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term>DATA</term>

              <listitem>
                <para>The form of this parameter is "&lt;geocolumn&gt; from
                &lt;tablename&gt; using srid=&lt;srid&gt; using unique &lt;primary key&gt;" where the column is the spatial column to
                be rendered to the map, the SRID is SRID used by the column and the primary key is the table primary key (or any
                other uniquely-valued column with an index).</para>
                <para>You can omit the "using srid" and "using unique" clauses and MapServer will automatically determine the
                correct values if possible, but at the cost of running a few extra queries on the server for each map
                draw.</para>
              </listitem>
            </varlistentry>

			<varlistentry>
              <term>PROCESSING</term>

              <listitem>
                <para>Putting in a CLOSE_CONNECTION=DEFER if you have multiple layers reuses existing connections instead of closing them.  This improves
					speed.  Refer to for <ulink url="http://blog.cleverelephant.ca/2008/10/mapserverpostgis-performance-tips.html">MapServer PostGIS Performance Tips</ulink> for a more detailed explanation.  </para>
              </listitem>
            </varlistentry>

            <varlistentry>
              <term>FILTER</term>

              <listitem>
                <para>The filter must be a valid SQL string corresponding to
                the logic normally following the "WHERE" keyword in a SQL
                query. So, for example, to render only roads with 6 or more
                lanes, use a filter of "num_lanes &gt;= 6".</para>
              </listitem>
            </varlistentry>
          </variablelist>
        </listitem>

        <listitem>
          <para>In your spatial database, ensure you have spatial (GiST)
          indexes built for any the layers you will be drawing.</para>

          <programlisting>CREATE INDEX [indexname] ON [tablename] USING GIST ( [geometrycolumn] );</programlisting>
        </listitem>

        <listitem>
          <para>If you will be querying your layers using MapServer you will
          also need to use the "using unique" clause in your DATA statement.</para>

          <para>MapServer requires unique identifiers for each spatial record
          when doing queries, and the PostGIS module of MapServer uses the
          unique value you specify in order to provide these unique
          identifiers. Using the table primary key is the best practice.</para>
        </listitem>
      </orderedlist>
    </sect2>

    <sect2>
      <title>Frequently Asked Questions</title>

      <qandaset>
        <qandaentry>
          <question>
            <para>When I use an <varname>EXPRESSION</varname> in my map file,
            the condition never returns as true, even though I know the values
            exist in my table.</para>
          </question>

          <answer>
            <para>Unlike shape files, PostGIS field names have to be
            referenced in EXPRESSIONS using <emphasis>lower
            case</emphasis>.</para>

            <programlisting>EXPRESSION ([numlanes] &gt;= 6)</programlisting>
          </answer>
        </qandaentry>

        <qandaentry>
          <question>
            <para>The FILTER I use for my Shapefiles is not working for my
            PostGIS table of the same data.</para>
          </question>

          <answer>
            <para>Unlike shape files, filters for PostGIS layers use SQL
            syntax (they are appended to the SQL statement the PostGIS
            connector generates for drawing layers in MapServer).</para>

            <programlisting>FILTER "type = 'highway' and numlanes &gt;= 4"</programlisting>
          </answer>
        </qandaentry>

        <qandaentry>
          <question>
            <para>My PostGIS layer draws much slower than my Shapefile layer,
            is this normal?</para>
          </question>

          <answer>
            <para>In general, the more features you are drawing into a given map,
            the more likely it is that PostGIS will be slower than Shapefiles.
            For maps with relatively few features (100s), PostGIS will often be faster.
            For maps with high feature density (1000s), PostGIS will always be slower.
            </para>

            <para>If you are finding substantial draw performance problems, it
            is possible that you have not built a spatial index on your
            table.</para>

            <programlisting>postgis# CREATE INDEX geotable_gix ON geotable USING GIST ( geocolumn );
postgis# VACUUM ANALYZE;</programlisting>
          </answer>
        </qandaentry>

        <qandaentry>
          <question>
            <para>My PostGIS layer draws fine, but queries are really slow.
            What is wrong?</para>
          </question>

          <answer>
            <para>For queries to be fast, you must have a unique key for your
            spatial table and you must have an index on that unique
            key.</para>

            <para>You can specify what unique key for mapserver to use with
            the <varname>USING UNIQUE</varname> clause in your
            <varname>DATA</varname> line:</para>

            <programlisting>DATA "geom FROM geotable USING UNIQUE gid"</programlisting>

          </answer>
        </qandaentry>

        <qandaentry>
          <question>
            <para>Can I use "geography" columns (new in PostGIS 1.5) as a source for
            MapServer layers?</para>
          </question>

          <answer>
            <para>Yes! MapServer understands geography columns as being the same as
            geometry columns, but always using an SRID of 4326. Just make sure to include
            a "using srid=4326" clause in your <varname>DATA</varname> statement. Everything else
            works exactly the same as with geometry.</para>

            <programlisting>DATA "geog FROM geogtable USING SRID=4326 USING UNIQUE gid"</programlisting>

          </answer>
        </qandaentry>


      </qandaset>
    </sect2>

    <sect2>
      <title>Advanced Usage</title>

      <para>The <varname>USING</varname> pseudo-SQL clause is used to add some
      information to help mapserver understand the results of more complex
      queries. More specifically, when either a view or a subselect is used as
      the source table (the thing to the right of "FROM" in a
      <varname>DATA</varname> definition) it is more difficult for mapserver
      to automatically determine a unique identifier for each row and also the
      SRID for the table. The <varname>USING</varname> clause can provide
      mapserver with these two pieces of information as follows:</para>

      <programlisting>DATA "geom FROM (
  SELECT
    table1.geom AS geom,
    table1.gid AS gid,
    table2.data AS data
  FROM table1
  LEFT JOIN table2
  ON table1.id = table2.id
) AS new_table USING UNIQUE gid USING SRID=4326"</programlisting>

      <variablelist>
        <varlistentry>
          <term>USING UNIQUE &lt;uniqueid&gt;</term>

          <listitem>
            <para>MapServer requires a unique id for each row in order to
            identify the row when doing map queries. Normally it identifies
            the primary key from the system tables. However, views and subselects don't
            automatically have an known unique column. If you want to use MapServer's
            query functionality, you need to ensure your view
            or subselect includes a uniquely valued column, and declare it with <varname>USING UNIQUE</varname>.
            For example, you could explicitly select nee of the table's primary key
            values for this purpose, or any other column which is guaranteed
            to be unique for the result set.</para>

            <note>
              <para>"Querying a Map" is the action of clicking on a map to ask
              for information about the map features in that location. Don't
              confuse "map queries" with the SQL query in a
              <varname>DATA</varname> definition.</para>
            </note>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term>USING SRID=&lt;srid&gt;</term>

          <listitem>
            <para>PostGIS needs to know which spatial referencing system is
            being used by the geometries in order to return the correct data
            back to MapServer. Normally it is possible to find this
            information in the "geometry_columns" table in the PostGIS
            database, however, this is not possible for tables which are
            created on the fly such as subselects and views. So the
            <varname>USING SRID=</varname> option allows the correct SRID to
            be specified in the <varname>DATA</varname> definition.</para>
          </listitem>
        </varlistentry>
      </variablelist>

    </sect2>

    <sect2>
      <title>Examples</title>

      <para>Lets start with a simple example and work our way up. Consider the
      following MapServer layer definition:</para>

      <programlisting>LAYER
  CONNECTIONTYPE postgis
  NAME "roads"
  CONNECTION "user=theuser password=thepass dbname=thedb host=theserver"
  DATA "geom from roads"
  STATUS ON
  TYPE LINE
  CLASS
    STYLE
      COLOR 0 0 0
    END
  END
END</programlisting>

      <para>This layer will display all the road geometries in the roads table
      as black lines.</para>

      <para>Now lets say we want to show only the highways until we get zoomed
      in to at least a 1:100000 scale - the next two layers will achieve this
      effect:</para>

      <programlisting>LAYER
  CONNECTIONTYPE postgis
  CONNECTION "user=theuser password=thepass dbname=thedb host=theserver"
  PROCESSING "CLOSE_CONNECTION=DEFER"
  DATA "geom from roads"
  MINSCALE 100000
  STATUS ON
  TYPE LINE
  FILTER "road_type = 'highway'"
  CLASS
    COLOR 0 0 0
  END
END
LAYER
  CONNECTIONTYPE postgis
  CONNECTION "user=theuser password=thepass dbname=thedb host=theserver"
  PROCESSING "CLOSE_CONNECTION=DEFER"
  DATA "geom from roads"
  MAXSCALE 100000
  STATUS ON
  TYPE LINE
  CLASSITEM road_type
  CLASS
    EXPRESSION "highway"
    STYLE
      WIDTH 2
      COLOR 255 0 0
    END
  END
  CLASS
    STYLE
      COLOR 0 0 0
    END
  END
END</programlisting>

      <para>The first layer is used when the scale is greater than 1:100000,
      and displays only the roads of type "highway" as black lines. The
      <varname>FILTER</varname> option causes only roads of type "highway" to
      be displayed.</para>

      <para>The second layer is used when the scale is less than 1:100000, and
      will display highways as double-thick red lines, and other roads as
      regular black lines.</para>

      <para>So, we have done a couple of interesting things using only
      MapServer functionality, but our <varname>DATA</varname> SQL statement
      has remained simple. Suppose that the name of the road is stored in
      another table (for whatever reason) and we need to do a join to get it
      and label our roads.</para>

      <programlisting>LAYER
  CONNECTIONTYPE postgis
  CONNECTION "user=theuser password=thepass dbname=thedb host=theserver"
  DATA "geom FROM (SELECT roads.gid AS gid, roads.geom AS geom,
        road_names.name as name FROM roads LEFT JOIN road_names ON
        roads.road_name_id = road_names.road_name_id)
        AS named_roads USING UNIQUE gid USING SRID=4326"
  MAXSCALE 20000
  STATUS ON
  TYPE ANNOTATION
  LABELITEM name
  CLASS
    LABEL
      ANGLE auto
      SIZE 8
      COLOR 0 192 0
      TYPE truetype
      FONT arial
    END
  END
END</programlisting>

      <para>This annotation layer adds green labels to all the roads when the
      scale gets down to 1:20000 or less. It also demonstrates how to use an
      SQL join in a <varname>DATA</varname> definition.</para>
    </sect2>
  </sect1>

  <sect1>
    <title>Java Clients (JDBC)</title>

    <para>Java clients can access PostGIS "geometry" objects in the PostgreSQL
    database either directly as text representations or using the JDBC
    extension objects bundled with PostGIS. In order to use the extension
    objects, the "postgis.jar" file must be in your CLASSPATH along with the
    "postgresql.jar" JDBC driver package.</para>

    <programlisting>import java.sql.*;
import java.util.*;
import java.lang.*;
import org.postgis.*;

public class JavaGIS {

public static void main(String[] args) {

  java.sql.Connection conn;

  try {
    /*
    * Load the JDBC driver and establish a connection.
    */
    Class.forName("org.postgresql.Driver");
    String url = "jdbc:postgresql://localhost:5432/database";
    conn = DriverManager.getConnection(url, "postgres", "");
    /*
    * Add the geometry types to the connection. Note that you
    * must cast the connection to the pgsql-specific connection
    * implementation before calling the addDataType() method.
    */
    ((org.postgresql.PGConnection)conn).addDataType("geometry",Class.forName("org.postgis.PGgeometry"));
    ((org.postgresql.PGConnection)conn).addDataType("box3d",Class.forName("org.postgis.PGbox3d"));
    /*
    * Create a statement and execute a select query.
    */
    Statement s = conn.createStatement();
    ResultSet r = s.executeQuery("select geom,id from geomtable");
    while( r.next() ) {
      /*
      * Retrieve the geometry as an object then cast it to the geometry type.
      * Print things out.
      */
      PGgeometry geom = (PGgeometry)r.getObject(1);
      int id = r.getInt(2);
      System.out.println("Row " + id + ":");
      System.out.println(geom.toString());
    }
    s.close();
    conn.close();
  }
catch( Exception e ) {
  e.printStackTrace();
  }
}
}</programlisting>

    <para>The "PGgeometry" object is a wrapper object which contains a
    specific topological geometry object (subclasses of the abstract class
    "Geometry") depending on the type: Point, LineString, Polygon, MultiPoint,
    MultiLineString, MultiPolygon.</para>

    <programlisting>PGgeometry geom = (PGgeometry)r.getObject(1);
if( geom.getType() == Geometry.POLYGON ) {
  Polygon pl = (Polygon)geom.getGeometry();
  for( int r = 0; r &lt; pl.numRings(); r++) {
    LinearRing rng = pl.getRing(r);
    System.out.println("Ring: " + r);
    for( int p = 0; p &lt; rng.numPoints(); p++ ) {
      Point pt = rng.getPoint(p);
      System.out.println("Point: " + p);
      System.out.println(pt.toString());
    }
  }
}</programlisting>

    <para>The JavaDoc for the extension objects provides a reference for the
    various data accessor functions in the geometric objects.</para>
  </sect1>

  <sect1>
    <title>C Clients (libpq)</title>

    <para>...</para>

    <sect2>
      <title>Text Cursors</title>

      <para>...</para>
    </sect2>

    <sect2>
      <title>Binary Cursors</title>

      <para>...</para>
    </sect2>
  </sect1>
</chapter>