Commit b53cbe31 authored by Darafei Praliaskouski's avatar Darafei Praliaskouski

SP-GiST documentation.

Patch provided by Esteban Zimanyi.

Closes #1847


git-svn-id: http://svn.osgeo.org/postgis/trunk@16642 b70326c6-7e19-0410-871a-916f4a2858ee
parent 587badb8
Pipeline #25744889 passed with stage
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for kNN searches at the moment.</para>
</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 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 GIS indexing,
SP-GiST is used to speed up searches on many kinds of data, such as phone
routing, ip routing, substring search, etc. </para>
<para>As it is the case for GiST indexes, SP-GiST indexes are lossy, in the
sense that they store the bounding box englobing the spatial objects.
SP-GiST indexes can be considered as an alternative to GiST indexes. The
performance tests reveal that SP-GiST indexes are especially beneficial
when there are many overlapping objects, that is, with so-called
“spaghetti data”.</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>&lt;&lt;, &amp;&lt;, &amp;&gt;, &gt;&gt;, &lt;&lt;|, &amp;&lt;|, |&amp;&gt;, |&gt;&gt;, &amp;&amp;, @&gt;, &lt;@, and ~=, for 2-dimensional indexes,</para></listitem>
<listitem><para> &amp;/&amp;, ~==, @&gt;&gt;, and &lt;&lt;@, for 3-dimensional indexes.</para></listitem>
</itemizedlist>
<para>There is no support for kNN searches at the moment.</para>
</sect2>
<sect2>
<title>Using Indexes</title>
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