[postgis-tickets] [SCM] PostGIS branch master updated. 3.1.0rc1-3-g49e96ec
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Mon Dec 14 18:30:08 PST 2020
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commit 49e96ec4dc650375f80fc2167c0dc375fe6e9c51
Author: Martin Davis <mtnclimb at gmail.com>
Date: Mon Dec 14 18:29:55 2020 -0800
Improve doc Using Indexes text
diff --git a/doc/using_postgis_query.xml b/doc/using_postgis_query.xml
index cd629b8..b140e13 100644
--- a/doc/using_postgis_query.xml
+++ b/doc/using_postgis_query.xml
@@ -466,8 +466,8 @@ WHERE a.geom && b.geom
and the distance operators used in nearest-neighbour queries
(the most common being <xref linkend="geometry_distance_knn" />.)
Index-aware functions include most of the named spatial predicates
- (such as <xref linkend="ST_Intersects" />), and most of the distance predicates such as
- <xref linkend="ST_DWithin" />.)
+ (such as <xref linkend="ST_Intersects" />), and most of the distance predicates
+ (such as <xref linkend="ST_DWithin" />.)
</para>
<para>
Functions such as
@@ -486,7 +486,7 @@ WHERE ST_Distance(the_geom, 'SRID=312;POINT(100000 200000)') < 100</programli
is computed for <emphasis role="bold">every</emphasis> row in the table.
</para>
<para>
- We can reduce the number of distance calculations required by using the
+ We can reduce the number of rows processed by using the
index-aware function <xref linkend="ST_DWithin" />:</para>
<programlisting>SELECT the_geom
@@ -495,15 +495,17 @@ WHERE ST_DWithin(the_geom, 'SRID=312;POINT(100000 200000)', 100)
</programlisting>
<para>This query selects the same geometries, but it does it in a more
- efficient way. If there is a GiST index on the_geom, the query
- planner will recognize that it can use the index to reduce the number of
- rows before calculating the distance.
+ efficient way.
This is enabled by <varname>ST_DWithin()</varname> using the
<varname>&&</varname> operator internally on an expanded bounding box
of the query geometry.
- That uses the spatial index to find only records with geometries
+ If there is a spatial index on <code>the_geom</code>, the query
+ planner will recognize that it can use the index to reduce the number of
+ rows scanned before calculating the distance.
+ The spatial index allows retrieving only records with geometries
whose bounding boxes overlap the expanded extent
- (or in other words, skips records with geometries which are guaranteed to be too far away.)
+ and hence which <emphasis>might</emphasis> be within the required distance.
+ The actual distance is then computed to confirm whether to include the record in the result set.
</para>
</sect2>
-----------------------------------------------------------------------
Summary of changes:
doc/using_postgis_query.xml | 18 ++++++++++--------
1 file changed, 10 insertions(+), 8 deletions(-)
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