[postgis-tickets] [SCM] PostGIS branch master updated. 3.2.0-463-g169d45243
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- Log -----------------------------------------------------------------
commit 169d452431cc11553a8db25025fe89b1d41aabb3
Author: Martin Davis <mtnclimb at gmail.com>
Date: Wed Feb 2 11:52:50 2022 -0800
Improve doc ST_Subdivide
diff --git a/doc/reference_overlay.xml b/doc/reference_overlay.xml
index 5f051ec1b..93baa1d97 100644
--- a/doc/reference_overlay.xml
+++ b/doc/reference_overlay.xml
@@ -620,13 +620,20 @@ GEOMETRYCOLLECTION(
<title>Description</title>
<para>
- Divides geometry into parts using rectilinear lines, until each part can be represented using no more than <code>max_vertices</code>.
- Point-in-polygon and other spatial operations are normally faster for indexed subdivided dataset:
- "miss" cases are faster to check as boxes for all parts typically cover smaller area than original geometry box,
- "hit" cases are faster because recheck operates on less points.
+ A set-returning function that divides geometry into parts using rectilinear lines,
+ with each part containing no more than <code>max_vertices</code>.
+ </para>
+ <para>
<code>max_vertices</code> must be 5 or more, as 5 points are needed to represent a closed box.
<code>gridSize</code> can be specified to have clipping work in fixed-precision space (requires GEOS-3.9.0+).
</para>
+ <para>
+ Point-in-polygon and other spatial operations are normally faster for indexed subdivided datasets.
+ Since the bounding boxes for parts usually cover a smaller area than original geometry bbox,
+ the index queries are more selective and produce fewer "hit" cases.
+ The "hit" cases are faster because the spatial operations
+ executed by the index recheck process fewer points.
+ </para>
<para>Performed by the GEOS module.</para>
<para>Availability: 2.2.0</para>
<para>Enhanced: 2.5.0 reuses existing points on polygon split, vertex count is lowered from 8 to 5.</para>
@@ -635,70 +642,92 @@ GEOMETRYCOLLECTION(
<refsection>
<title>Examples</title>
- <programlisting>-- Subdivide complex geometries in table, in place
-with complex_areas_to_subdivide as (
- delete from polygons_table
- where ST_NPoints(geom) > 255
- returning id, column1, column2, column3, geom
-)
-insert into polygons_table (fid, column1, column2, column3, geom)
- select
- fid, column1, column2, column3,
- ST_Subdivide(geom, 255) as geom
- from complex_areas_to_subdivide;
- </programlisting>
-
- <programlisting>-- Create a new subdivided table suitable for joining to the original
-CREATE TABLE subdivided_geoms AS
-SELECT pkey, ST_Subdivide(geom) AS geom
-FROM original_geoms;
- </programlisting>
- <informaltable>
- <tgroup cols="1">
- <tbody>
- <row>
- <entry><para><informalfigure>
- <mediaobject>
- <imageobject>
- <imagedata fileref="images/st_subdivide01.png" />
- </imageobject>
- <caption><para>Subdivide max 10 vertices</para></caption>
- </mediaobject>
- </informalfigure>
- <programlisting>SELECT row_number() OVER() As rn, ST_AsText(geom) As wkt
-FROM ( SELECT ST_SubDivide('POLYGON((132 10,119 23,85 35,68 29,66 28,49 42,32 56,22 64,32 110,40 119,36 150,
-57 158,75 171,92 182,114 184,132 186,146 178,176 184,179 162,184 141,190 122,
-190 100,185 79,186 56,186 52,178 34,168 18,147 13,132 10))'::geometry,10)) As f(geom);</programlisting>
+ <para><emphasis role="bold">Example:</emphasis>
+ Subdivide a polygon into parts with no more than 10 vertices,
+ and assign each part a unique id.
+ </para>
+
+ <informalfigure>
+ <mediaobject>
+ <imageobject>
+ <imagedata fileref="images/st_subdivide01.png" />
+ </imageobject>
+ <caption><para>Subdivided to maximum 10 vertices</para></caption>
+ </mediaobject>
+ </informalfigure>
+<programlisting>
+SELECT row_number() OVER() As rn, ST_AsText(geom) As wkt
+ FROM (SELECT ST_SubDivide(
+ 'POLYGON((132 10,119 23,85 35,68 29,66 28,49 42,32 56,22 64,32 110,40 119,36 150,
+ 57 158,75 171,92 182,114 184,132 186,146 178,176 184,179 162,184 141,190 122,
+ 190 100,185 79,186 56,186 52,178 34,168 18,147 13,132 10))'::geometry,10)) AS f(geom);
+</programlisting>
<screen> rn │ wkt
────┼────────────────────────────────────────────────────────────────────────────────────────────────────────────────
1 │ POLYGON((119 23,85 35,68 29,66 28,32 56,22 64,29.8260869565217 100,119 100,119 23))
2 │ POLYGON((132 10,119 23,119 56,186 56,186 52,178 34,168 18,147 13,132 10))
3 │ POLYGON((119 56,119 100,190 100,185 79,186 56,119 56))
4 │ POLYGON((29.8260869565217 100,32 110,40 119,36 150,57 158,75 171,92 182,114 184,114 100,29.8260869565217 100))
- 5 │ POLYGON((114 184,132 186,146 178,176 184,179 162,184 141,190 122,190 100,114 100,114 184))</screen>
- </para></entry>
- </row>
- <row>
- <entry><para><informalfigure>
- <mediaobject>
- <imageobject>
- <imagedata fileref="images/st_subdivide02.png" />
- </imageobject>
- <caption><para>Useful in conjunction with ST_Segmentize(geography) to create additional vertices that can then be used for splitting.</para></caption>
- </mediaobject>
- </informalfigure>
- <programlisting>SELECT ST_AsText(ST_Subdivide(ST_Segmentize('LINESTRING(0 0, 85 85)'::geography,1200000)::geometry,8));</programlisting>
- <screen>
+ 5 │ POLYGON((114 184,132 186,146 178,176 184,179 162,184 141,190 122,190 100,114 100,114 184))
+ </screen>
+
+ <para><emphasis role="bold">Example:</emphasis>
+ Densify a long geography line using ST_Segmentize(geography, distance),
+ and use ST_Subdivide to split the resulting line into sublines of 8 vertices.
+ </para>
+ <informalfigure>
+ <mediaobject>
+ <imageobject>
+ <imagedata fileref="images/st_subdivide02.png" />
+ </imageobject>
+ <caption><para>The densified and split lines.</para></caption>
+ </mediaobject>
+ </informalfigure>
+<programlisting>
+SELECT ST_AsText( ST_Subdivide(
+ ST_Segmentize('LINESTRING(0 0, 85 85)'::geography,
+ 1200000)::geometry, 8));
+</programlisting>
+<screen>
LINESTRING(0 0,0.487578359029357 5.57659056746196,0.984542144675897 11.1527721155093,1.50101059639722 16.7281035483571,1.94532113630331 21.25)
LINESTRING(1.94532113630331 21.25,2.04869538062779 22.3020741387339,2.64204641967673 27.8740533545155,3.29994062412787 33.443216802941,4.04836719489742 39.0084282520239,4.59890468420694 42.5)
LINESTRING(4.59890468420694 42.5,4.92498503922732 44.5680389206321,5.98737409390639 50.1195229244701,7.3290919767674 55.6587646879025,8.79638749938413 60.1969505994924)
LINESTRING(8.79638749938413 60.1969505994924,9.11375579533779 61.1785363177625,11.6558166691368 66.6648504160202,15.642041247655 72.0867690601745,22.8716627200212 77.3609628116894,24.6991785131552 77.8939011989848)
LINESTRING(24.6991785131552 77.8939011989848,39.4046096622744 82.1822848017636,44.7994523421035 82.5156766227011)
-LINESTRING(44.7994523421035 82.5156766227011,85 85)</screen>
- </para></entry>
- </row>
- </tbody></tgroup></informaltable>
+LINESTRING(44.7994523421035 82.5156766227011,85 85)
+</screen>
+
+ <para><emphasis role="bold">Example:</emphasis>
+ Subdivide the complex geometries of a table in-place.
+ The original geometry records are deleted from the source table,
+ and new records for each subdivided result geometry are inserted.
+ </para>
+
+<programlisting>
+WITH complex_areas_to_subdivide AS (
+ DELETE from polygons_table
+ WHERE ST_NPoints(geom) > 255
+ RETURNING id, column1, column2, column3, geom
+)
+INSERT INTO polygons_table (fid, column1, column2, column3, geom)
+ SELECT fid, column1, column2, column3,
+ ST_Subdivide(geom, 255) as geom
+ FROM complex_areas_to_subdivide;
+</programlisting>
+
+ <para><emphasis role="bold">Example:</emphasis>
+ Create a new table containing subdivided geometries,
+ retaining the key of the original geometry so that the new table
+ can be joined to the source table.
+ </para>
+
+<programlisting>-- Create a new subdivided table suitable for joining to the original
+CREATE TABLE subdivided_geoms AS
+SELECT pkey, ST_Subdivide(geom) AS geom
+FROM original_geoms;
+</programlisting>
+
</refsection>
<refsection>
<title>See Also</title>
@@ -976,7 +1005,7 @@ on that same grid. (Requires GEOS-3.9.0 or higher)
<para>Performed by the GEOS module.</para>
<para>ST_Union creates MultiLineString and does not sew LineStrings into a single LineString.
Use <xref linkend="ST_LineMerge" /> to sew LineStrings.</para>
-
+
<para>NOTE: this function was formerly called GeomUnion(), which
was renamed from "Union" because UNION is an SQL reserved
word.</para>
-----------------------------------------------------------------------
Summary of changes:
doc/reference_overlay.xml | 143 ++++++++++++++++++++++++++++------------------
1 file changed, 86 insertions(+), 57 deletions(-)
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