#2242: Clarify behavior of ST_Union with NULLS

git-svn-id: http://svn.osgeo.org/postgis/trunk@11383 b70326c6-7e19-0410-871a-916f4a2858ee

diff --git a/doc/reference_processing.xml b/doc/reference_processing.xml
index af17d0db4ba94d15babcce6250e2a43380c0fd58..bd50c450913514ba3d60d9decffabf52aad0d74a 100644 (file)
--- a/doc/reference_processing.xml
+++ b/doc/reference_processing.xml
@@ -2875,11 +2875,11 @@ MULTILINESTRING((1 3 2.75,1 4 2),(1 1 3,1 2 2.25))
                from a set of geometries. The ST_Union() function is an "aggregate"
                function in the terminology of PostgreSQL. That means that it
                operates on rows of data, in the same way the SUM() and AVG()
-               functions do.</para>
+               functions do and like most aggregates, it also ignores NULL geometries.</para>
 
        <para>Non-Aggregate version: This function returns a geometry being a union of two
                input geometries. Output type can be a MULTI*, NON-MULTI or
-               GEOMETRYCOLLECTION.</para>
+               GEOMETRYCOLLECTION. If any are NULL, then NULL is returned.</para>
 
        <note><para>ST_Collect and ST_Union are often interchangeable.
                ST_Union is in general orders of magnitude slower than ST_Collect