Doc: improve description of trim() and related functions.

Per bug #14441 from Mark Pether, the documentation could be misread,
mainly because some of the examples failed to show what happens with
a multicharacter "characters to trim" string.  Also, while the text
description in most of these entries was fairly clear that the
"characters" argument is a set of characters not a substring to match,
some of them used variant wording that was a bit less clear.
trim() itself suffered from both deficiencies and was thus pretty
misinterpretable.

Also fix failure to explain which of LEADING/TRAILING/BOTH is the
default.

Discussion: https://postgr.es/m/20161130011710.6539.53657@wrigleys.postgresql.org

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 5752c0f9e75768b24fbcb2e8d344448319eea150..c37ef80026e6971c6ed59aaab7d50b5db239f6bb 100644 (file)
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -1306,11 +1306,12 @@
        </entry>
        <entry><type>text</type></entry>
        <entry>
-        Remove the longest string containing only the
+        Remove the longest string containing only characters from
         <parameter>characters</parameter> (a space by default) from the
-        start/end/both ends of the <parameter>string</parameter>
+        start, end, or both ends (<literal>both</> is the default)
+        of <parameter>string</parameter>
        </entry>
-       <entry><literal>trim(both 'x' from 'xTomxx')</literal></entry>
+       <entry><literal>trim(both 'xyz' from 'yxTomxx')</literal></entry>
        <entry><literal>Tom</literal></entry>
       </row>
 
@@ -1382,7 +1383,7 @@
         in <parameter>characters</parameter> (a space by default)
         from the start and end of <parameter>string</parameter>
        </entry>
-       <entry><literal>btrim('xyxtrimyyx', 'xy')</literal></entry>
+       <entry><literal>btrim('xyxtrimyyx', 'xyz')</literal></entry>
        <entry><literal>trim</literal></entry>
       </row>
 
@@ -1657,8 +1658,8 @@
         <parameter>characters</parameter> (a space by default) from the start of
         <parameter>string</parameter>
        </entry>
-       <entry><literal>ltrim('zzzytrim', 'xyz')</literal></entry>
-       <entry><literal>trim</literal></entry>
+       <entry><literal>ltrim('zzzytest', 'xyz')</literal></entry>
+       <entry><literal>test</literal></entry>
       </row>
 
       <row>
@@ -1940,8 +1941,8 @@
         <parameter>characters</parameter> (a space by default) from the end of
         <parameter>string</parameter>
        </entry>
-       <entry><literal>rtrim('trimxxxx', 'x')</literal></entry>
-       <entry><literal>trim</literal></entry>
+       <entry><literal>rtrim('testxxzx', 'xyz')</literal></entry>
+       <entry><literal>test</literal></entry>
       </row>
 
       <row>
@@ -2985,11 +2986,11 @@
        </entry>
        <entry><type>bytea</type></entry>
        <entry>
-        Remove the longest string containing only the bytes in
+        Remove the longest string containing only bytes appearing in
         <parameter>bytes</parameter> from the start
         and end of <parameter>string</parameter>
        </entry>
-       <entry><literal>trim(E'\\000'::bytea from E'\\000Tom\\000'::bytea)</literal></entry>
+       <entry><literal>trim(E'\\000\\001'::bytea from E'\\000Tom\\001'::bytea)</literal></entry>
        <entry><literal>Tom</literal></entry>
       </row>
      </tbody>
@@ -3028,11 +3029,11 @@
        </entry>
        <entry><type>bytea</type></entry>
        <entry>
-        Remove the longest string consisting only of bytes
-        in <parameter>bytes</parameter> from the start and end of
+        Remove the longest string containing only bytes appearing in
+        <parameter>bytes</parameter> from the start and end of
         <parameter>string</parameter>
       </entry>
-      <entry><literal>btrim(E'\\000trim\\000'::bytea, E'\\000'::bytea)</literal></entry>
+      <entry><literal>btrim(E'\\000trim\\001'::bytea, E'\\000\\001'::bytea)</literal></entry>
       <entry><literal>trim</literal></entry>
      </row>