docs: document behavior of CHAR() comparisons with chars < space

Space trimming rather than space-padding causes unusual behavior, which
might not be standards-compliant.

Also remove recently-added now-redundant C comment.

diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml
index 00ccbe1bb5f6a7f59256d8124d6aae27ab35bf0c..ebcadf617aa80af3520e856545c7e648586c210a 100644 (file)
--- a/doc/src/sgml/datatype.sgml
+++ b/doc/src/sgml/datatype.sgml
@@ -1078,10 +1078,13 @@ SELECT '52093.89'::money::numeric::float8;
    
     Values of type character are physically padded
     with spaces to the specified width n, and are
-    stored and displayed that way.  However, the padding spaces are
-    treated as semantically insignificant.  Trailing spaces are
-    disregarded when comparing two values of type character,
-    and they will be removed when converting a character value
+    stored and displayed that way.  However, trailing spaces are treated as
+    semantically insignificant and disregarded when comparing two values
+    of type character.  In collations where whitespace
+    is significant, this behavior can produce unexpected results,
+    e.g. SELECT 'a '::CHAR(2) collate "C" < 'a\n'::CHAR(2)
+    returns true.
+    Trailing spaces are removed when converting a character value
     to one of the other string types.  Note that trailing spaces
     are semantically significant in
     character varying and text values, and

diff --git a/src/backend/utils/adt/varchar.c b/src/backend/utils/adt/varchar.c
index 284b5d13093a2bf2de0320aa60f465e95822f0af..502ca44e04a961a9e7ce256ed4cb39636b4a1831 100644 (file)
--- a/src/backend/utils/adt/varchar.c
+++ b/src/backend/utils/adt/varchar.c
@@ -846,18 +846,6 @@ bpcharcmp(PG_FUNCTION_ARGS)
                len2;
    int         cmp;
 
-   /*
-    * Trimming trailing spaces off of both strings can cause a string
-    * with a character less than a space to compare greater than a
-    * space-extended string, e.g. this returns false:
-    *      SELECT E'ab\n'::CHAR(10) < E'ab '::CHAR(10);
-    * even though '\n' is less than the space if CHAR(10) was
-    * space-extended.  The correct solution would be to trim only
-    * the longer string to be the same length of the shorter, if
-    * possible, then do the comparison.  However, changing this
-    * might break existing indexes, breaking binary upgrades.
-    * For details, see http://www.postgresql.org/message-id/CAK+WP1xdmyswEehMuetNztM4H199Z1w9KWRHVMKzyyFM+hV=zA@mail.gmail.com
-    */
    len1 = bcTruelen(arg1);
    len2 = bcTruelen(arg2);