Add C comment about problems with CHAR() space trimming

diff --git a/src/backend/utils/adt/varchar.c b/src/backend/utils/adt/varchar.c
index 502ca44e04a961a9e7ce256ed4cb39636b4a1831..284b5d13093a2bf2de0320aa60f465e95822f0af 100644 (file)
--- a/src/backend/utils/adt/varchar.c
+++ b/src/backend/utils/adt/varchar.c
@@ -846,6 +846,18 @@ 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);