C comment: mention no doc for negative start of substring(text)

Also add URL to hackers discussion.

Backpatch-through: master

diff --git a/src/backend/utils/adt/varlena.c b/src/backend/utils/adt/varlena.c
index 8d28dd42ce1bbe3ac04bf4b2bbfdf6af04241352..d1b09dedfd4a8d8d5add824513bc38fab89297d9 100644 (file)
--- a/src/backend/utils/adt/varlena.c
+++ b/src/backend/utils/adt/varlena.c
@@ -891,8 +891,9 @@ text_substring(Datum str, int32 start, int32 length, bool length_not_specified)
    int32       E;              /* end position */
 
    /*
-    * SQL99 says S can be zero or negative, but we still must fetch from the
-    * start of the string.
+    * SQL99 says S can be zero or negative (which we don't document), but we
+    * still must fetch from the start of the string.
+    * https://www.postgresql.org/message-id/170905442373.643.11536838320909376197%40wrigleys.postgresql.org
     */
    S1 = Max(S, 1);