Fix old oversight in const-simplification of COALESCE() expressions.

Once we have found a non-null constant argument, there is no need to
examine additional arguments of the COALESCE.  The previous coding got it
right only if the constant was in the first argument position; otherwise
it tried to simplify following arguments too, leading to unexpected
behavior like this:

regression=# select coalesce(f1, 42, 1/0) from int4_tbl;
ERROR:  division by zero

It's a minor corner case, but a bug is a bug, so back-patch all the way.

diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c
index aba496253b9801fcb4d2912033fc2bedcf87b0f3..e046632d6edbaf4c1a65ec6570ae21f68d33b80f 100644 (file)
--- a/src/backend/optimizer/util/clauses.c
+++ b/src/backend/optimizer/util/clauses.c
@@ -2758,7 +2758,9 @@ eval_const_expressions_mutator(Node *node,
            /*
             * We can remove null constants from the list. For a non-null
             * constant, if it has not been preceded by any other
-            * non-null-constant expressions then that is the result.
+            * non-null-constant expressions then it is the result.  Otherwise,
+            * it's the next argument, but we can drop following arguments
+            * since they will never be reached.
             */
            if (IsA(e, Const))
            {
@@ -2766,6 +2768,8 @@ eval_const_expressions_mutator(Node *node,
                    continue;   /* drop null constant */
                if (newargs == NIL)
                    return e;   /* first expr */
+               newargs = lappend(newargs, e);
+               break;
            }
            newargs = lappend(newargs, e);
        }