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 e948bccc1a2fcb7864df063b52eed49063dee4f4..de2e66b0f160ddb1356e545ba09fa28e4abca706 100644 (file)
--- a/src/backend/optimizer/util/clauses.c
+++ b/src/backend/optimizer/util/clauses.c
@@ -2746,7 +2746,9 @@ 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))
                        {
@@ -2754,6 +2756,8 @@ 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);
                }