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

tglsfdc · tglsfdc · commit 5c85d1122b93 · 2010-11-12T15:18:27.000-05:00
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
@@ -2733,14 +2733,18 @@ 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))
 			{
 				if (((Const *) e)->constisnull)
 					continue;	/* drop null constant */
 				if (newargs == NIL)
 					return e;	/* first expr */
+				newargs = lappend(newargs, e);
+				break;
 			}
 			newargs = lappend(newargs, e);
 		}
