plpgsql's exec_simple_cast_value() mistakenly supposed that it could bypass

casting effort whenever the input value was NULL.  However this prevents
application of not-null domain constraints in the cases that use this
function, as illustrated in bug #4741.  Since this function isn't meant
for use in performance-critical paths anyway, this certainly seems like
another case of "premature optimization is the root of all evil".

Back-patch as far as 8.2; older versions made no effort to enforce
domain constraints here anyway.

src/pl/plpgsql/src/pl_exec.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/pl/plpgsql/src/pl_exec.c,v 1.236 2009/03/26 22:26:08 petere Exp $
+ *	  $PostgreSQL: pgsql/src/pl/plpgsql/src/pl_exec.c,v 1.237 2009/04/02 01:16:11 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -4754,8 +4754,6 @@ exec_simple_cast_value(Datum value, Oid valtype,
 					   Oid reqtype, int32 reqtypmod,
 					   bool isnull)
 {
-	if (!isnull)
-	{
 	if (valtype != reqtype || reqtypmod != -1)
 	{
 		Oid			typinput;
@@ -4774,7 +4772,6 @@ exec_simple_cast_value(Datum value, Oid valtype,
 								reqtypmod,
 								isnull);
 	}
-	}
 
 	return value;
 }