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 $
  *
  *-------------------------------------------------------------------------
  */
@@ -4754,26 +4754,23 @@ exec_simple_cast_value(Datum value, Oid valtype,
 					   Oid reqtype, int32 reqtypmod,
 					   bool isnull)
 {
-	if (!isnull)
+	if (valtype != reqtype || reqtypmod != -1)
 	{
-		if (valtype != reqtype || reqtypmod != -1)
-		{
-			Oid			typinput;
-			Oid			typioparam;
-			FmgrInfo	finfo_input;
+		Oid			typinput;
+		Oid			typioparam;
+		FmgrInfo	finfo_input;
 
-			getTypeInputInfo(reqtype, &typinput, &typioparam);
+		getTypeInputInfo(reqtype, &typinput, &typioparam);
 
-			fmgr_info(typinput, &finfo_input);
+		fmgr_info(typinput, &finfo_input);
 
-			value = exec_cast_value(value,
-									valtype,
-									reqtype,
-									&finfo_input,
-									typioparam,
-									reqtypmod,
-									isnull);
-		}
+		value = exec_cast_value(value,
+								valtype,
+								reqtype,
+								&finfo_input,
+								typioparam,
+								reqtypmod,
+								isnull);
 	}
 
 	return value;