Fix plpgsql's exec_eval_expr() to ensure it returns a sane type OID

even when the expression is a query that returns no rows.

So far as I can tell, the only caller that actually fails when a garbage
OID is returned is exec_stmt_case(), which is new in 8.4 --- in all other
cases, we might make a useless trip through casting logic, but we won't
fail since the isnull flag will be set.  Hence, backpatch only to 8.4,
just in case there are apps out there that aren't expecting an error to
be thrown if the query returns more or less than one column.  (Which seems
unlikely, since the error would be thrown if the query ever did return a
row; but it's possible there's some never-exercised code out there.)

Per report from Mario Splivalo.

src/pl/plpgsql/src/pl_exec.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/pl/plpgsql/src/pl_exec.c,v 1.256 2010/02/26 02:01:34 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/pl/plpgsql/src/pl_exec.c,v 1.257 2010/04/14 23:52:10 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -4158,7 +4158,24 @@ exec_eval_expr(PLpgSQL_execstate *estate,
 				 errmsg("query \"%s\" did not return data", expr->query)));
 
 	/*
-	 * If there are no rows selected, the result is NULL.
+	 * Check that the expression returns exactly one column...
+	 */
+	if (estate->eval_tuptable->tupdesc->natts != 1)
+		ereport(ERROR,
+				(errcode(ERRCODE_SYNTAX_ERROR),
+				 errmsg_plural("query \"%s\" returned %d column",
+							   "query \"%s\" returned %d columns",
+							   estate->eval_tuptable->tupdesc->natts,
+							   expr->query,
+							   estate->eval_tuptable->tupdesc->natts)));
+
+	/*
+	 * ... and get the column's datatype.
+	 */
+	*rettype = SPI_gettypeid(estate->eval_tuptable->tupdesc, 1);
+
+	/*
+	 * If there are no rows selected, the result is a NULL of that type.
 	 */
 	if (estate->eval_processed == 0)
 	{
@@ -4167,26 +4184,17 @@ exec_eval_expr(PLpgSQL_execstate *estate,
 	}
 
 	/*
-	 * Check that the expression returned one single Datum
+	 * Check that the expression returned no more than one row.
 	 */
-	if (estate->eval_processed > 1)
+	if (estate->eval_processed != 1)
 		ereport(ERROR,
 				(errcode(ERRCODE_CARDINALITY_VIOLATION),
 				 errmsg("query \"%s\" returned more than one row",
 						expr->query)));
-	if (estate->eval_tuptable->tupdesc->natts != 1)
-		ereport(ERROR,
-				(errcode(ERRCODE_SYNTAX_ERROR),
-				 errmsg_plural("query \"%s\" returned %d column",
-							   "query \"%s\" returned %d columns",
-							   estate->eval_tuptable->tupdesc->natts,
-							   expr->query,
-							   estate->eval_tuptable->tupdesc->natts)));
 
 	/*
-	 * Return the result and its type
+	 * Return the single result Datum.
 	 */
-	*rettype = SPI_gettypeid(estate->eval_tuptable->tupdesc, 1);
 	return SPI_getbinval(estate->eval_tuptable->vals[0],
 						 estate->eval_tuptable->tupdesc, 1, isNull);
 }