Fix for a bug with EXPLAIN and (....) UNION (....) order by

mysql-test/r/union.result

@@ -84,6 +84,11 @@ a	b
 3	c
 2	b
 1	a
+explain (select a,b from t1 limit 2)  union all (select a,b from t2 order by a limit 1) order by b desc;
+table	type	possible_keys	key	key_len	ref	rows	Extra
+t1	ALL	NULL	NULL	NULL	NULL	4	
+t2	ALL	NULL	NULL	NULL	NULL	4	Using filesort
+t1	ALL	NULL	NULL	NULL	NULL	4	
 explain select a,b from t1 union all select a,b from t2;
 table	type	possible_keys	key	key_len	ref	rows	Extra
 t1	ALL	NULL	NULL	NULL	NULL	4	

mysql-test/t/union.test

@@ -20,6 +20,7 @@ select 't1',b,count(*) from t1 group by b UNION select 't2',b,count(*) from t2 g
 (select a,b from t1 limit 2)  union all (select a,b from t2 order by a) limit 4;
 (select a,b from t1 limit 2)  union all (select a,b from t2 order by a limit 1);
 (select a,b from t1 limit 2)  union all (select a,b from t2 order by a limit 1) order by b desc;
+explain (select a,b from t1 limit 2)  union all (select a,b from t2 order by a limit 1) order by b desc;
 
 # Test some error conditions with UNION
 explain select a,b from t1 union all select a,b from t2;

sql/sql_union.cc

@@ -34,6 +34,7 @@ int mysql_union(THD *thd, LEX *lex,select_result *result)
   int describe=(lex->select_lex.options & SELECT_DESCRIBE) ? 1 : 0;
   int res;
   TABLE_LIST result_table_list;
+  TABLE_LIST *first_table=(TABLE_LIST *)lex->select_lex.table_list.first;
   TMP_TABLE_PARAM tmp_table_param;
   select_union *union_result;
   DBUG_ENTER("mysql_union");
@@ -58,8 +59,9 @@ int mysql_union(THD *thd, LEX *lex,select_result *result)
       the ORDER BY and LIMIT parameter for the whole UNION
     */
     lex_sl= sl;
-    last_sl->next=0;				// Remove this extra element
     order=  (ORDER *) lex_sl->order_list.first;
+    if (!order || !describe) 
+      last_sl->next=0;				// Remove this extra element
   }
   else if (!last_sl->braces)
   {
@@ -136,7 +138,7 @@ int mysql_union(THD *thd, LEX *lex,select_result *result)
     if (thd->select_limit == HA_POS_ERROR)
       sl->options&= ~OPTION_FOUND_ROWS;
 
-    res=mysql_select(thd, (TABLE_LIST*) sl->table_list.first,
+    res=mysql_select(thd, (describe && sl->linkage==NOT_A_SELECT) ? first_table :  (TABLE_LIST*) sl->table_list.first,
 		     sl->item_list,
 		     sl->where,
 		     (sl->braces) ? (ORDER *)sl->order_list.first : (ORDER *) 0,
@@ -193,7 +195,7 @@ int mysql_union(THD *thd, LEX *lex,select_result *result)
       if (describe)
 	thd->select_limit= HA_POS_ERROR;		// no limit
       res=mysql_select(thd,&result_table_list,
-		       item_list, NULL, /*ftfunc_list,*/ order,
+		       item_list, NULL, (describe) ? 0 : order,
 		       (ORDER*) NULL, NULL, (ORDER*) NULL,
 		       thd->options, result);
     }