A fix (Bug #4980: union statement with () union () order by produces wrong explain).

mysql-test/r/union.result

@@ -88,7 +88,6 @@ explain (select a,b from t1 limit 2)  union all (select a,b from t2 order by a l
 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	
 (select sql_calc_found_rows  a,b from t1 limit 2)  union all (select a,b from t2 order by a) limit 2;
 a	b
 1	a
@@ -424,8 +423,16 @@ create table t1 select a from t1 union select a from t2;
 INSERT TABLE 't1' isn't allowed in FROM table list
 select a from t1 union select a from t2 order by t2.a;
 Unknown column 't2.a' in 'ORDER BY'
+drop table t1;
 drop table t1,t2;
 select length(version()) > 1 as `*` UNION select 2;
 *
 1
 2
+create table t1 (a int);
+insert into t1 values (0), (3), (1), (2);
+explain (select * from t1) union (select * from t1) order by a;
+table	type	possible_keys	key	key_len	ref	rows	Extra
+t1	ALL	NULL	NULL	NULL	NULL	4	
+t1	ALL	NULL	NULL	NULL	NULL	4	
+drop table t1;

mysql-test/t/union.test

@@ -228,6 +228,7 @@ create temporary table t1 select a from t1 union select a from t2;
 create table t1 select a from t1 union select a from t2;
 --error 1054
 select a from t1 union select a from t2 order by t2.a;
+drop table t1;   # Drop temporary table
 drop table t1,t2;
 
 #
@@ -236,3 +237,11 @@ drop table t1,t2;
 
 select length(version()) > 1 as `*` UNION select 2;
 
+#
+# Bug #4980: problem with explain
+#
+
+create table t1 (a int);
+insert into t1 values (0), (3), (1), (2);
+explain (select * from t1) union (select * from t1) order by a;
+drop table t1;

sql/sql_union.cc

@@ -148,6 +148,10 @@ int mysql_union(THD *thd, LEX *lex,select_result *result)
   {
     ha_rows records_at_start;
     lex->select=sl;
+#if MYSQL_VERSION_ID < 40100
+    if (describe && sl->linkage == NOT_A_SELECT)
+      break;      // Skip extra item in case of 'explain'
+#endif
     /* Don't use offset for the last union if there is no braces */
     if (sl != lex_sl)
     {