MDEV-25682 Explain shows an execution plan different from actually executed

If a select query contained an ORDER BY clause that followed a LIMIT clause
or an ORDER BY clause or ORDER BY with LIMIT the EXPLAIN output for the
query showed an execution plan different from that was actually executed.

Approved by Roman Nozdrin <roman.nozdrin@mariadb.com>

mysql-test/main/order_by.result

@@ -3460,6 +3460,31 @@ SET max_length_for_sort_data=@save_max_length_for_sort_data;
 SET max_sort_length= @save_max_sort_length;
 SET sql_select_limit= @save_sql_select_limit;
 DROP TABLE t1;
+#
+# MDEV-25682: EXPLAIN for SELECT with ORDER BY after [ORDER BY] LIMIT
+#
+create table t1 (a int);
+insert into t1 values (3), (7), (1);
+explain (select a from t1 limit 2) order by a desc;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	
+NULL	UNIT RESULT	<unit1>	ALL	NULL	NULL	NULL	NULL	NULL	Using filesort
+(select a from t1 limit 2) order by a desc;
+a
+7
+3
+create table t2 (a int, b int);
+insert into t2 values (3,70), (7,10), (1,40), (4,30);
+explain (select b,a from t2 order by a limit 3) order by b desc;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	4	Using filesort
+NULL	UNIT RESULT	<unit1>	ALL	NULL	NULL	NULL	NULL	NULL	Using filesort
+(select b,a from t2 order by a limit 3) order by b desc;
+b	a
+70	3
+40	1
+30	4
+drop table t1,t2;
 # End of 10.2 tests
 #
 # MDEV-16214: Incorrect plan taken by the optimizer , uses INDEX instead of ref access with ORDER BY

mysql-test/main/order_by.test

@@ -2293,6 +2293,22 @@ SET max_sort_length= @save_max_sort_length;
 SET sql_select_limit= @save_sql_select_limit;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-25682: EXPLAIN for SELECT with ORDER BY after [ORDER BY] LIMIT
+--echo #
+
+create table t1 (a int);
+insert into t1 values (3), (7), (1);
+explain (select a from t1 limit 2) order by a desc;
+(select a from t1 limit 2) order by a desc;
+
+create table t2 (a int, b int);
+insert into t2 values (3,70), (7,10), (1,40), (4,30);
+explain (select b,a from t2 order by a limit 3) order by b desc;
+(select b,a from t2 order by a limit 3) order by b desc;
+
+drop table t1,t2;
+
 --echo # End of 10.2 tests
 
 --echo #

sql/item_subselect.cc

@@ -274,6 +274,8 @@ bool Item_subselect::fix_fields(THD *thd_param, Item **ref)
         res= TRUE;
         goto end;
       }
+      if (sl ==  unit->first_select() && !sl->next_select())
+        unit->fake_select_lex= 0;
     }
   }
   

sql/sql_select.cc

@@ -26370,7 +26370,7 @@ bool mysql_explain_union(THD *thd, SELECT_LEX_UNIT *unit, select_result *result)
     sl->options|= SELECT_DESCRIBE;
   }
 
-  if (unit->is_unit_op())
+  if (unit->is_unit_op() || unit->fake_select_lex)
   {
     if (unit->union_needs_tmp_table() && unit->fake_select_lex)
     {