make JOIN::prepare, JOIN::optimize only once for EXPLAIN of derived table

(BUG#2120 sfter merge)

mysql-test/r/derived.result

@@ -141,7 +141,7 @@ a	t
 explain select count(*) from t1 as tt1, (select * from t1) as tt2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
-2	DERIVED	t1	index	NULL	a	4	NULL	10000	Using index
+2	DERIVED	t1	ALL	NULL	NULL	NULL	NULL	10000	
 drop table t1;
 SELECT * FROM (SELECT (SELECT * FROM (SELECT 1 as a) as a )) as b;
 (SELECT * FROM (SELECT 1 as a) as a )
@@ -189,13 +189,13 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	m2	ALL	NULL	NULL	NULL	NULL	9	
 1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	6	Using where
 2	DERIVED	mp	ALL	NULL	NULL	NULL	NULL	9	Using temporary; Using filesort
-2	DERIVED	m1	index	NULL	PRIMARY	3	NULL	9	Using index
+2	DERIVED	m1	eq_ref	PRIMARY	PRIMARY	3	test.mp.mat_id	1	
 explain SELECT STRAIGHT_JOIN d.pla_id, m2.test FROM t1 m2  INNER JOIN (SELECT mp.pla_id, MIN(m1.matintnum) AS matintnum FROM t2 mp INNER JOIN t1 m1 ON mp.mat_id=m1.mat_id GROUP BY mp.pla_id) d ON d.matintnum=m2.matintnum;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	m2	ALL	NULL	NULL	NULL	NULL	9	
 1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	6	Using where
 2	DERIVED	mp	ALL	NULL	NULL	NULL	NULL	9	Using temporary; Using filesort
-2	DERIVED	m1	index	NULL	PRIMARY	3	NULL	9	Using index
+2	DERIVED	m1	eq_ref	PRIMARY	PRIMARY	3	test.mp.mat_id	1	
 drop table t1,t2;
 SELECT a.x FROM (SELECT 1 AS x) AS a HAVING a.x = 1;
 x
@@ -229,8 +229,8 @@ explain select count(*) from t1 INNER JOIN (SELECT A.E1, A.E2, A.E3 FROM t1 AS A
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	2	
 1	PRIMARY	t1	eq_ref	PRIMARY	PRIMARY	4	THEMAX.E2	1	Using where
-2	DERIVED	A	index	NULL	PRIMARY	4	NULL	2	Using where; Using index
-3	DEPENDENT SUBQUERY	B	index	NULL	PRIMARY	4	NULL	2	Using where; Using index
+2	DERIVED	A	ALL	NULL	NULL	NULL	NULL	2	Using where
+3	DEPENDENT SUBQUERY	B	ALL	NULL	NULL	NULL	NULL	2	Using where
 drop table t1;
 create table t1 (a int);
 insert into t1 values (1),(2);
@@ -309,3 +309,14 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	<derived2>	ALL	NULL	NULL	NULL	NULL	3	Using temporary; Using filesort
 2	DERIVED	x	ALL	NULL	NULL	NULL	NULL	17	Using temporary; Using filesort
 2	DERIVED	y	ALL	NULL	NULL	NULL	NULL	17	Using where
+drop table t1;
+create table t2 (a int, b int, primary key (a));
+insert into t2 values (1,7),(2,7);
+explain select a from t2 where a>1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	range	PRIMARY	PRIMARY	4	NULL	2	Using where; Using index
+explain select a from (select a from t2 where a>1) tt;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	<derived2>	system	NULL	NULL	NULL	NULL	1	
+2	DERIVED	t2	range	PRIMARY	PRIMARY	4	NULL	2	Using where; Using index
+drop table t2;

mysql-test/t/derived.test

@@ -192,3 +192,13 @@ CREATE TABLE t1 (name char(1) default NULL, val int(5) default NULL);
 INSERT INTO t1 VALUES ('a',1),  ('a',2),  ('a',2),  ('a',2),  ('a',3),  ('a',6), ('a',7), ('a',11), ('a',11), ('a',12), ('a',13), ('a',13), ('a',20), ('b',2), ('b',3), ('b',4), ('b',5);
 SELECT s.name, AVG(s.val) AS median FROM (SELECT x.name, x.val FROM t1 x, t1 y WHERE x.name=y.name GROUP BY x.name, x.val HAVING SUM(y.val <= x.val) >= COUNT(*)/2 AND SUM(y.val >= x.val) >= COUNT(*)/2) AS s GROUP BY s.name;
 explain SELECT s.name, AVG(s.val) AS median FROM (SELECT x.name, x.val FROM t1 x, t1 y WHERE x.name=y.name GROUP BY x.name, x.val HAVING SUM(y.val <= x.val) >= COUNT(*)/2 AND SUM(y.val >= x.val) >= COUNT(*)/2) AS s GROUP BY s.name;
+drop table t1;
+
+#
+# "Using index" in explain
+#
+create table t2 (a int, b int, primary key (a));
+insert into t2 values (1,7),(2,7);
+explain select a from t2 where a>1;
+explain select a from (select a from t2 where a>1) tt;
+drop table t2;

sql/mysql_priv.h

@@ -247,7 +247,9 @@ extern CHARSET_INFO *national_charset_info, *table_alias_charset;
 // uncachable cause
 #define UNCACHEABLE_DEPENDENT   1
 #define UNCACHEABLE_RAND        2
-#define UNCACHEABLE_SIDEEFFECT 4
+#define UNCACHEABLE_SIDEEFFECT	4
+// forcing to save JOIN for explain
+#define UNCACHEABLE_EXPLAIN     8
 
 #ifdef EXTRA_DEBUG
 /*

sql/sql_derived.cc

@@ -62,6 +62,15 @@ mysql_handle_derived(LEX *lex)
 	  return 1;
 	}
       }
+      if (lex->describe)
+      {
+	/*
+	  Force join->join_tmp creation, because we will use this JOIN
+	  twice for EXPLAIN and we have to have unchanged join for EXPLAINing
+	*/
+	sl->uncacheable|= UNCACHEABLE_EXPLAIN;
+	sl->master_unit()->uncacheable|= UNCACHEABLE_EXPLAIN;
+      }
     }
   }
   return 0;

sql/sql_lex.h

@@ -232,6 +232,7 @@ class st_select_lex_node {
       UNCACHEABLE_DEPENDENT
       UNCACHEABLE_RAND
       UNCACHEABLE_SIDEEFFECT
+      UNCACHEABLE_EXPLAIN
   */
   uint8 uncacheable;
   enum sub_select_type linkage;

sql/sql_select.cc

@@ -1579,8 +1579,8 @@ mysql_select(THD *thd, Item ***rref_pointer_array,
 	  goto err;
 	}
       }
-      free_join= 0;
     }
+    free_join= 0;
     join->select_options= select_options;
   }
   else
@@ -3769,11 +3769,6 @@ JOIN::join_free(bool full)
       {
 	if (tab->table)
 	{
-	  if (tab->table->key_read)
-	  {
-	    tab->table->key_read= 0;
-	    tab->table->file->extra(HA_EXTRA_NO_KEYREAD);
-	  }
 	  /* Don't free index if we are using read_record */
 	  if (!tab->read_record.table)
 	    tab->table->file->index_end();
@@ -9128,6 +9123,9 @@ int mysql_explain_union(THD *thd, SELECT_LEX_UNIT *unit, select_result *result)
        sl;
        sl= sl->next_select())
   {
+    // drop UNCACHEABLE_EXPLAIN, because it is for internal usage only
+    uint8 uncacheable= (sl->uncacheable & ~UNCACHEABLE_EXPLAIN);
+
     res= mysql_explain_select(thd, sl,
 			      (((&thd->lex->select_lex)==sl)?
 			       ((thd->lex->all_selects_list != sl) ? 
@@ -9135,13 +9133,13 @@ int mysql_explain_union(THD *thd, SELECT_LEX_UNIT *unit, select_result *result)
 			       ((sl == first)?
 				((sl->linkage == DERIVED_TABLE_TYPE) ?
 				 "DERIVED":
-				((sl->uncacheable & UNCACHEABLE_DEPENDENT) ?
+				((uncacheable & UNCACHEABLE_DEPENDENT) ?
 				 "DEPENDENT SUBQUERY":
-				 (sl->uncacheable?"UNCACHEABLE SUBQUERY":
+				 (uncacheable?"UNCACHEABLE SUBQUERY":
 				   "SUBQUERY"))):
-				((sl->uncacheable & UNCACHEABLE_DEPENDENT) ?
+				((uncacheable & UNCACHEABLE_DEPENDENT) ?
 				 "DEPENDENT UNION":
-				 sl->uncacheable?"UNCACHEABLE UNION":
+				 uncacheable?"UNCACHEABLE UNION":
 				  "UNION"))),
 			      result);
     if (res)