Fix LP BUG#680005

Analysis:
This another instance of the problem fixed in LP BUG#675981 -
evaluation of subqueries during EXPLAIN when the subquery plan
is incomplete because JOIN::optimize() generally doesn't create
complete execution plans for EXPLAIN statements.

In this case the call path is:
mysql_explain_union -> outer_join.exec -> outer_join.init_execution ->
create_sort_index -> filesort -> find_all_keys ->
SQL_SELECT::skip_record -> outer_where_clause.val_int -> ...
-> subselect_join.exec -> ... -> sub_select_cache

When calling sub_select_cache JOIN_TAB::cache is NULL because the cache
objects are not created for EXPLAIN statements.

Solution:
Delay the call to init_execution() after all EXPLAIN related processing
is completed. Thus init_execution() is not called at all during EXPLAIN.

mysql-test/r/subselect4.result

@@ -456,6 +456,38 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 2	DEPENDENT SUBQUERY	t3	ALL	NULL	NULL	NULL	NULL	2	Using where; Using join buffer
 drop table t1, t2, t3;
 #
+# LP BUG#680005 Second assertion `cache != __null' failed in
+#                sub_select_cache() on EXPLAIN
+#
+CREATE TABLE t1 (f1 int,f2 int,f4 int,f6 int,KEY (f4)) ;
+INSERT IGNORE INTO t1 VALUES
+('1','5','1','0'),('2','1','1','0'),('2','2','2','0'),('0',NULL,'0','0'),
+('2','1','2','0'),('2','0','0','0'),('2','2','2','0'),('2','8','2','0'),
+('2','7','2','0'),('2','5','2','0'),('2',NULL,'1','0');
+CREATE TABLE t2 (f3 int) ;
+INSERT IGNORE INTO t2 VALUES ('7');
+CREATE TABLE t3 (f3 int) ;
+INSERT IGNORE INTO t3 VALUES ('2');
+EXPLAIN
+SELECT t1.f4
+FROM t2 JOIN t1 ON t1.f6
+WHERE
+( t1.f2 ) IN (SELECT SUBQUERY2_t1.f3
+FROM t3 AS SUBQUERY2_t1
+JOIN
+(t1 AS SUBQUERY2_t2
+JOIN
+t1 AS SUBQUERY2_t3 ON SUBQUERY2_t3.f1)
+ON SUBQUERY2_t3.f2)
+GROUP BY t1.f4 ORDER BY t1.f1 LIMIT 10;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	system	NULL	NULL	NULL	NULL	1	Using temporary; Using filesort
+1	PRIMARY	t1	index	NULL	f4	5	NULL	10	Using where
+2	DEPENDENT SUBQUERY	SUBQUERY2_t1	system	NULL	NULL	NULL	NULL	1	
+2	DEPENDENT SUBQUERY	SUBQUERY2_t2	index	NULL	f4	5	NULL	11	Using where; Using index
+2	DEPENDENT SUBQUERY	SUBQUERY2_t3	ALL	NULL	NULL	NULL	NULL	11	Using where; Using join buffer
+drop table t1, t2, t3;
+#
 # BUG#52317: Assertion failing in Field_varstring::store() 
 # 	    at field.cc:6833
 #

mysql-test/t/subselect4.test

@@ -418,6 +418,38 @@ WHERE f1 IN (SELECT t1.f2 FROM t1 JOIN t3 ON t3.f4);
 
 drop table t1, t2, t3;
 
+--echo #
+--echo # LP BUG#680005 Second assertion `cache != __null' failed in
+--echo #                sub_select_cache() on EXPLAIN
+--echo #
+
+CREATE TABLE t1 (f1 int,f2 int,f4 int,f6 int,KEY (f4)) ;
+INSERT IGNORE INTO t1 VALUES
+('1','5','1','0'),('2','1','1','0'),('2','2','2','0'),('0',NULL,'0','0'),
+('2','1','2','0'),('2','0','0','0'),('2','2','2','0'),('2','8','2','0'),
+('2','7','2','0'),('2','5','2','0'),('2',NULL,'1','0');
+
+CREATE TABLE t2 (f3 int) ;
+INSERT IGNORE INTO t2 VALUES ('7');
+
+CREATE TABLE t3 (f3 int) ;
+INSERT IGNORE INTO t3 VALUES ('2');
+
+EXPLAIN
+SELECT t1.f4
+FROM t2 JOIN t1 ON t1.f6
+WHERE
+( t1.f2 ) IN (SELECT SUBQUERY2_t1.f3
+              FROM t3 AS SUBQUERY2_t1
+                   JOIN
+                   (t1 AS SUBQUERY2_t2
+                      JOIN
+                    t1 AS SUBQUERY2_t3 ON SUBQUERY2_t3.f1)
+                   ON SUBQUERY2_t3.f2)
+GROUP BY t1.f4 ORDER BY t1.f1 LIMIT 10;
+
+drop table t1, t2, t3;
+
 --echo #
 --echo # BUG#52317: Assertion failing in Field_varstring::store() 
 --echo # 	    at field.cc:6833

sql/sql_select.cc

@@ -1747,9 +1747,6 @@ JOIN::exec()
   int      tmp_error;
   DBUG_ENTER("JOIN::exec");
 
-  if (!initialized && init_execution())
-    DBUG_VOID_RETURN;
-
   thd_proc_info(thd, "executing");
   error= 0;
   if (procedure)
@@ -1880,6 +1877,9 @@ JOIN::exec()
     DBUG_VOID_RETURN;
   }
 
+  if (!initialized && init_execution())
+    DBUG_VOID_RETURN;
+
   JOIN *curr_join= this;
   List<Item> *curr_all_fields= &all_fields;
   List<Item> *curr_fields_list= &fields_list;