Fixed LP BUG#675981

Cause:
The optimize() phase for the subquery selected to use join buffering via setting
JOIN_TAB::next_select= sub_select_cache in make_join_readinfo, however, the call
to check_join_cache_usage() from make_join_readinfo didn't create the corresponding
JOIN_CACHE_BNL object because of the condition:
    if ((options & SELECT_DESCRIBE) ||
        (((tab->cache= new JOIN_CACHE_BNL(join, tab, prev_cache))) &&
         !tab->cache->init()))
Since EXPLAIN for subqueries runs regular execution, the constant predicates that
were delayed to be evaluated at the exec() phase, were evaluated during EXPLAIN.
As a result the outer JOIN::exec called JOIN::exec for the subquery, while the
subquery execution plan was no properly created, which resulted in an failed ASSERT.

Fix:
The patch blocks evaluation of constant expensive conditions during EXPLAIN. Notice
that these conditions are "constant" with respect to the outer query, thus in
general they could be arbitrarily expensive, which may result in very slow EXPLAINs.

mysql-test/r/subselect4.result

@@ -437,6 +437,25 @@ SELECT * FROM t4 LEFT JOIN t2 ON t4.c1 WHERE 's' IN (SELECT c1 FROM t2);
 c1	c1
 drop table t1, t2, t3, t4;
 #
+# LP BUG#675981 Assertion `cache != __null' failed in sub_select_cache()
+#               on EXPLAIN
+#
+CREATE TABLE t1 (f1 int,f2 int) ;
+INSERT IGNORE INTO t1 VALUES ('2','5'),('2',NULL);
+CREATE TABLE t2 (f1 int, f5 int) ;
+INSERT IGNORE INTO t2 VALUES (1,0);
+CREATE TABLE t3 (f4 int) ;
+INSERT IGNORE INTO t3 VALUES (0),(0);
+set @@optimizer_switch='in_to_exists=on,materialization=off,semijoin=off';
+EXPLAIN
+SELECT * FROM t2
+WHERE f1 IN (SELECT t1.f2 FROM t1 JOIN t3 ON t3.f4);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	system	NULL	NULL	NULL	NULL	1	
+2	DEPENDENT SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
+2	DEPENDENT SUBQUERY	t3	ALL	NULL	NULL	NULL	NULL	2	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

@@ -397,6 +397,27 @@ SELECT * FROM t4 LEFT JOIN t2 ON t4.c1 WHERE 's' IN (SELECT c1 FROM t2);
 SELECT * FROM t4 LEFT JOIN t2 ON t4.c1 WHERE 's' IN (SELECT c1 FROM t2);
 drop table t1, t2, t3, t4;
 
+--echo #
+--echo # LP BUG#675981 Assertion `cache != __null' failed in sub_select_cache()
+--echo #               on EXPLAIN
+--echo #
+
+CREATE TABLE t1 (f1 int,f2 int) ;
+INSERT IGNORE INTO t1 VALUES ('2','5'),('2',NULL);
+
+CREATE TABLE t2 (f1 int, f5 int) ;
+INSERT IGNORE INTO t2 VALUES (1,0);
+
+CREATE TABLE t3 (f4 int) ;
+INSERT IGNORE INTO t3 VALUES (0),(0);
+
+set @@optimizer_switch='in_to_exists=on,materialization=off,semijoin=off';
+EXPLAIN
+SELECT * FROM t2
+WHERE f1 IN (SELECT t1.f2 FROM t1 JOIN t3 ON t3.f4);
+
+drop table t1, t2, t3;
+
 --echo #
 --echo # BUG#52317: Assertion failing in Field_varstring::store() 
 --echo # 	    at field.cc:6833

sql/sql_select.cc

@@ -1821,7 +1821,14 @@ JOIN::exec()
   if (tables)
     thd->limit_found_rows= 0;
 
-  if (exec_const_cond && !exec_const_cond->val_int())
+  /*
+    Evaluate expensive constant conditions that were not evaluated during
+    optimization. Do not evaluate them for EXPLAIN statements as these
+    condtions may be arbitrarily costly, and because the optimize phase
+    might not have produced a complete executable plan for EXPLAINs.
+  */
+  if (exec_const_cond && !(select_options & SELECT_DESCRIBE) &&
+      !exec_const_cond->val_int())
     zero_result_cause= "Impossible WHERE noticed after reading const tables";
 
   if (zero_result_cause)