Fix bug lp:827416

Analysis:
Constant table optimization of the outer query finds that
the right side of the equality is a constant that can
be used for an eq_ref access to fetch one row from t1,
and substitute t1 with a constant. Thus constant optimization
triggers evaluation of the subquery during the optimize
phase of the outer query.

The innermost subquery requires a plan with a temporary
table because with InnoDB tables the exact count of rows
is not known, and the empty tables cannot be optimzied
way. JOIN::exec for the innermost subquery substitutes
the subquery tables with a temporary table.

When EXPLAIN gets to print the tables in the innermost
subquery, EXPLAIN needs to print the name of each table
through the corresponding TABLE_LIST object. However,
the temporary table created during execution doesn't
have a corresponding TABLE_LIST, so we get a null
pointer exception.

Solution:
The solution is to forbid using expensive constant
expressions for eq_ref access for contant table
optimization. Notice that eq_ref with a subquery
providing the value is still possible during regular
execution.

1 files changed, 23 insertions, 1 deletions

diff --git a/mysql-test/t/subselect_innodb.test b/mysql-test/t/subselect_innodb.test
index da7524c1427..e108aecfd36 100644
--- a/mysql-test/t/subselect_innodb.test
+++ b/mysql-test/t/subselect_innodb.test
@@ -241,5 +241,27 @@ call p1();
 drop procedure p1;
 drop tables t1,t2,t3;
 
-set optimizer_switch=@subselect_innodb_tmp;
+--echo #
+--echo # LP BUG#827416: Crash in select_describe() on EXPLAIN with DISTINCT in nested subqueries
+--echo #
+
+CREATE TABLE t3 ( b int) ENGINE=InnoDB;
+CREATE TABLE t2 ( c int) ENGINE=InnoDB;
+CREATE TABLE t1 ( a int NOT NULL , PRIMARY KEY (a)) ENGINE=InnoDB;
+
+EXPLAIN SELECT *
+FROM t1
+WHERE t1.a = (
+        SELECT SUM( c )
+        FROM t2
+        WHERE (SELECT DISTINCT b FROM t3) > 0);
+SELECT *
+FROM t1
+WHERE t1.a = (
+        SELECT SUM( c )
+        FROM t2
+        WHERE (SELECT DISTINCT b FROM t3) > 0);
 
+DROP TABLE t1, t2, t3;
+
+set optimizer_switch=@subselect_innodb_tmp;