Re: bool close_thread_table(THD*, TABLE**): Assertion `table->key_read == 0' failed in maria-5.3-mwl128
Test case:

CREATE TABLE t1 (f1 int,f3 int,f4 int) ;
INSERT IGNORE INTO t1 VALUES (NULL,1,0);

CREATE TABLE t2 (f2 int,f4 int,f5 int) ;
INSERT IGNORE INTO t2 VALUES (8,0,0),(5,0,0);

CREATE TABLE t3 (f4 int,KEY (f4)) ;
INSERT IGNORE INTO t3 VALUES (0),(0);

EXPLAIN
SELECT f3 FROM t1 WHERE
(
SELECT SUBQUERY2_t1.f1
FROM t1 AS SUBQUERY2_t1
RIGHT JOIN t1 AS SUBQUERY2_t2 JOIN t2 AS SUBQUERY2_t3 ON /* SUBQUERY2_t3.f5 = SUBQUERY2_t2.f4 AND */ SUBQUERY2_t3.f4 <= ALL
(
SELECT CHILD_SUBQUERY1_t1.f4 AS CHILD_SUBQUERY1_field1
FROM t3 AS CHILD_SUBQUERY1_t1 JOIN t3 AS CHILD_SUBQUERY1_t3 ON CHILD_SUBQUERY1_t3.f4
GROUP BY child_subquery1_field1 )
ON SUBQUERY2_t3.f2 )
;

Re: bool close_thread_table(THD*, TABLE**): Assertion `table->key_read == 0' failed in EXPLAIN in maria-5.3-mwl89
Reduced test case:

set @@optimizer_switch='semijoin=off';

EXPLAIN
SELECT * FROM t1 WHERE
(SELECT f1 FROM t2
WHERE f4 <= ALL
(SELECT SQ1_t1.f4
FROM t3 AS SQ1_t1 JOIN t3 AS SQ1_t3 ON SQ1_t3.f4
GROUP BY SQ1_t1.f4));

Re: bool close_thread_table(THD*, TABLE**): Assertion `table->key_read == 0' failed in EXPLAIN in maria-5.3-mwl89
Analysis:
The cause for the failed ASSERT is in JOIN::join_free, where we set
bool full= (!select_lex->uncacheable && !thd->lex->describe);
Thus for EXPLAIN statements full == FALSE, and as a result the call to
JOIN::cleanup doesn't call JOIN_TAB::cleanup which should have
called table->disable_keyread().

Re: bool close_thread_table(THD*, TABLE**): Assertion `table->key_read == 0' failed in EXPLAIN in maria-5.3-mwl89
The reason that subqueries are evaluated during EXPLAIN in this case, is
because single-row subqueries are not currently considered expensive.

A proper fix of this bug should consist of two parts:
a) make all subqueries expensive, because the assumption that single-row
ones are not expensive is generally wrong.
b) make it possible to execute subqueries and do proper cleanup after them,
because in the future we may properly detect that some subqueries are
really cheap, and decide to execute them during EXPLAIN.

Launchpad bug id: 680038