maria-developers team mailing list archive

[Sergey Petrunya <psergey@xxxxxxxxxxxx>]

Hi Kristian,

On Tue, Mar 09, 2010 at 04:11:26PM +0100, Kristian Nielsen wrote:
> Thanks for offering to help with the memory leak. Here are the details.
> 
> Tree is here:
> 
>     lp:~maria-captains/maria/mariadb-5.1-knielsen
> 
> This tree is the merge with MySQL-5.1.44, including your fix for the
> uninitialised variable in table elimination.
> 
> The test case is the following, which is a simplified version of main.union
> (so main.union shows the same memory leak):
> 
> -------------------------------- cut here --------------------------------
> CREATE TABLE t1 (a VARCHAR(10), FULLTEXT KEY a (a));
> INSERT INTO t1 VALUES (1),(2);
> CREATE TABLE t2 (b INT);
> INSERT INTO t2 VALUES (1),(2);
> 
> EXPLAIN EXTENDED
> SELECT * FROM t1 UNION SELECT * FROM t1
>   ORDER BY (SELECT a FROM t2 WHERE b = 12);
> 
> DROP TABLE t1,t2;
> -------------------------------- cut here --------------------------------

Please find the fix below. The fix has a relatively high chance of breaking
something (I've discovered that this kind of subqueries is processed in a way
that would surprise pretty much everyone I think), so it's better to wait for
a full buildbot run before we include it in the main tree.
 
----- Forwarded message from Sergey Petrunya <psergey@xxxxxxxxxxxx> -----

From: Sergey Petrunya <psergey@xxxxxxxxxxxx>
To: maria-developers@xxxxxxxxxxxxxxxxxxx
X-Mailer: mail (GNU Mailutils 1.2)
Date: Tue,  9 Mar 2010 21:53:58 +0300 (MSK)
Subject: [Maria-developers] Rev 2798: Fix a buildbot memory leak due to
	JOIN::destroy() not being called for EXPLAIN in
	file:///home/psergey/dev/mariadb-5.1-knielsen/

At file:///home/psergey/dev/mariadb-5.1-knielsen/

------------------------------------------------------------
revno: 2798
revision-id: psergey@xxxxxxxxxxxx-20100309185356-ns7o30p75xjjby35
parent: knielsen@xxxxxxxxxxxxxxx-20100309150559-07zcnnz8dh54m2ug
committer: Sergey Petrunya <psergey@xxxxxxxxxxxx>
branch nick: mariadb-5.1-knielsen
timestamp: Tue 2010-03-09 21:53:56 +0300
message:
  Fix a buildbot memory leak due to JOIN::destroy() not being called for EXPLAIN 
  query:
  - When subquery is located in ORDER BY, EXPLAIN will run as follows:
    select_describe() will run JOIN::prepare()/optimize() for the subquery;
    then at some point subselect_single_select_engine::prepare() will be called,
    which will create another join and run join->prepare().
  
    In mainline mysql this is not a problem because subquery's join will be 
    destroyed after the first call. 
    In MariaDB, it won't (table elimination needs to keep JOIN objects around
    for longer in order to know which tables were eliminated when constructing
    EXPLAIN EXTENDED warning).
    
    Fix the problem of memory leak by calling select_lex->cleanup() in 
    subselect_single_select_engine::prepare().
=== modified file 'sql/item_subselect.cc'
--- a/sql/item_subselect.cc	2010-03-09 15:03:54 +0000
+++ b/sql/item_subselect.cc	2010-03-09 18:53:56 +0000
@@ -1776,6 +1776,10 @@
 {
   if (prepared)
     return 0;
+  if (select_lex->join)
+  {
+    select_lex->cleanup();
+  }
   join= new JOIN(thd, select_lex->item_list,
 		 select_lex->options | SELECT_NO_UNLOCK, result);
   if (!join || !result)


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----- End forwarded message -----

BR
 Sergey
-- 
Sergey Petrunia, Software Developer
Monty Program AB, http://askmonty.org
Blog: http://s.petrunia.net/blog