Bug #26867 LOCK TABLES + REPAIR + merge table result in memory/cpu hogging
Submitted: 6 Mar 2007 10:24 Modified: 3 Dec 2007 9:34
Reporter: Dmitry Lenev Email Updates:
Status: Closed Impact on me:
Category:MySQL Server: Merge storage engine Severity:S3 (Non-critical)
Version:5.0.38-bk, 5.1 OS:Linux (Linux)
Assigned to: Ingo Strüwing CPU Architecture:Any
Tags: bfsm_2007_03_15, bfsm_2007_04_19, DoS, lock tables, locking, memory leak, merge

[6 Mar 2007 10:24] Dmitry Lenev
This problem was originally reported as part of bug #25966 but since it has different roots than main problem mentioned there it is reported here as separate bug.

Attempt to execute statement which deals with merge table (and therefore tries to lock it) for which one of underlying tables has been locked with LOCK TABLES and later has been REPAIRed in the same connection leads to memory and cpu hogging.

See How-to-repeat section for details.

How to repeat:
#connection 1
drop table if exists t1;
drop table if exists m1;
create table t1(id int) engine=myisam;
create table m1(id int) engine=merge union=(t1) insert_method=last;

#connection 2
lock table t1 write;

#connection 1
insert into m1 values (1);

#connection 2
repair table t1;

#now watch memory using task manager or top command.
also, check processlist of mysqld, the state of the insert query will be
changing from Opening tables to NULL to Locked....
[18 Apr 2007 8:05] Calvin Sun
It is likely a duplicate of 26379, according to Ingo.
[16 Nov 2007 8:08] Ingo Strüwing
For the patch see Bug#26379 - Combination of FLUSH TABLE and REPAIR TABLE corrupts a MERGE table.
[28 Nov 2007 10:35] Ingo Strüwing
For documentation see Bug#26379 - Combination of FLUSH TABLE and REPAIR TABLE corrupts a MERGE table.
[3 Dec 2007 9:34] MC Brown
A note has been added to the 5.1.23 and 6.0.4 changelogs: 

Important Change: Incompatible Change: A number of problems existed in the implementation of MERGE tables that could cause problems. The problems are summarized below:

Bug#26379 - Combination of FLUSH TABLE and REPAIR TABLE corrupts a MERGE table. This was caused in a number of situations:

A thread trying to lock a MERGE table performs busy waiting while REPAIR TABLE or a similar table administration task is ongoing on one or more of its MyISAM tables.

A thread trying to lock a MERGE table performs busy waiting until all threads that did REPAIR TABLE or similar table administration tasks on one or more of its MyISAM tables in LOCK TABLES segments do UNLOCK TABLES. The difference against problem #1 is that the busy waiting takes place after the administration task. It is terminated by UNLOCK TABLES only.

Two FLUSH TABLES within a LOCK TABLES segment can invalidate the lock. This does not require a MERGE table. The first FLUSH TABLES can be replaced by any statement that requires other threads to reopen the table. In 5.0 and 5.1 a single FLUSH TABLES can provoke the problem.

Bug#26867 - Simultaneously executing LOCK TABLES and REPAIR TABLE on a MERGE table would result in memory/cpu hogging.

Trying DML on a MERGE table, which has a child locked and repaired by another thread, made an infinite loop in the server.

Bug#26377 - Deadlock with MERGE and FLUSH TABLE

Locking a MERGE table and its children in parent-child order and flushing the child deadlocked the server.

Bug#25038 - Waiting TRUNCATE

Truncating a MERGE child, while the MERGE table was in use, let the truncate fail instead of waiting for the table to become free.

Bug#25700 - MERGE base tables get corrupted by OPTIMIZE/ANALYZE/REPAIR TABLE

Repairing a child of an open MERGE table corrupted the child. It was necessary to FLUSH the child first.

Bug#30275 - MERGE tables: FLUSH TABLES or UNLOCK TABLES causes server to crash.

Flushing and optimizing locked MERGE children crashed the server.

Bug#19627 - temporary merge table locking

Use of a temporary MERGE table with non-temporary children could corrupt the children.

Temporary tables are never locked. Creation of tables with non-temporary chidlren of a temporary MERGE table is now prohibited.

Bug#27660 - Falcon: MERGE table possible

It was possible to create a MERGE table with non-MyISAM children.

Bug#30273 - MERGE tables: Can't lock file (errno: 155)

This was a Windows-only bug. Table administration statements sometimes failed with "Can't lock file (errno: 155)".

The fix introduces the following changes in behavior:

This patch changes the behavior of temporary MERGE tables. Temporary MERGE must have temporary children. The old behavior was wrong. A temporary table is not locked. Hence even non-temporary children were not locked. See Bug#19627.

You cannot change the union list of a non-temporary MERGE table when LOCK TABLES is in effect. The following does not work:

      ALTER TABLE m1 ... UNION=(t1,t2) ...;
However, you can do this with a temporary MERGE table.

You cannot create a MERGE table with CREATE ... SELECT, neither as a temporary MERGE table, nor as a non-temporary MERGE table. For example:

Gives error message: table is not BASE TABLE.