I think what is strange to me is that InnoDB is locking on the subquery
table at all. Here's another example:
DELETE
FROM Vers
WHERE (
Vers.elementID IN (
SELECT Elems.ID
FROM Elems
WHERE (Elems.nodeID = ?)))
Disregarding whether performance would be better or worse with a JOIN,
what I find odd is that this DELETE statement on Vers seems to be
putting locks on Elems. Might this be a bug in InnoDB? Innotop has this
to say:
____________________ Locks Held and Waited For ____________________
Txn What Mode DB Tbl Index Heap Special Ins Intent
1 waits_for X te elems PRIMARY 2 rec but not gap 0
Not that Txn 1 is an UPDATE on a single row of the ELEMS table and it is
waiting for the LOCK from the above DELETE FROM Vers to be released. I'm
not sure why the DELETE statement is locking the subquery table ELEMS
which is simply being queried. Do I *really* need to change all of these
to write the subquery to a temporary table in order to gain better
concurrency?
R.
-----Original Message-----
From: Baron Schwartz [mailto:[EMAIL PROTECTED]
Sent: Tuesday, October 03, 2006 1:39 PM
To: Rick James
Cc: Robert DiFalco; mysql@lists.mysql.com; [EMAIL PROTECTED]
Subject: Re: Innodb Locks
There is a detailed write-up on how locking works in the manual:
http://dev.mysql.com/doc/refman/5.0/en/innodb-transaction-model.html
If you are not doing replication, you might check out
innodb_locks_unsafe_for_binlog as mentioned in
http://dev.mysql.com/doc/refman/5.0/en/innodb-parameters.html. Peter Z
also wrote an article on this:
http://www.mysqlperformanceblog.com/category/replication/
You may get better performance from using a JOIN instead of an IN()
subquery. You will have to test. Sometimes it is much better,
sometimes worse. Usually better in my experience. Making the
long-running query as short as possible is probably a good idea. Maybe
you can break it up into several queries so it doesn't try to lock so
many rows at once. There could be many other approaches too, it just
depends on your needs and data.
Without altering how locks are handled with startup options, the
temporary table approach will avoid the locks only if you COMMIT after
the CREATE... SELECT. The other subquery approach will not avoid them.
I'm not sure if I should be replying to both the 'internals' and 'lists'
mailing lists, since this was cross-posted. Feel free to give me
guidance :-)
Baron
Rick James wrote:
> Can't answer your question directly. But I wonder if this would trick
> it into avoiding the lock:
>
> UPDATE AnotherTable
> SET...
> WHERE id IN (SELECT id FROM SomeTable);
>
> And the real workaround would be
>
> CREATE TEMPORARY TABLE t
> SELECT id ...;
> UPDATE AnotherTable
> SET...
> WHERE id IN (SELECT id FROM t);
>
>> -----Original Message-----
>> From: Robert DiFalco [mailto:[EMAIL PROTECTED]
>> Sent: Tuesday, October 03, 2006 9:26 AM
>> To: mysql@lists.mysql.com; [EMAIL PROTECTED]
>> Subject: RE: Innodb Locks
>>
>> Any thoughts on this? Should SomeTable be locked when performing the
>> UPDATE on AnotherTable?
>>
>> -------
>>
>> Is there a detailed source for when innodb creates row or table
locks?
>>
>> I have a situation where one thread is performing this in one
>> transaction:
>>
>> UPDATE SomeTable SET .... WHERE SomeTable.id = N;
>>
>>
>> This is invoked after another thread has kicked off this long running
>> query in another transaction:
>>
>> UPDATE AnotherTable
>> SET ...
>> WHERE EXISTS(
>> SELECT null
>> FROM SomeTable
>> WHERE SomeTable.id = AnotherTable.id );
>>
>>
>> Would this create a conflicting lock? I am getting "Lock wait timeout
>> exceeded" on SomeTable fro the UPDATE to SomeTable.
>>
>> TIA,
>>
>> R.
>>
>>
>> --
>> MySQL General Mailing List
>> For list archives: http://lists.mysql.com/mysql To unsubscribe:
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>>
>>
>>
>>
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>>
>>
>>
>
>
--
Baron Schwartz
http://www.xaprb.com/
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