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From:Dan Nelson Date:August 28 2008 4:04pm
Subject:Re: MyIsam Locking Questions
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In the last episode (Aug 28), Jose Estuardo Avila said:
> On Aug 28, 2008, at 5:48 AM, "Perrin Harkins" wrote:
> > On Wed, Aug 27, 2008 at 8:21 PM, Jose Estuardo Avila wrote:
> >> Hi, I've been trying to find information on how myisam handles
> >> locks. I though myisam had locking only on writes and not on
> >> reads.
> >
> > No, readers block writers.  This true of any system that only has
> > read and write locks to work with.  The only reason they don't
> > block in InnoDB tables is the MVCC system that lets readers use
> > snapshots while writers modify the data.  This is all in the MyISAM
> > section of the docs.
>
> I understand that reads are locked by writes but nowhere does of
> mention that reads also block reads.  Boy queries y posted to the
> list are selects.

Are you sure there wasn't an UPDATE, INSERT, or DELETE somewhere in
your process list?  Readers and writers block each other, and when
blocked, readers queue up behind writers.  So an UPDATE query sent just
after a long-running SELECT starts up will cause all other SELECTs to
queue up behind it.  You can force specific reads to queue up in front
of writes with the HIGH_PRIORITY attribute, and you can force specific
writes to queue up behind reads with the LOW_PRIORITY attribute.

http://dev.mysql.com/doc/refman/5.1/en/locking-issues.html
 
-- 
	Dan Nelson
	dnelson@stripped
Thread
MyIsam Locking QuestionsJose Estuardo Avila28 Aug
  • Re: MyIsam Locking QuestionsPerrin Harkins28 Aug
    • Re: MyIsam Locking QuestionsJose Estuardo Avila28 Aug
      • Re: MyIsam Locking QuestionsDan Nelson28 Aug
      • Re: MyIsam Locking QuestionsPerrin Harkins28 Aug
        • Re: MyIsam Locking QuestionsJose Estuardo Avila28 Aug
          • Re: MyIsam Locking QuestionsPerrin Harkins28 Aug