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From:Jocelyn Fournier Date:February 18 2009 11:03pm
Subject:Re: comments and query cache
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Actually there's a bug opened to avoid a query with SQL_NO_CACHE keyword 
looking into the QC :

   Jocelyn Fournier

Le 18/02/2009 18:22, Rick James a écrit :
> It is possible that someone, today, is changing a comment in order to
> bypass the QC.  This would be an incompatible change for them.  However,
> the benefit might outweigh the drawback.
> It would be nice if your change could cut back on the frequency or time
> taken to lock the QC; this is becoming a non-trivial performance issue
> in these days of 8-core SMP machines.
> Rick James
> MySQL Geeks - Consulting&  Review
>> -----Original Message-----
>> From: Michael Widenius [mailto:monty@stripped]
>> Sent: Wednesday, February 18, 2009 5:39 AM
>> To: Eric
>> Cc: internals@stripped
>> Subject: re: comments and query cache
>> Hi!
>>>>>>> "Eric" == Eric<esquid2@stripped>  writes:
>> Eric>  Hi All-
>> Eric>  AFAICT, query comments are used as part of the hash key
>> for query cache (at
>> Eric>  least in 5.0).  Is this intentional?  Still the case in
>> newer versions of
>> Eric>  mysql?
>> Yes;  It's mainly for speed but also because comments can affect the
>> way queries are executed (if you use optimizer hints)
>> <cut>
>> Eric>  I have a patch (attached) to strip comments before
>> checking and storing the
>> Eric>  query into the query cache but wanted to see if this
>> was a reasonable way to
>> Eric>  do it.  Also, I couldn't find a very reasonable
>> function to strip comments.
>> Eric>  (The one in innobase/hash/hash0hash.c seemed the best,
>> but I imagine it is
>> Eric>  not fit to include so I copied it just to get my
>> proof-of-concept working
>> Eric>  until/if something better is decided.)
>> Don't know of any other function to do that.
>> Eric>  If this seems useful and reasonable, is there a better
>> way to get the
>> Eric>  comments stripped?  Should I implement my own?  (Should
>> be able to be done
>> Eric>  in the existing char* without allocating any new memory, right?)
>> If you *only* strip safe comments (not using the /*!...*/ syntax you
>> can strip it in the original memory.
>> Eric>  Does
>> Eric>  InnoDB (what we use) or any other engine depend on the
>> comments in any way
>> Eric>  (I know oracle puts query hints in there)?  How should
>> this behavior be
>> Eric>  enabled since I imagine not everyone will want the
>> runtime v. cache hit
>> Eric>  tradeoff.  ./configure option?  my.cnf?  a dynamic variable?
>> You should have this either as a dynamic variable or a connection
>> option.  This is to not cause an extra parsing overhead in the case
>> where comments are static for the query.
>> Regards,
>> Monty
>> --
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comments and query cacheEric17 Feb
  • Re: comments and query cacheAntony T Curtis18 Feb
  • re: comments and query cacheMichael Widenius18 Feb
    • RE: comments and query cacheRick James18 Feb
      • Re: comments and query cacheJocelyn Fournier19 Feb