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From:Revathi Rangachari Date:February 25 2012 4:01pm
Subject:re: memcache vs.query_cache
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Hi Monty

Thanks for the explanation.
Regards
Revathi R

--- On Fri, 2/24/12, Michael Widenius <monty@stripped> wrote:

> From: Michael Widenius <monty@stripped>
> Subject: re: memcache vs.query_cache
> To: "Revathi Rangachari" <masrrev@stripped>
> Cc: "Sergei Golubchik" <serg@stripped>, internals@stripped
> Date: Friday, February 24, 2012, 5:06 PM
> 
> Hi!
> 
> >>>>> "Revathi" == Revathi Rangachari <masrrev@stripped>
> writes:
> 
> Revathi> Hi
> Revathi> (1) If memcache is available  - in an
> apache-php-mysql environment, does enabling or disabling of
> query_cache_type in mysql configuration, matter in improving
> performance? 
> 
> It depends on how memcache is used and how often you
> disregard things
> from it.
> 
> In memcache it's better to store things that doesn't matter
> if it's a
> bit old.
> 
> Query cache is better for things that changes 'relatively
> seldom' (not
> more than once a second) as it's 'never out of date'.
> 
> It's not useful to cache the same kind of data with both
> caches...
> 
> Revathi> (2) If memcache is enabled and also
> query_cache_type is enabled, will both cache mechanism work
> or does one take precedence over the other, in serving
> requests.
> 
> As you normally ask memcache first if a query is there, it
> will take
> precedence.
> 
> Revathi> (2) I just wanted to understand how these
> variables query_cache_type, tmp_table_size,
> max_heap_table_size, memory temporary tables influence each
> other?
> 
> tmp_table_size and max_heap_table_size have nothing to with
> query
> cache. These are just restriction for how much memory that
> MySQL/MariaDB will use for resolving a query.
> 
> Revathi> In a replicated environment where
> query_cache_type is enabled and the tmp_table_size (64MB)
> and max_heap_table_size ( 1GB) the number of memory
> temporary tables created over a period of half-an-hour is
> around 2000.
> 
> This is depending on your queries.
> 
> Revathi> Whereas if query_cache_type is disabled and the
> tmp_table_size(64 MB) and max_heap_table_size 91 GB) the
> number of temporary tables created over a period of
> half-an-hour is around 6000.
> 
> This happens because many of your queries are resolved by
> the query
> cache and thus there is no need to run the query (that
> apparently is
> using temporary tables to resolve the query).
> 
> Revathi> Can anyone please let me know what would be the
> explanation for the above? 
> Revathi> Should I bring down values of tmp_table_size and
> max_heap_table_size?
> 
> No.
> 
> Regards,
> Monty
> 
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