Re: Memcached for Database server

2011/5/17 Craig Ringer <craig(at)postnewspapers(dot)com(dot)au>:
> On 05/17/2011 01:38 PM, Satoshi Nagayasu wrote:
>>
>> Hi,
>>
>> 2011/05/17 14:31, Adarsh Sharma wrote:
>>>
>>> Rick Genter wrote:
>>>>
>>>> On May 16, 2011, at 10:09 PM, Adarsh Sharma wrote:
>>>>
>>>>
>>>>> Dear all,
>>>>>
>>>>> I need to research on Memcache in the next few days.
>>>>>
>>>>> What I want to know is it worth to have memcahed enable in our
>>>>> Mysql/ Postgres Production Servers.
>>>>> We have databases from 20 to 230 GB and it's not the OLTP just a
>>>>> simple OLAP where data is fetched and stored in some meaningful format.
>>>>>
>>>>>
>>>>> What are benefits& why we used memcahed?
>>>>>
>>>>> What are the bottlenecks to meet?
>>>>>
>>>>
>>>> You need to read about memcached. Memcached is not something you
>>>> "enable". You have to program to it.
>>>>
>>>
>>> Thanks Rick, just one question..
>>>
>>> At what stage we need memcached & what is the purpose of using it.
>>>
>>> I just want to know whether it is worth to use memcahced or not as per
>>> our requirements.
>>
>> I just built a software to enable query caching for PostgreSQL
>> with using memcached, which adds a proxy layer.
>>
>> http://pgsnaga.blogspot.com/2011/03/postgresql-query-cache-pqc.html
>
> Much like with memcached, it looks like you still have to handle your own
> cache invalidation with your cache daemon, and it can return outdated or
> inconsistent results. Your examples clearly show that. It'd be nice if the
> google code front page clearly pointed out that it's not a fully transparent
> cache in that it can return stale or inconsistent data and the app has to be
> aware of that.
>
> How do you handle statements that rely on current_timestamp, random(), etc?
> What about if their reliance is via a function? Is that just an understood
> limitation of the cache, that it'll cache even queries that don't really
> make sense to cache?

there is also pgmemcache
http://pgfoundry.org/projects/pgmemcache/

It is not a proxy but an extension to access memcache from within
postgresql. You can use it to build your own querycache.

>
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> Craig Ringer
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Cédric Villemain               2ndQuadrant
http://2ndQuadrant.fr/     PostgreSQL : Expertise, Formation et Support