Re: How to find if a SELECT is reading from buffer or disk ?

2010/5/25 Chirag Dave <cdave(at)ca(dot)afilias(dot)info>:
>
>
> On Tue, May 25, 2010 at 1:48 PM, Balkrishna Sharma <b_ki(at)hotmail(dot)com> wrote:
>>
>> Hi,
>> I am increasing the shared_buffer size in postgresql.conf and want to
>> measure its effect on READ. In essence I want to know if the SELECT queries
>> I am firing repeatedly is reading from the buffer or going directly to the
>> disk.
>> I am expecting the first SELECT to go to disk and the subsequent call of
>> the same SELECT to read from buffer .
>> Right now I am just looking at execution time of the SELECTs and trying to
>> conclude. But there should be a direct way to see where the SELECT reads
>> from.
>
> You can also use pg_stat_database view. you can compute cache reads
> percentage of the total number of reads (cache and physical) between the two
> snapshots using  pg_stat_database.blks_hit  and pg_stat_database.blks_read.

views does not reflect this exact behavior : hit and read are relative
to hit shared buffers and request a block (from OS page cache or from
disk).

>
> Chirag Dave 416-673-4102
> Database Administrator, Afilias Canada Corp.
> cdave(at)ca(dot)afilias(dot)info
>
>
>>
>> How can I accomplish this ?
>> Thanks
>> Bala
>> ________________________________
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>

--
Cédric Villemain 2ndQuadrant
http://2ndQuadrant.fr/ PostgreSQL : Expertise, Formation et Support