Re: Query timing stable?

On 03/14/2012 11:24 AM, Cheng wrote:
> Hi,
>
> I turned on "\timing" in the command line. It seems that the timing is
> not stable. For the same query, the query's executing time is longer
> for the first time I run it than for the second and third time run it.
> I think it might be "cached" automatically.
>
> Is there a way to turn the "automatic caching" off so that I can get
> the same "timing" when I run the same query? Or is there some other
> way to get stable query time whenever the query is run?
>
> Thanks,
> Cheng
>

Depends on your OS. Things almost always run faster on subsequent access
- usually due to OS caching of disk reads but data can be cached in
PostgreSQL and your storage subsystems as well.

Ensuring that *everything* is flushed almost requires restarting your
system. Depending on your OS, you may be able to flush the disk cache
and restart PostgreSQL and get "close enough".

Depending on your query, you may be altering the underlying tables so
getting exact repeatability is unlikely.

Cheers,
Steve