Re: Oracle v. Postgres 9.0 query performance

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "Tony Capobianco" <tcapobianco(at)prospectiv(dot)com>
Cc: <pgsql-performance(at)postgresql(dot)org>,"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: Oracle v. Postgres 9.0 query performance
Date: 2011-06-08 19:30:50
Message-ID: 4DEF879A020000250003E36C@gw.wicourts.gov
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Tony Capobianco <tcapobianco(at)prospectiv(dot)com> wrote:

> According to some documentation, I want to set
> effective_cache_size to my OS disk cache + shared_buffers.

That seems reasonable, and is what has worked well for me.

> In this case, I have 4 quad-core processors with 512K cache (8G)
> and my shared_buffers is 7680M. Therefore my effective_cache_size
> should be approximately 16G?

I didn't follow that at all. Can you run `free` or `vmstat`? If
so, go by what those say your cache size is.

> Most of our other etl processes are running fine, however I'm
> curious if I could see a significant performance boost by reducing
> the effective_cache_size.

Since it is an optimizer costing parameter and has no affect on
memory allocation, you can set it on a connection and run a query on
that connection to test the impact. Why wonder about it when you
can easily test it?

-Kevin

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