Message-ID: <4CD73686.4090807@2ndquadrant.com>
Date: Sun, 07 Nov 2010 18:30:14 -0500
From: Greg Smith <greg@2ndquadrant.com>
User-Agent: Thunderbird 2.0.0.24 (X11/20100317)
MIME-Version: 1.0
To: Mark Rostron <mrostron@ql2.com>
CC: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: questions regarding shared_buffers behavior
References: <FD020D3E50E7FA479567872E5F5F31E3045A218A28@ex01.corp.ql2.com>
In-Reply-To: <FD020D3E50E7FA479567872E5F5F31E3045A218A28@ex01.corp.ql2.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

Mark Rostron wrote:
>
> What is the procedure that postgres uses to decide whether or not a 
> table/index block will be left in the shared_buffers cache at the end 
> of the operation?
>

There is no such procedure.  When a table or index page is used, its 
usage count goes up, which means it's more likely to stay in the cache 
for longer afterwards.  Processing trying to allocate pages are 
constantly circling the buffer cache looking for pages where the usage 
count is at 0 to re-use.  The only special cases are for sequential 
scans and VACUUM, which use continuously re-use a small section of the 
buffer cache in some cases instead.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services and Support        www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books