Message-ID: <7b12ce640429a5b286b39342998b47fed74fafec.camel@cybertec.at>
Subject: Re: shared buffers
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Greg Rychlewski <greg.rychlewski@gmail.com>
Cc: pgsql-novice@postgresql.org
Date: Tue, 30 Mar 2021 13:14:55 +0200
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On Mon, 2021-03-29 at 14:04 -0400, Greg Rychlewski wrote:
> > > Will every page touched during a table or index scan, even if it's
> > >  not going to be used in the final result, be loaded into shared buffers? 
> > > 
> > > i.e. if you need to evaluate a filter condition, will it load that page
> > >  into shared buffers and then evaluate it from there?
> > 
> > Even if a value does not appear in a query result, the page containing it
> > has to be read, if the value is used for calculating the query result.
> > 
> > All pages read are loaded into shared buffers.  So yes, they will be loaded.
> > 
> > Note that there is an optimization for big sequential scans:  if the table
> > scanned is bigger than a quarter of shared buffers, PostgreSQL will use a
> > small ring buffer to read the table.  This prevents a large sequential scan
> > from blowing out your cache, since it uses the same buffers to scan
> > the whole table.
> 
> Oh that's really interesting about the ring buffer. So if you're doing an
> update/delete/insert that requires the ring buffer, does that mean the
> backend itself will write to disk instead of the
> checkpoint process?

That is only used for reading.

All reading and writing is *always* done through shared buffers.

Yours,
Laurenz Albe
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