Re: shared buffers

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?

On Mon, Mar 29, 2021 at 1:09 PM Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
wrote:

> On Mon, 2021-03-29 at 11:27 -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.
>
> Yours,
> Laurenz Albe
> --
> Cybertec | https://www.cybertec-postgresql.com
>
>