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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