| From: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com> |
|---|---|
| To: | Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Peter Geoghegan <pg(at)bowt(dot)ie>, Shijia Wei <shijiawei(at)utexas(dot)edu>, Pgsql Performance <pgsql-performance(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: Consecutive Query Executions with Increasing Execution Time |
| Date: | 2019-12-17 16:11:12 |
| Message-ID: | CAMkU=1w2i=Ph7840GYtf2eWta858Y3Eyb0zRKJd84cyEEctBZg@mail.gmail.com |
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| Lists: | pgsql-performance |
On Tue, Dec 17, 2019 at 8:08 AM Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
wrote:
> On Mon, 2019-12-16 at 15:50 -0500, Tom Lane wrote:
> > Peter Geoghegan <pg(at)bowt(dot)ie> writes:
> > > Why do the first and the twentieth executions of the query have almost
> > > identical "buffers shared/read" numbers? That seems odd.
> >
> > It's repeat execution of the same query, so that doesn't seem odd to me.
>
> Really? Shouldn't the blocks be in shared buffers after a couple
> of executions?
>
If it is doing a seq scan (I don't know if it is) they intentionally use a
small ring buffer to, so they evict their own recently used blocks, rather
than evicting other people's blocks. So these blocks won't build up in
shared_buffers very rapidly just on the basis of repeated seq scans.
Cheers,
Jeff
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