Re: pg_buffercache: Add per-relation summary stats

On Sun, Mar 1, 2026 at 12:59 AM Lukas Fittl <lukas@fittl.com> wrote:
>
> Hi,
>
> See attached a patch that implements a new function,
> pg_buffercache_relation_stats(), which returns per-relfilenode
> statistics on the number of buffers, how many are dirtied/pinned, and
> their avg usage count.
>
> This can be used in monitoring scripts to know which relations are
> kept in shared buffers, to understand performance issues better that
> occur due to relations getting evicted from the cache. In our own
> monitoring tool (pganalyze) we've offered a functionality like this
> based on the existing pg_buffercache() function for a bit over a year
> now [0], and people have found this very valuable - but it doesn't
> work for larger database servers.
[..]
> (3 rows)
>
> Time: 20.991 ms
[..vs]
>
> Time: 2.912 ms

Hi Lukas, I have glanced at the patch briefly and couldn't find any
issues - patch looks solid, however I'm not sure if e.g. launching whole
NBuffers scan let's say every 5mins doesn't cause latency spikes on the
system? I mean introducing such function seems to invite users to use
pg_buffercache and I'm wondering if such regular pattern doesn't cause
issues? (this is not FUD :), just more like a question based on Your's
obervation)

Also have you quantified what was the breaking point of previous query?
(You wrote "larger database servers", but was that like 128GB+ shared_buffers?
and if so what would be the difference in terms of runtime there -- also
like ~7x?)

-J.