Re: Should we update the random_page_cost default value?

Hi,

On 2025-10-08 21:25:53 +0200, Tomas Vondra wrote:
> On 10/8/25 19:23, Robert Haas wrote:
> >> I think in the past we mostly assumed we can't track cache size per
> >> table, because we have no visibility into page cache. But maybe direct
> >> I/O would change this?
> >
> > I think it's probably going to work out really poorly to try to use
> > cache contents for planning. The plan may easily last much longer than
> > the cache contents.
> >
>
> Why wouldn't that trigger invalidations / replanning just like other
> types of stats? I imagine we'd regularly collect stats about what's
> cached, etc. and we'd invalidate stale plans just like after ANALYZE.

You can't just handle it like other such stats - the contents of
shared_buffers can differ between primary and standby and other stats that
trigger replanning are all in system tables that can't differ between primary
and hot standby instances.

We IIRC don't currently use shared memory stats for planning and thus have no
way to trigger invalidation for relevant changes. While it seems plausible to
drive this via shared memory stats, the current cumulative counters aren't
really suitable, we'd either need something that removes the influence of
olders hits/misses or a new field tracking the current number of buffers for a
relation [fork].

Greetings,

Andres Freund