On Thursday 11 November 2010 19:58:49 Tom Lane wrote:
> I wrote:
> > I do think that something based around a settable-per-table caching
> > percentage might be a reasonable way to proceed.
>
> BTW ... on reflection it seems that this would *not* solve the use-case
> Kevin described at the start of this thread. What he's got AIUI is some
> large tables whose recent entries are well-cached, and a lot of queries
> that tend to hit that well-cached portion, plus a few queries that hit
> the whole table and so see largely-not-cached behavior. We can't
> represent that very well with a caching knob at the table level. Either
> a high or a low setting will be wrong for one set of queries or the
> other.
>
> It might work all right if he were to partition the table and then have
> a different caching value attached to the currently-latest partition,
> but that doesn't sound exactly maintenance-free either. Also, that only
> works with the current statically-planned approach to partitioned
> tables. I think where we're trying to go with partitioning is that
> the planner doesn't consider the individual partitions, but the executor
> just hits the right one at runtime --- so cost modifiers attached to
> individual partitions aren't going to work in that environment.
>
> The most practical solution for his case still seems to be to twiddle
> some GUC or other locally in the maintenance scripts that do the
> full-table-scan queries. Unfortunately we don't have an equivalent
> of per-session SET (much less SET LOCAL) for per-relation attributes.
> Not sure if we want to go there.
As dicussed in another thread some time ago another possibility is to probe
how well the data i cached using mincore() or similar...
While it presents problem with cache ramp-up it quite cool for other use-cases
(like this one).
Andre