Re: pg_stat_statements max size clarification

I will mull over this and see if I can suggest a short phrase that points
to the nuances without explaining them. I think the current phrasing draws
attention to itself by seeming dubious, which creates a kind of cognitive
dissonance for the reader. This report isn't mean to be a bug report on the
functionality.

On Sat, Jul 15, 2017 at 10:17 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Peter Geoghegan <pg(at)bowt(dot)ie> writes:
> > On Sat, Jul 15, 2017 at 5:31 PM, <baron(at)xaprb(dot)com> wrote:
> >> I wonder if &quot;least-executed&quot; is correct. I&#39;m not an
> expert and haven&#39;t
> >> convinced myself of this by examining the code, but I think after N
> distinct
> >> queryid&#39;s have been seen, then any additional ones are ignored. But
> that may
> >> not be &quot;least-executed&quot; at all. It&#39;s
> &quot;most-recent&quot; instead. I think we need
> >> a new phrase here.
>
> > It's most executed since tracking for the entry began, with a special
> > heuristic for queries that take a long time to execute, and might
> > therefore consistently be evicted before execution finishes and costs
> > are tallied (see "sticky entries" stuff for full details). Most
> > executed means the total number of calls, which may not be the best
> > thing to evict on the basis of, but certainly isn't too bad.
>
> > The way it actually works is that either 5% of all entries or 10
> > entries are evicted (whichever amount is greatest) once
> > pg_stat_statements.max entries are reached. You're right that this
> > means that the most marginal of entries cannot be usefully tracked,
> > but I doubt that that's much of a problem in practice. It's the usual
> > "recency versus frequency" cache eviction problem, but for query cost
> > tracking purposes if 5,000 entries or 10,000 entries is truly
> > insufficient, then pg_stat_statements probably isn't the right tool.
>
> The short answer, really, is that the algorithm is too complicated to be
> worth explaining in the documentation --- and it's subject to change,
> anyway. But "least-executed" is a reasonable short description, since
> frequency of use is a major factor in the decisions. Certainly
> "most-recent" is flat out wrong.
>
> I am not sure whether this complaint is actually meant as a bug report
> that the algorithm didn't seem to work well on the OP's use case. If so,
> we'd need a lot more details to have any hope of improving it (and the
> documentation comments aren't the right submission forum, either).
>
> regards, tom lane
>