Re: How reliable are the stats collector stats?

Tom Lane wrote:

> Eric Ridge <ebr(at)tcdi(dot)com> writes:
>> Could pg_stats_user_indexes be lying?
>
> Jan probably knows this stuff better than I, but my guess is that if the
> counter type you are looking at is incrementing at all, then it's not
> too far off. I certainly can't think of a failure mechanism that would
> cause some indexes to be shown with zero hits when other indexes do
> get hits.

As described before in various threads, the messages from the backend to
the stats collector are unreliable INET UDP on purpose, so that a
clogged collector never slows down a backend.

If that happens, usually an entire bunch of not necessarily related
counter increments on a per transaction base would get lost.

>
>> I realize the real question is "why aren't these indexes being used",
>
> Up to a point. If it's a unique index then you may want the
> uniqueness-check functionality even if the index is never used for
> searches. (I think that pg_stats only counts search probes, not
> accesses made in connection with insertions, but I'm too tired to
> go double-check this.)

That is right. Only scans are counted for. A not scanned non-unique
index is obsolete or indicates a planner/casting problem.

Jan

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