Re: How reliable are the stats collector stats?

Tom Lane wrote:

> Eric Ridge <ebr@tcdi.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index is obsolete or indicates a planner/casting problem.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

--
#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#================================================== JanWieck@Yahoo.com #