Re: [GENERAL] Re: [GENERAL] FTS query, statistics and planner estimations…

2016-11-09 10:40 GMT+01:00 Francisco Olarte <folarte(at)peoplecall(dot)com>:

> Pierre:
>
> On Wed, Nov 9, 2016 at 10:22 AM, Pierre Ducroquet
> <pierre(dot)ducroquet(at)people-doc(dot)com> wrote:
> > The query does a few joins «after» running a FTS query on a main table.
> > The FTS query returns a few thousand rows, but the estimations are wrong,
> > leading the optimizer to terrible plans compared to what should happen,
> and
> > thus creates a far higher execution time.
> ....
> > but the issue remain the same. The table contains about 295,000
> documents, and
> ....
> > Request | Estimated rows | Real rows
> > ----------------------------------+----------------+-----------
> > 'word1' | 38050 | 37500
> > 'word1 word2' | 4680 | 32000
> > 'word1 word2 word3' | 270 | 12300
> > 'word1 word2 word3 word4' | 10 | 9930
> > 'word1 word2 word3 word4 word5' | 1 | 9930
> >
> > You can see that with more words in query, the estimation falls far
> behind
> > reality.
>
> I'm not really familiar with FTS but, doing a few division of
> estimations and rows it seems it estimates as uncorrelated words, and
> you real rows clearly indicate some of them are clearly correlated (
> like w1/w2 and w4/s5, and partially w3/w45 ) and very common.
>
> > Is that a known limitation of the FTS indexing ? Am I missing something
> > obvious, or a poor configuration ?
>
> Someone more familiar with it needed for that, but what I've found
> several times is FTS does not mix too well with relational queries at
> the optimizer level ( as FTS terms can have very diverse degrees of
> correlation, which is very difficult to store in the statistics a
> relational optimizer normally uses ).
>

there is workaround - the FTS query can be wrapped to immutable function -
then can be executed in planner time, and the estimations can be better

http://postgres.cz/wiki/PostgreSQL_SQL_Tricks_II#Using_IMMUTABLE_functions_as_hints_for_the_optimizer

Regards

Pavel

> Francisco Olarte.
>
>
> --
> Sent via pgsql-general mailing list (pgsql-general(at)postgresql(dot)org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-general
>