2016-11-09 10:40 GMT+01:00 Francisco Olarte <fola...@peoplecall.com>:
> Pierre: > > On Wed, Nov 9, 2016 at 10:22 AM, Pierre Ducroquet > <pierre.ducroq...@people-doc.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@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-general >
