On Thu, Apr 5, 2012 at 2:47 PM, Julien Cigar <jcigar@xxxxxxxxx> wrote: > - http://www.pastie.org/3731956 : with default config > - http://www.pastie.org/3731960 : this is with enable_seq_scan = off It looks like the join selectivity of (context_to_context_links, ancestors) is being overestimated by almost two orders of magnitude. The optimizer thinks that there are 564 rows in the context_to_context_links table for each taxon_id, while in fact for this query the number is 9. To confirm that this, you can force the selectivity estimate to be 200x lower by adding a geo_id = geod_id where clause to the subquery. If it does help, then the next question would be why is the estimate so much off. It could be either because the stats for context_to_context_links.taxon_id are wrong or because ancestors.taxon_id(subphylum_id = 18830) is a special case. To help figuring this is out, you could run the following to queries and post the results: SELECT floor(log(num,2)) AS nmatch, COUNT(*) AS freq FROM (SELECT COUNT(*) AS num FROM context_to_context_links GROUP BY taxon_id) AS dist GROUP BY 1 ORDER BY 1; SELECT floor(log(num,2)) AS nmatch, COUNT(*) AS freq FROM (SELECT COUNT(*) AS num FROM context_to_context_links WHERE NOT geo_id IS NULL and taxon_id= ANY ( select taxon_id from rab.ancestors where ancestors.subphylum_id = 18830) GROUP BY taxon_id) AS dist GROUP BY 1 ORDER BY 1; If the second distribution has a significantly different shape then cross column statistics are necessary to get good plans. As it happens I'm working on adding this functionality to PostgreSQL and would love to hear more details about your use-case to understand if it would be solved by this work. Regards, Ants Aasma -- Cybertec Schönig & Schönig GmbH Gröhrmühlgasse 26 A-2700 Wiener Neustadt Web: http://www.postgresql-support.de -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
