Re: 512,600ms query becomes 7500ms... but why? Postgres 8.3 query planner quirk?

"Exists" can be quite slow.  So can "not exists"

See if you can re-write it using a sub-select - just replace the "exists ...." with "(select ...) is not null"

Surprisingly this often results in a MUCH better query plan under Postgresql.  Why the planner evaluates it "better" eludes me (it shouldn't) but the differences are often STRIKING - I've seen factor-of-10 differences in execution performance.


Kevin Grittner wrote:

Bryce Nesbitt <bryce2@obviously.com> wrote: 

I've got a very slow query, which I can make faster by doing
something seemingly trivial.    

 
Out of curiosity, what kind of performance do you get with?:
EXPLAIN ANALYZE
SELECT contexts.context_key FROM contexts JOIN articles ON (articles.context_key = contexts.context_key) JOIN matview_82034 ON (matview_82034.context_key =                        contexts.context_key) WHERE EXISTS       (         SELECT *           FROM article_words           JOIN words using (word_key)           WHERE context_key = contexts.context_key             AND word = 'insider'       )   AND EXISTS       (         SELECT *           FROM article_words           JOIN words using (word_key)           WHERE context_key = contexts.context_key             AND word = 'trading'       )   AND EXISTS       (         SELECT *           FROM virtual_ancestors a           JOIN bp_categories ON (bp_categories.context_key =                                  a.ancestor_key)           WHERE a.context_key = contexts.context_key             AND lower(bp_categories.category) = 'law'       )   AND articles.indexed
;
(You may have to add some table aliases in the subqueries.)
If you are able to make a copy on 8.4 and test the various forms,
that would also be interesting.  I suspect that the above might do
pretty well in 8.4.
-Kevin