"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
