"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@xxxxxxxxxxxxx> 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
|
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