Thread
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BUG #4940: Performance regression between 8.2 and 8.3.7
Robert Treat <xzilla@users.sourceforge.net> — 2009-07-24T06:28:34Z
The following bug has been logged online: Bug reference: 4940 Logged by: Robert Treat Email address: xzilla@users.sourceforge.net PostgreSQL version: 8.3.7 Operating system: Linux 2.6.18-128.el5 #1 SMP Wed Dec 17 11:41:38 EST 2008 x86_64 x86_64 x86_64 GNU/Linux Description: Performance regression between 8.2 and 8.3.7 Details: Upgrading from 8.2 to 8.3.7, I get a worse plan... verified settings / hardware / analyze all good. if i push the ORDER BY to an outer select, I get 8.2's plan. new bad plan: # explain analyze SELECT forum_thread_id FROM forums_threads WHERE NOT is_locked AND active AND (to_tsvector('english', title) @@ to_tsquery('english', 'mario')) ORDER BY last_updated_tsz DESC OFFSET 0 LIMIT 20; QUERY PLAN ---------------------------------------------------------------------------- ---------------------------------------------------------------------------- ---------------------------- Limit (cost=0.00..3222.14 rows=20 width=12) (actual time=1634.956..15483.223 rows=20 loops=1) -> Index Scan Backward using forums_threads_last_updated_tsz_idx on forums_threads (cost=0.00..202511.62 rows=1257 width=12) (actual time=1634.951..15483.145 rows=20 loops=1) Filter: ((NOT is_locked) AND active AND (to_tsvector('english'::regconfig, title) @@ '''mario'''::tsquery)) Total runtime: 15483.367 ms (4 rows) better plan that matches 8.2: # explain analyze select * from (SELECT forum_thread_id, last_updated_tsz FROM forums_threads WHERE NOT is_locked AND active AND (to_tsvector('english', title) @@ to_tsquery('english', 'mario')) OFFSET 0 LIMIT 20) x order by last_updated_tsz DESC; QUERY PLAN ---------------------------------------------------------------------------- ---------------------------------------------------------------------------- --------- Sort (cost=61.47..61.52 rows=20 width=12) (actual time=0.284..0.300 rows=20 loops=1) Sort Key: forums_threads.last_updated_tsz Sort Method: quicksort Memory: 25kB -> Limit (cost=0.00..60.84 rows=20 width=12) (actual time=0.083..0.230 rows=20 loops=1) -> Index Scan using forums_threads_title_gin_idx on forums_threads (cost=0.00..3823.73 rows=1257 width=12) (actual time=0.080..0.192 rows=20 loops=1) Index Cond: (to_tsvector('english'::regconfig, title) @@ '''mario'''::tsquery) Filter: ((NOT is_locked) AND active) Total runtime: 0.391 ms (8 rows) Not sure there is anything I can do settings wise to fix this, seems like performance regression; any thoughts?