Some info:
PostgreSQL version: 9.1.2
Table "cache":
Rows count: 3 471 081
Column "tsv" tsvector
Index "cache_tsv" USING gin (tsv)
If i do query like THIS:
SELECT id FROM table WHERE tsv @@ to_tsquery('test:*');
It uses index and returns results immediately:
explain analyze
'Bitmap Heap Scan on cache (cost=1441.78..63802.63 rows=19843 width=4) (actual time=29.309..31.518 rows=1358 loops=1)'
' Recheck Cond: (tsv @@ to_tsquery('test:*'::text))'
' -> Bitmap Index Scan on cache_tsv (cost=0.00..1436.82 rows=19843 width=0) (actual time=
28.966..28.966 rows=1559 loops=1)'
' Index Cond: (tsv @@ to_tsquery('test:*'::text))'
'Total runtime: 31.789 ms'
But the performance problems starts when i do the same query specifying LIMIT.
SELECT id FROM cache WHERE tsv @@ to_tsquery('test:*') limit 20;
By some reason index is not used.
explain analyze
'Limit (cost=0.00..356.23 rows=20 width=4) (actual time=7.984..765.550 rows=20 loops=1)'
' -> Seq Scan on cache (cost=0.00..353429.50 rows=19843 width=4) (actual time=7.982..765.536 rows=20 loops=1)'
' Filter: (tsv @@ to_tsquery('test:*'::text))'
'Total runtime: 765.620 ms'
Some more debug notes:
1) If i set SET enable_seqscan=off; then query uses indexes correctly
2) Also i notified, if i use: to_tsquery('test') without wildcard search :*, then index is used correctly in both queries, with or without LIMIT
Any ideas how to fix the problem?
Thank you