Обсуждение: Query planner doesn't use index scan on tsvector GIN index if LIMIT is specified

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:

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. 

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