Question

I have a words table with an index on (language_id, state). Here are the results for EXPLAIN ANALYZE:

No limit

explain analyze SELECT "words".* FROM "words" WHERE (words.language_id = 27) AND (state IS NULL);

Bitmap Heap Scan on words  (cost=10800.38..134324.10 rows=441257 width=96) (actual time=233.257..416.026 rows=540556 loops=1)
Recheck Cond: ((language_id = 27) AND (state IS NULL))
->  Bitmap Index Scan on ls  (cost=0.00..10690.07 rows=441257 width=0) (actual time=230.849..230.849 rows=540556 loops=1)
Index Cond: ((language_id = 27) AND (state IS NULL))
Total runtime: 460.277 ms
(5 rows)

Limit 100

explain analyze SELECT "words".* FROM "words" WHERE (words.language_id = 27) AND (state IS NULL) LIMIT 100;

Limit  (cost=0.00..51.66 rows=100 width=96) (actual time=0.081..0.184 rows=100 loops=1)
->  Seq Scan on words  (cost=0.00..227935.59 rows=441257 width=96) (actual time=0.080..0.160 rows=100 loops=1)
Filter: ((state IS NULL) AND (language_id = 27))
Total runtime: 0.240 ms
(4 rows)

Why is this happening? How can I get the index to be used in all cases?

Thanks.

Answer 1

I think that the PostreSQL query planner just thinks that in the second case – the one with the LIMIT – it’s not worth applying the index as it [the LIMIT] is too small. So it’s not an issue.