Greetings, Consider this:
create table t10 (c1 int, c2 int); create index on t10 (c1) where c2 > 5; \d t10 Table "sfrost.t10" Column | Type | Modifiers --------+---------+----------- c1 | integer | c2 | integer | Indexes: "t10_c1_idx" btree (c1) WHERE c2 > 5 insert into t10 select * from generate_series(1,10000) a, generate_series(1,10) b; (repeat a bunch of times, if desired) vacuum analyze t10; set work_mem = '64kB'; set enable_indexscan = false; set enable_seqscan = false; =*> explain analyze select * from t10 where c2 > 6; QUERY PLAN ---------------------------------------------------------------------------------------------------------------------------------- Bitmap Heap Scan on t10 (cost=6496.49..15037.50 rows=318653 width=8) (actual time=34.682..116.236 rows=320000 loops=1) Recheck Cond: (c2 > 5) Rows Removed by Index Recheck: 327502 Filter: (c2 > 6) Rows Removed by Filter: 80000 Heap Blocks: exact=642 lossy=2898 -> Bitmap Index Scan on t10_c1_idx (cost=0.00..6416.83 rows=400081 width=0) (actual time=34.601..34.601 rows=400000 loops=1) Planning time: 0.087 ms Execution time: 124.229 ms (9 rows) Perhaps I'm missing something obvious, but isn't it a bit redundant to have both a Recheck condition (which is the predicate of the index) and a Filter condition (which is the user's predicate) when we've already decided that the user's predicate must result in a subset of the index's, as, otherwise, we wouldn't be able to use the index in the first place? In other words, it seems like we shouldn't need a Filter in the above Bitmap Heap Scan, instead we should just make the Recheck be (c2 > 6). I've not looked into the code side of this at all and there may be reasons why this is hard to do, but it seems like a worthwhile improvement to consider doing, though perhaps I'm missing some reason why we need both the Recheck and the Filter in such cases for correctness. Thanks! Stephen
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