Hi, I think that lossy-heap-block information for a bitmap heap scan, not just "Rows Removed by Index Recheck" information, would also be a clue used to tune work_mem for better performance especially when the bitmap heap scan uses an index such as gin or gist, not btree.
So here's a patch that adds the information to the EXPLAIN ANALYZE output. The
following shows an example. The number of lossy-heap-block fetches (ie
tbmres->ntuples = -1) as well as that of exact-heap-block fetches (ie
tbmres->ntuples >= 0) are shown in the "Heap Blocks" line.
postgres=# EXPLAIN ANALYZE SELECT * FROM demo WHERE col2 between 0.01 and 0.02;
QUERY PLAN
--------------------------------------------------------------------------------
------------------------------------------------
Bitmap Heap Scan on demo (cost=2716.54..92075.46 rows=105766 width=34) (actual
time=24.907..1119.961 rows=100047 loops=1)
Recheck Cond: ((col2 >= 0.01::double precision) AND (col2 <= 0.02::double
precision))
Rows Removed by Index Recheck: 5484114
Heap Blocks: exact=11975 lossy=46388
-> Bitmap Index Scan on demo_idx (cost=0.00..2690.09 rows=105766 width=0)
(actual time=22.821..22.821 rows=100047 loops=1)
Index Cond: ((col2 >= 0.01::double precision) AND (col2 <= 0.02::double
precision))
Total runtime: 1129.334 ms
(7 rows)
Comments welcome.
Thanks,
Best regards,
Etsuro Fujita
explain-bitmapscan-20131031.patch
Description: Binary data
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