Re: Show lossy heap block info in EXPLAIN ANALYZE for bitmap heap scan - Mailing list pgsql-hackers

Reconsidering that, I wish to know your opinion.  The patch shows the number of exact/lossy pages that has been fetched in a bitmap heap scan.  But the number varies with the fraction of tuples to be retrieved like the following.

postgres=# EXPLAIN ANALYZE SELECT * FROM demo WHERE col2 between 0.01 and 0.02;
                                                             QUERY PLAN

------------------------------------------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on demo  (cost=2187.35..101419.96 rows=102919 width=42) (actual time=23.684..1302.382 rows=99803 loops=1)

   Recheck Cond: ((col2 >= 0.01::double precision) AND (col2 <= 0.02::double precision))

   Rows Removed by Index Recheck: 6279502
   Heap Blocks: exact=1990 lossy=59593
   ->  Bitmap Index Scan on demo_col2_idx  (cost=0.00..2161.62 rows=102919 width=0) (actual time=23.330..23.330 rows=99803 loops=1)

         Index Cond: ((col2 >= 0.01::double precision) AND (col2 <= 0.02::double precision))

 Total runtime: 1311.949 ms
(7 rows)

postgres=# EXPLAIN ANALYZE SELECT * FROM demo WHERE col2 between 0.01 and 0.02 LIMIT 5000;
                                                                QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=2187.35..7008.26 rows=5000 width=42) (actual time=23.543..86.093 rows=5000 loops=1)
   ->  Bitmap Heap Scan on demo  (cost=2187.35..101419.96 rows=102919 width=42) (actual time=23.542..85.196 rows=5000 loops=1)

         Recheck Cond: ((col2 >= 0.01::double precision) AND (col2 <= 0.02::double precision))

         Rows Removed by Index Recheck: 312179
         Heap Blocks: exact=99 lossy=2963
         ->  Bitmap Index Scan on demo_col2_idx  (cost=0.00..2161.62 rows=102919 width=0) (actual time=23.189..23.189 rows=99803 loops=1)

               Index Cond: ((col2 >= 0.01::double precision) AND (col2 <= 0.02::double precision))

 Total runtime: 86.626 ms
(8 rows)

So, my question is, we should show the number of exact/lossy pages in a TIDBitmap, not the number of these pages that has been fetched in the bitmap heap scan?

Thanks,

Best regards,
Etsuro Fujita