On 25 November 2013 13:37, Etsuro Fujita <fujita.ets...@lab.ntt.co.jp>wrote:
> > 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? > Yes, I agree that rather than looking at the bitmap heap scan to track the number of pages, we should look somewhere in the underlying index scan. Yes, we should get a constant number of index pages regardless of the actual parent table rows. I can see that btgetbitmap() adds all the tuples into the bitmap, so somewhere below under btgetbitmap() might be the right place to track. Somewhere in tbm_create_pagetable(), but not sure. > Thanks, > > Best regards, > Etsuro Fujita > >
