database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-07-07 00:36:50 +03:00
Code Activity

Account for the effect of lossy pages when costing bitmap scans.

Browse Source 
Dilip Kumar, reviewed by Alexander Kumenkov, Amul Sul, and me.
Some final adjustments by me.

Discussion: http://postgr.es/m/CAFiTN-sYtqUOXQ4SpuhTv0Z9gD0si3YxZGv_PQAAMX8qbOotcg@mail.gmail.com
This commit is contained in:
Robert Haas
2017-11-10 16:50:50 -05:00
parent 0c98d0dd5c
commit 5edc63bda6
3 changed files with 75 additions and 22 deletions
Download Patch File Download Diff File Expand all files Collapse all files

59
src/backend/optimizer/path/costsize.c
View File

@ -5171,6 +5171,8 @@ compute_bitmap_pages(PlannerInfo *root, RelOptInfo *baserel, Path *bitmapqual,
double T;
double pages_fetched;
double tuples_fetched;
double heap_pages;
long maxentries;
/*
* Fetch total cost of obtaining the bitmap, as well as its total
@ -5185,6 +5187,24 @@ compute_bitmap_pages(PlannerInfo *root, RelOptInfo *baserel, Path *bitmapqual,
T = (baserel->pages > 1) ? (double) baserel->pages : 1.0;
/*
* For a single scan, the number of heap pages that need to be fetched is
* the same as the Mackert and Lohman formula for the case T <= b (ie, no
* re-reads needed).
*/
pages_fetched = (2.0 * T * tuples_fetched) / (2.0 * T + tuples_fetched);
/*
* Calculate the number of pages fetched from the heap. Then based on
* current work_mem estimate get the estimated maxentries in the bitmap.
* (Note that we always do this calculation based on the number of pages
* that would be fetched in a single iteration, even if loop_count > 1.
* That's correct, because only that number of entries will be stored in
* the bitmap at one time.)
*/
heap_pages = Min(pages_fetched, baserel->pages);
maxentries = tbm_calculate_entries(work_mem * 1024L);
if (loop_count > 1)
{
/*
@ -5199,22 +5219,41 @@ compute_bitmap_pages(PlannerInfo *root, RelOptInfo *baserel, Path *bitmapqual,
root);
pages_fetched /= loop_count;
}
else
{
/*
* For a single scan, the number of heap pages that need to be fetched
* is the same as the Mackert and Lohman formula for the case T <= b
* (ie, no re-reads needed).
*/
pages_fetched =
(2.0 * T * tuples_fetched) / (2.0 * T + tuples_fetched);
}
if (pages_fetched >= T)
pages_fetched = T;
else
pages_fetched = ceil(pages_fetched);
if (maxentries < heap_pages)
{
double exact_pages;
double lossy_pages;
/*
* Crude approximation of the number of lossy pages. Because of the
* way tbm_lossify() is coded, the number of lossy pages increases
* very sharply as soon as we run short of memory; this formula has
* that property and seems to perform adequately in testing, but it's
* possible we could do better somehow.
*/
lossy_pages = Max(0, heap_pages - maxentries / 2);
exact_pages = heap_pages - lossy_pages;
/*
* If there are lossy pages then recompute the number of tuples
* processed by the bitmap heap node. We assume here that the chance
* of a given tuple coming from an exact page is the same as the
* chance that a given page is exact. This might not be true, but
* it's not clear how we can do any better.
*/
if (lossy_pages > 0)
tuples_fetched =
clamp_row_est(indexSelectivity *
(exact_pages / heap_pages) * baserel->tuples +
(lossy_pages / heap_pages) * baserel->tuples);
}
if (cost)
*cost = indexTotalCost;
if (tuple)