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Commit2609e91

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Fix regression in parallel planning against inheritance tables.
Commit51ee6f3 accidentally changedthe behavior around inheritance hierarchies; before, we alwaysconsidered parallel paths even for very small inheritance children,because otherwise an inheritance hierarchy with even one small childwouldn't be eligible for parallelism. That exception was inadverentlyremoved; put it back.In passing, also adjust the degree-of-parallelism comptuation forindex-only scans not to consider the number of heap pages fetched.Otherwise, we'll avoid parallel index-only scans on tables that aremostly all-visible, which isn't especially logical.Robert Haas and Amit Kapila, per a report from Ashutosh Sharma.Discussion:http://postgr.es/m/CAE9k0PmgSoOHRd60SHu09aRVTHRSs8s6pmyhJKWHxWw9C_x+XA@mail.gmail.com
1 parentc11453c commit2609e91

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3 files changed

+36
-25
lines changed

3 files changed

+36
-25
lines changed

‎src/backend/optimizer/path/allpaths.c

Lines changed: 25 additions & 21 deletions
Original file line numberDiff line numberDiff line change
@@ -692,7 +692,7 @@ create_plain_partial_paths(PlannerInfo *root, RelOptInfo *rel)
692692
{
693693
intparallel_workers;
694694

695-
parallel_workers=compute_parallel_worker(rel,rel->pages,0);
695+
parallel_workers=compute_parallel_worker(rel,rel->pages,-1);
696696

697697
/* If any limit was set to zero, the user doesn't want a parallel scan. */
698698
if (parallel_workers <=0)
@@ -2938,7 +2938,7 @@ create_partial_bitmap_paths(PlannerInfo *root, RelOptInfo *rel,
29382938
pages_fetched=compute_bitmap_pages(root,rel,bitmapqual,1.0,
29392939
NULL,NULL);
29402940

2941-
parallel_workers=compute_parallel_worker(rel,pages_fetched,0);
2941+
parallel_workers=compute_parallel_worker(rel,pages_fetched,-1);
29422942

29432943
if (parallel_workers <=0)
29442944
return;
@@ -2953,16 +2953,16 @@ create_partial_bitmap_paths(PlannerInfo *root, RelOptInfo *rel,
29532953
* be scanned and the size of the index to be scanned, then choose a minimum
29542954
* of those.
29552955
*
2956-
* "heap_pages" is the number of pages from the table that we expect to scan.
2957-
* "index_pages" is the number of pages from the index that we expect to scan.
2956+
* "heap_pages" is the number of pages from the table that we expect to scan, or
2957+
* -1 if we don't expect to scan any.
2958+
*
2959+
* "index_pages" is the number of pages from the index that we expect to scan, or
2960+
* -1 if we don't expect to scan any.
29582961
*/
29592962
int
2960-
compute_parallel_worker(RelOptInfo*rel,BlockNumberheap_pages,
2961-
BlockNumberindex_pages)
2963+
compute_parallel_worker(RelOptInfo*rel,doubleheap_pages,doubleindex_pages)
29622964
{
29632965
intparallel_workers=0;
2964-
intheap_parallel_workers=1;
2965-
intindex_parallel_workers=1;
29662966

29672967
/*
29682968
* If the user has set the parallel_workers reloption, use that; otherwise
@@ -2972,23 +2972,24 @@ compute_parallel_worker(RelOptInfo *rel, BlockNumber heap_pages,
29722972
parallel_workers=rel->rel_parallel_workers;
29732973
else
29742974
{
2975-
intheap_parallel_threshold;
2976-
intindex_parallel_threshold;
2977-
29782975
/*
2979-
* If this relation is too small to be worth a parallel scan, just
2980-
* return without doing anything ... unless it's an inheritance child.
2981-
* In that case, we want to generate a parallel path here anyway. It
2982-
* might not be worthwhile just for this relation, but when combined
2983-
* with all of its inheritance siblings it may well pay off.
2976+
* If the number of pages being scanned is insufficient to justify a
2977+
* parallel scan, just return zero ... unless it's an inheritance
2978+
* child. In that case, we want to generate a parallel path here
2979+
* anyway. It might not be worthwhile just for this relation, but
2980+
* when combined with all of its inheritance siblings it may well pay
2981+
* off.
29842982
*/
2985-
if (heap_pages< (BlockNumber)min_parallel_table_scan_size&&
2986-
index_pages< (BlockNumber)min_parallel_index_scan_size&&
2987-
rel->reloptkind==RELOPT_BASEREL)
2983+
if (rel->reloptkind==RELOPT_BASEREL&&
2984+
((heap_pages >=0&&heap_pages<min_parallel_table_scan_size)||
2985+
(index_pages >=0&&index_pages<min_parallel_index_scan_size)))
29882986
return0;
29892987

2990-
if (heap_pages>0)
2988+
if (heap_pages >=0)
29912989
{
2990+
intheap_parallel_threshold;
2991+
intheap_parallel_workers=1;
2992+
29922993
/*
29932994
* Select the number of workers based on the log of the size of
29942995
* the relation. This probably needs to be a good deal more
@@ -3008,8 +3009,11 @@ compute_parallel_worker(RelOptInfo *rel, BlockNumber heap_pages,
30083009
parallel_workers=heap_parallel_workers;
30093010
}
30103011

3011-
if (index_pages>0)
3012+
if (index_pages >=0)
30123013
{
3014+
intindex_parallel_workers=1;
3015+
intindex_parallel_threshold;
3016+
30133017
/* same calculation as for heap_pages above */
30143018
index_parallel_threshold=Max(min_parallel_index_scan_size,1);
30153019
while (index_pages >= (BlockNumber) (index_parallel_threshold*3))

‎src/backend/optimizer/path/costsize.c

Lines changed: 9 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -662,15 +662,22 @@ cost_index(IndexPath *path, PlannerInfo *root, double loop_count,
662662

663663
if (partial_path)
664664
{
665+
/*
666+
* For index only scans compute workers based on number of index pages
667+
* fetched; the number of heap pages we fetch might be so small as
668+
* to effectively rule out parallelism, which we don't want to do.
669+
*/
670+
if (indexonly)
671+
rand_heap_pages=-1;
672+
665673
/*
666674
* Estimate the number of parallel workers required to scan index. Use
667675
* the number of heap pages computed considering heap fetches won't be
668676
* sequential as for parallel scans the pages are accessed in random
669677
* order.
670678
*/
671679
path->path.parallel_workers=compute_parallel_worker(baserel,
672-
(BlockNumber)rand_heap_pages,
673-
(BlockNumber)index_pages);
680+
rand_heap_pages,index_pages);
674681

675682
/*
676683
* Fall out if workers can't be assigned for parallel scan, because in

‎src/include/optimizer/paths.h

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -54,8 +54,8 @@ extern RelOptInfo *standard_join_search(PlannerInfo *root, int levels_needed,
5454
List*initial_rels);
5555

5656
externvoidgenerate_gather_paths(PlannerInfo*root,RelOptInfo*rel);
57-
externintcompute_parallel_worker(RelOptInfo*rel,BlockNumberheap_pages,
58-
BlockNumberindex_pages);
57+
externintcompute_parallel_worker(RelOptInfo*rel,doubleheap_pages,
58+
doubleindex_pages);
5959
externvoidcreate_partial_bitmap_paths(PlannerInfo*root,RelOptInfo*rel,
6060
Path*bitmapqual);
6161

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