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Postpone generate_gather_paths for topmost scan/join rel.

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Don't call generate_gather_paths for the topmost scan/join relation
when it is initially populated with paths.  Instead, do the work in
grouping_planner.  By itself, this gains nothing; in fact it loses
slightly because we end up calling set_cheapest() for the topmost
scan/join rel twice rather than once.  However, it paves the way for
a future commit which will postpone generate_gather_paths for the
topmost scan/join relation even further, allowing more accurate
costing of parallel paths.

Amit Kapila and Robert Haas.  Earlier versions of this patch (which
different substantially) were reviewed by Dilip Kumar, Amit
Khandekar, Marina Polyakova, and Ashutosh Bapat.
This commit is contained in:
Robert Haas
2018-03-12 16:45:15 -04:00
parent d7c19e62a8
commit 3f90ec8597
3 changed files with 42 additions and 14 deletions
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21
src/backend/optimizer/geqo/geqo_eval.c
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@@ -40,7 +40,7 @@ typedef struct
} Clump;
static List *merge_clump(PlannerInfo *root, List *clumps, Clump *new_clump,
bool force);
int num_gene, bool force);
static bool desirable_join(PlannerInfo *root,
RelOptInfo *outer_rel, RelOptInfo *inner_rel);
@@ -196,7 +196,7 @@ gimme_tree(PlannerInfo *root, Gene *tour, int num_gene)
cur_clump->size = 1;
/* Merge it into the clumps list, using only desirable joins */
clumps = merge_clump(root, clumps, cur_clump, false);
clumps = merge_clump(root, clumps, cur_clump, num_gene, false);
}
if (list_length(clumps) > 1)
@@ -210,7 +210,7 @@ gimme_tree(PlannerInfo *root, Gene *tour, int num_gene)
{
Clump *clump = (Clump *) lfirst(lc);
fclumps = merge_clump(root, fclumps, clump, true);
fclumps = merge_clump(root, fclumps, clump, num_gene, true);
}
clumps = fclumps;
}
@@ -235,7 +235,8 @@ gimme_tree(PlannerInfo *root, Gene *tour, int num_gene)
* "desirable" joins.
*/
static List *
merge_clump(PlannerInfo *root, List *clumps, Clump *new_clump, bool force)
merge_clump(PlannerInfo *root, List *clumps, Clump *new_clump, int num_gene,
bool force)
{
ListCell *prev;
ListCell *lc;
@@ -267,8 +268,14 @@ merge_clump(PlannerInfo *root, List *clumps, Clump *new_clump, bool force)
/* Create paths for partitionwise joins. */
generate_partitionwise_join_paths(root, joinrel);
/* Create GatherPaths for any useful partial paths for rel */
generate_gather_paths(root, joinrel, false);
/*
* Except for the topmost scan/join rel, consider gathering
* partial paths. We'll do the same for the topmost scan/join
* rel once we know the final targetlist (see
* grouping_planner).
*/
if (old_clump->size + new_clump->size < num_gene)
generate_gather_paths(root, joinrel, false);
/* Find and save the cheapest paths for this joinrel */
set_cheapest(joinrel);
@@ -286,7 +293,7 @@ merge_clump(PlannerInfo *root, List *clumps, Clump *new_clump, bool force)
* others. When no further merge is possible, we'll reinsert
* it into the list.
*/
return merge_clump(root, clumps, old_clump, force);
return merge_clump(root, clumps, old_clump, num_gene, force);
}
}
prev = lc;