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Add a Gather Merge executor node.

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Like Gather, we spawn multiple workers and run the same plan in each
one; however, Gather Merge is used when each worker produces the same
output ordering and we want to preserve that output ordering while
merging together the streams of tuples from various workers.  (In a
way, Gather Merge is like a hybrid of Gather and MergeAppend.)

This works out to a win if it saves us from having to perform an
expensive Sort.  In cases where only a small amount of data would need
to be sorted, it may actually be faster to use a regular Gather node
and then sort the results afterward, because Gather Merge sometimes
needs to wait synchronously for tuples whereas a pure Gather generally
doesn't.  But if this avoids an expensive sort then it's a win.

Rushabh Lathia, reviewed and tested by Amit Kapila, Thomas Munro,
and Neha Sharma, and reviewed and revised by me.

Discussion: http://postgr.es/m/CAGPqQf09oPX-cQRpBKS0Gq49Z+m6KBxgxd_p9gX8CKk_d75HoQ@mail.gmail.com
This commit is contained in:
Robert Haas
2017-03-09 07:40:36 -05:00
parent a72f0365db
commit 355d3993c5
27 changed files with 1355 additions and 16 deletions
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36
src/backend/optimizer/path/allpaths.c
View File

@ -2084,39 +2084,51 @@ set_worktable_pathlist(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
/*
* generate_gather_paths
* Generate parallel access paths for a relation by pushing a Gather on
* top of a partial path.
* Generate parallel access paths for a relation by pushing a Gather or
* Gather Merge on top of a partial path.
*
* This must not be called until after we're done creating all partial paths
* for the specified relation. (Otherwise, add_partial_path might delete a
* path that some GatherPath has a reference to.)
* path that some GatherPath or GatherMergePath has a reference to.)
*/
void
generate_gather_paths(PlannerInfo *root, RelOptInfo *rel)
{
Path *cheapest_partial_path;
Path *simple_gather_path;
ListCell *lc;
/* If there are no partial paths, there's nothing to do here. */
if (rel->partial_pathlist == NIL)
return;
/*
* The output of Gather is currently always unsorted, so there's only one
* partial path of interest: the cheapest one. That will be the one at
* the front of partial_pathlist because of the way add_partial_path
* works.
*
* Eventually, we should have a Gather Merge operation that can merge
* multiple tuple streams together while preserving their ordering. We
* could usefully generate such a path from each partial path that has
* non-NIL pathkeys.
* The output of Gather is always unsorted, so there's only one partial
* path of interest: the cheapest one. That will be the one at the front
* of partial_pathlist because of the way add_partial_path works.
*/
cheapest_partial_path = linitial(rel->partial_pathlist);
simple_gather_path = (Path *)
create_gather_path(root, rel, cheapest_partial_path, rel->reltarget,
NULL, NULL);
add_path(rel, simple_gather_path);
/*
* For each useful ordering, we can consider an order-preserving Gather
* Merge.
*/
foreach (lc, rel->partial_pathlist)
{
Path *subpath = (Path *) lfirst(lc);
GatherMergePath *path;
if (subpath->pathkeys == NIL)
continue;
path = create_gather_merge_path(root, rel, subpath, rel->reltarget,
subpath->pathkeys, NULL, NULL);
add_path(rel, &path->path);
}
}
/*