Consider materializing the cheapest inner path in parallel nestloop

When generating non-parallel nestloop paths for each available outer
path, we always consider materializing the cheapest inner path if
feasible.  Similarly, in this patch, we also consider materializing
the cheapest inner path when building partial nestloop paths.  This
approach potentially reduces the need to rescan the inner side of a
partial nestloop path for each outer tuple.

Author: Tender Wang
Reviewed-by: Richard Guo, Robert Haas, David Rowley, Alena Rybakina
Reviewed-by: Tomasz Rybak, Paul Jungwirth, Yuki Fujii
Discussion: https://postgr.es/m/CAHewXNkPmtEXNfVQMou_7NqQmFABca9f4etjBtdbbm0ZKDmWvw@mail.gmail.com

1 files changed, 25 insertions, 0 deletions

diff --git a/src/backend/optimizer/path/joinpath.c b/src/backend/optimizer/path/joinpath.c
index 40eb58341c1..02dd9724924 100644
--- a/src/backend/optimizer/path/joinpath.c
+++ b/src/backend/optimizer/path/joinpath.c
@@ -2021,11 +2021,31 @@ consider_parallel_nestloop(PlannerInfo *root,
 						   JoinPathExtraData *extra)
 {
 	JoinType	save_jointype = jointype;
+	Path	   *inner_cheapest_total = innerrel->cheapest_total_path;
+	Path	   *matpath = NULL;
 	ListCell   *lc1;
 
 	if (jointype == JOIN_UNIQUE_INNER)
 		jointype = JOIN_INNER;
 
+	/*
+	 * Consider materializing the cheapest inner path, unless: 1) we're doing
+	 * JOIN_UNIQUE_INNER, because in this case we have to unique-ify the
+	 * cheapest inner path, 2) enable_material is off, 3) the cheapest inner
+	 * path is not parallel-safe, 4) the cheapest inner path is parameterized
+	 * by the outer rel, or 5) the cheapest inner path materializes its output
+	 * anyway.
+	 */
+	if (save_jointype != JOIN_UNIQUE_INNER &&
+		enable_material && inner_cheapest_total->parallel_safe &&
+		!PATH_PARAM_BY_REL(inner_cheapest_total, outerrel) &&
+		!ExecMaterializesOutput(inner_cheapest_total->pathtype))
+	{
+		matpath = (Path *)
+			create_material_path(innerrel, inner_cheapest_total);
+		Assert(matpath->parallel_safe);
+	}
+
 	foreach(lc1, outerrel->partial_pathlist)
 	{
 		Path	   *outerpath = (Path *) lfirst(lc1);
@@ -2082,6 +2102,11 @@ consider_parallel_nestloop(PlannerInfo *root,
 				try_partial_nestloop_path(root, joinrel, outerpath, mpath,
 										  pathkeys, jointype, extra);
 		}
+
+		/* Also consider materialized form of the cheapest inner path */
+		if (matpath != NULL)
+			try_partial_nestloop_path(root, joinrel, outerpath, matpath,
+									  pathkeys, jointype, extra);
 	}
 }