Estimate cost of elided SubqueryScan, Append, MergeAppend nodes better.

setrefs.c contains logic to discard no-op SubqueryScan nodes, that is,
ones that have no qual to check and copy the input targetlist unchanged.
(Formally it's not very nice to be applying such optimizations so late
in the planner, but there are practical reasons for it; mostly that we
can't unify relids between the subquery and the parent query until we
flatten the rangetable during setrefs.c.)  This behavior falsifies our
previous cost estimates, since we would've charged cpu_tuple_cost per
row just to pass data through the node.  Most of the time that's little
enough to not matter, but there are cases where this effect visibly
changes the plan compared to what you would've gotten with no
sub-select.

To improve the situation, make the callers of cost_subqueryscan tell
it whether they think the targetlist is trivial.  cost_subqueryscan
already has the qual list, so it can check the other half of the
condition easily.  It could make its own determination of tlist
triviality too, but doing so would be repetitive (for callers that
may call it several times) or unnecessarily expensive (for callers
that can determine this more cheaply than a general test would do).

This isn't a 100% solution, because createplan.c also does things
that can falsify any earlier estimate of whether the tlist is
trivial.  However, it fixes nearly all cases in practice, if results
for the regression tests are anything to go by.

setrefs.c also contains logic to discard no-op Append and MergeAppend
nodes.  We did have knowledge of that behavior at costing time, but
somebody failed to update it when a check on parallel-awareness was
added to the setrefs.c logic.  Fix that while we're here.

These changes result in two minor changes in query plans shown in
our regression tests.  Neither is relevant to the purposes of its
test case AFAICT.

Patch by me; thanks to Richard Guo for review.

Discussion: https://postgr.es/m/2581077.1651703520@sss.pgh.pa.us

1 files changed, 44 insertions, 12 deletions

diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index 483c4f41373..dd64b460865 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -1326,19 +1326,28 @@ create_append_path(PlannerInfo *root,
 	Assert(!parallel_aware || pathnode->path.parallel_safe);
 
 	/*
-	 * If there's exactly one child path, the Append is a no-op and will be
-	 * discarded later (in setrefs.c); therefore, we can inherit the child's
-	 * size and cost, as well as its pathkeys if any (overriding whatever the
-	 * caller might've said).  Otherwise, we must do the normal costsize
+	 * If there's exactly one child path then the output of the Append is
+	 * necessarily ordered the same as the child's, so we can inherit the
+	 * child's pathkeys if any, overriding whatever the caller might've said.
+	 * Furthermore, if the child's parallel awareness matches the Append's,
+	 * then the Append is a no-op and will be discarded later (in setrefs.c).
+	 * Then we can inherit the child's size and cost too, effectively charging
+	 * zero for the Append.  Otherwise, we must do the normal costsize
 	 * calculation.
 	 */
 	if (list_length(pathnode->subpaths) == 1)
 	{
 		Path	   *child = (Path *) linitial(pathnode->subpaths);
 
-		pathnode->path.rows = child->rows;
-		pathnode->path.startup_cost = child->startup_cost;
-		pathnode->path.total_cost = child->total_cost;
+		if (child->parallel_aware == parallel_aware)
+		{
+			pathnode->path.rows = child->rows;
+			pathnode->path.startup_cost = child->startup_cost;
+			pathnode->path.total_cost = child->total_cost;
+		}
+		else
+			cost_append(pathnode, root);
+		/* Must do this last, else cost_append complains */
 		pathnode->path.pathkeys = child->pathkeys;
 	}
 	else
@@ -1476,10 +1485,13 @@ create_merge_append_path(PlannerInfo *root,
 
 	/*
 	 * Now we can compute total costs of the MergeAppend.  If there's exactly
-	 * one child path, the MergeAppend is a no-op and will be discarded later
-	 * (in setrefs.c); otherwise we do the normal cost calculation.
+	 * one child path and its parallel awareness matches that of the
+	 * MergeAppend, then the MergeAppend is a no-op and will be discarded
+	 * later (in setrefs.c); otherwise we do the normal cost calculation.
 	 */
-	if (list_length(subpaths) == 1)
+	if (list_length(subpaths) == 1 &&
+		((Path *) linitial(subpaths))->parallel_aware ==
+		pathnode->path.parallel_aware)
 	{
 		pathnode->path.startup_cost = input_startup_cost;
 		pathnode->path.total_cost = input_total_cost;
@@ -1986,9 +1998,15 @@ create_gather_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
  * create_subqueryscan_path
  *	  Creates a path corresponding to a scan of a subquery,
  *	  returning the pathnode.
+ *
+ * Caller must pass trivial_pathtarget = true if it believes rel->reltarget to
+ * be trivial, ie just a fetch of all the subquery output columns in order.
+ * While we could determine that here, the caller can usually do it more
+ * efficiently (or at least amortize it over multiple calls).
  */
 SubqueryScanPath *
 create_subqueryscan_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
+						 bool trivial_pathtarget,
 						 List *pathkeys, Relids required_outer)
 {
 	SubqueryScanPath *pathnode = makeNode(SubqueryScanPath);
@@ -2005,7 +2023,8 @@ create_subqueryscan_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
 	pathnode->path.pathkeys = pathkeys;
 	pathnode->subpath = subpath;
 
-	cost_subqueryscan(pathnode, root, rel, pathnode->path.param_info);
+	cost_subqueryscan(pathnode, root, rel, pathnode->path.param_info,
+					  trivial_pathtarget);
 
 	return pathnode;
 }
@@ -3901,10 +3920,23 @@ reparameterize_path(PlannerInfo *root, Path *path,
 		case T_SubqueryScan:
 			{
 				SubqueryScanPath *spath = (SubqueryScanPath *) path;
+				Path	   *subpath = spath->subpath;
+				bool		trivial_pathtarget;
+
+				/*
+				 * If existing node has zero extra cost, we must have decided
+				 * its target is trivial.  (The converse is not true, because
+				 * it might have a trivial target but quals to enforce; but in
+				 * that case the new node will too, so it doesn't matter
+				 * whether we get the right answer here.)
+				 */
+				trivial_pathtarget =
+					(subpath->total_cost == spath->path.total_cost);
 
 				return (Path *) create_subqueryscan_path(root,
 														 rel,
-														 spath->subpath,
+														 subpath,
+														 trivial_pathtarget,
 														 spath->path.pathkeys,
 														 required_outer);
 			}