Consider explicit incremental sort for Append and MergeAppend

For an ordered Append or MergeAppend, we need to inject an explicit
sort into any subpath that is not already well enough ordered.
Currently, only explicit full sorts are considered; incremental sorts
are not yet taken into account.

In this patch, for subpaths of an ordered Append or MergeAppend, we
choose to use explicit incremental sort if it is enabled and there are
presorted keys.

The rationale is based on the assumption that incremental sort is
always faster than full sort when there are presorted keys, a premise
that has been applied in various parts of the code.  In addition, the
current cost model tends to favor incremental sort as being cheaper
than full sort in the presence of presorted keys, making it reasonable
not to consider full sort in such cases.

No backpatch as this could result in plan changes.

Author: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Andrei Lepikhov <lepihov@gmail.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/CAMbWs4_V7a2enTR+T3pOY_YZ-FU8ZsFYym2swOz4jNMqmSgyuw@mail.gmail.com

1 files changed, 40 insertions, 14 deletions

diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 3d44815ed5a..1f04a2c182c 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -2247,7 +2247,7 @@ append_nonpartial_cost(List *subpaths, int numpaths, int parallel_workers)
  *	  Determines and returns the cost of an Append node.
  */
 void
-cost_append(AppendPath *apath)
+cost_append(AppendPath *apath, PlannerInfo *root)
 {
 	ListCell   *l;
 
@@ -2309,26 +2309,52 @@ cost_append(AppendPath *apath)
 			foreach(l, apath->subpaths)
 			{
 				Path	   *subpath = (Path *) lfirst(l);
-				Path		sort_path;	/* dummy for result of cost_sort */
+				int			presorted_keys;
+				Path		sort_path;	/* dummy for result of
+										 * cost_sort/cost_incremental_sort */
 
-				if (!pathkeys_contained_in(pathkeys, subpath->pathkeys))
+				if (!pathkeys_count_contained_in(pathkeys, subpath->pathkeys,
+												 &presorted_keys))
 				{
 					/*
 					 * We'll need to insert a Sort node, so include costs for
-					 * that.  We can use the parent's LIMIT if any, since we
+					 * that.  We choose to use incremental sort if it is
+					 * enabled and there are presorted keys; otherwise we use
+					 * full sort.
+					 *
+					 * We can use the parent's LIMIT if any, since we
 					 * certainly won't pull more than that many tuples from
 					 * any child.
 					 */
-					cost_sort(&sort_path,
-							  NULL, /* doesn't currently need root */
-							  pathkeys,
-							  subpath->disabled_nodes,
-							  subpath->total_cost,
-							  subpath->rows,
-							  subpath->pathtarget->width,
-							  0.0,
-							  work_mem,
-							  apath->limit_tuples);
+					if (enable_incremental_sort && presorted_keys > 0)
+					{
+						cost_incremental_sort(&sort_path,
+											  root,
+											  pathkeys,
+											  presorted_keys,
+											  subpath->disabled_nodes,
+											  subpath->startup_cost,
+											  subpath->total_cost,
+											  subpath->rows,
+											  subpath->pathtarget->width,
+											  0.0,
+											  work_mem,
+											  apath->limit_tuples);
+					}
+					else
+					{
+						cost_sort(&sort_path,
+								  root,
+								  pathkeys,
+								  subpath->disabled_nodes,
+								  subpath->total_cost,
+								  subpath->rows,
+								  subpath->pathtarget->width,
+								  0.0,
+								  work_mem,
+								  apath->limit_tuples);
+					}
+
 					subpath = &sort_path;
 				}