On Wed, Nov 18, 2015 at 7:25 AM, Amit Kapila <amit.kapil...@gmail.com> wrote:
> Don't we need the startup cost incase we need to build partial paths for
> joinpaths like mergepath?
> Also, I think there are other cases for single relation scan where startup
> cost can matter like when there are psuedoconstants in qualification
> (refer cost_qual_eval_walker()) or let us say if someone has disabled
> seq scan (disable_cost is considered as startup cost.)

I'm not saying that we don't need to compute it.  I'm saying we don't
need to take it into consideration when deciding which paths have
merit.   Note that consider_statup is set this way:

    rel->consider_startup = (root->tuple_fraction > 0);

And for a joinrel:

    joinrel->consider_startup = (root->tuple_fraction > 0);

root->tuple_fraction is 0 when we expect all tuples to be retrieved,
and parallel query can currently only be used when all tuples will be
retrieved.

> B. I think partial path is an important concept and desrves some
> explanation in src/backend/optimizer/README.
> There is already a good explanation about Paths, so I think it
> seems that it is better to add explanation about partial paths.

Good idea.  In the attached, revised version of the patch, I've added
a large new section to that README.

> 2.
> + *  costs: parallelism is only used for plans that will be run to
> completion.
> + *    Therefore, this
> routine is much simpler than add_path: it needs to
> + *    consider only pathkeys and total cost.
>
> There seems to be some spacing issue in last two lines.

Fixed.

> A.
> This means that for inheritance child relations for which rel pages are
> less than parallel_threshold, it will always consider the cost shared
> between 1 worker and leader as per below calc in cost_seqscan:
> if (path->parallel_degree > 0)
> run_cost = run_cost / (path->parallel_degree + 0.5);
>
> I think this might not be the appropriate cost model for even for
> non-inheritence relations which has pages more than parallel_threshold,
> but it seems to be even worst for inheritance children which have
> pages less than parallel_threshold

Why?  I'm certainly open to patches to improve the cost model, but I
don't see why this patch needs to do that.

> B.
> Will it be possible that if none of the inheritence child rels (or very few
> of them) are big enough for parallel scan, then considering Append
> node for parallelism of any use or in other words, won't it be better
> to generate plan as it is done now without this patch for such cases
> considering current execution model of Gather node?

I think we should instead extend the Append node as suggested by
KaiGai, so that it can have both partial and non-partial children.  I
think we can leave that to another patch, though.  Aside from
questions of what the fastest plan are, it's very bad to have Gather
nodes under the Append, because the Append could have many children
and we could end up with a ton of Gather nodes, each using a DSM and a
bunch of workers.  That's important to avoid.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index 012c14b..fe07176 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -1588,6 +1588,7 @@ _outPathInfo(StringInfo str, const Path *node)
 	else
 		_outBitmapset(str, NULL);
 	WRITE_BOOL_FIELD(parallel_aware);
+	WRITE_INT_FIELD(parallel_degree);
 	WRITE_FLOAT_FIELD(rows, "%.0f");
 	WRITE_FLOAT_FIELD(startup_cost, "%.2f");
 	WRITE_FLOAT_FIELD(total_cost, "%.2f");
@@ -1764,7 +1765,6 @@ _outGatherPath(StringInfo str, const GatherPath *node)
 	_outPathInfo(str, (const Path *) node);
 
 	WRITE_NODE_FIELD(subpath);
-	WRITE_INT_FIELD(num_workers);
 	WRITE_BOOL_FIELD(single_copy);
 }
 
@@ -1887,6 +1887,7 @@ _outRelOptInfo(StringInfo str, const RelOptInfo *node)
 	WRITE_NODE_FIELD(reltargetlist);
 	WRITE_NODE_FIELD(pathlist);
 	WRITE_NODE_FIELD(ppilist);
+	WRITE_NODE_FIELD(partial_pathlist);
 	WRITE_NODE_FIELD(cheapest_startup_path);
 	WRITE_NODE_FIELD(cheapest_total_path);
 	WRITE_NODE_FIELD(cheapest_unique_path);
diff --git a/src/backend/optimizer/README b/src/backend/optimizer/README
index 916a518..5019804 100644
--- a/src/backend/optimizer/README
+++ b/src/backend/optimizer/README
@@ -851,4 +851,57 @@ lateral reference.  (Perhaps now that that stuff works, we could relax the
 pullup restriction?)
 
 
--- bjm & tgl
+Parallel Query and Partial Paths
+--------------------------------
+
+Parallel query involves dividing up the work that needs to be performed
+either by an entire query or some portion of the query in such a way that
+some of that work can be done by one or more worker processes, which are
+called parallel workers.  Parallel workers are a subtype of dynamic
+background workers; see src/backend/access/transam/README.parallel for a
+fuller description.  Academic literature on parallel query suggests that
+that parallel execution strategies can be divided into essentially two
+categories: pipelined parallelism, where the execution of the query is
+divided into multiple stages and each stage is handled by a separate
+process; and partitioning parallelism, where the data is split between
+multiple processes and each process handles a subset of it.  The
+literature, however, suggests that gains from pipeline parallelism are
+often very limited due to the difficulty of avoiding pipeline stalls.
+Consequently, we do not currently attempt to generate query plans that
+use this technique.
+
+Instead, we focus on partitioning paralellism, which does not require
+that the underlying table be partitioned.  It only requires that (1)
+there is some method of dividing the data from at least one of the base
+tables involved in the relation across multiple processes, (2) allowing
+each process to handle its own portion of the data, and then (3)
+collecting the results.  Requirements (2) and (3) is satisfied by the
+executor node Gather, which launches any number of worker processes and
+executes its single child plan in all of them (and perhaps in the leader
+also, if the children aren't generating enough data to keep the leader
+busy).  Requirement (1) is handled by the SeqScan node: when invoked
+with parallel_aware = true, this node will, in effect, partition the
+table on a block by block basis, returning a subset of the tuples from
+the relation in each worker where that SeqScan is executed.  A similar
+scheme could be (and probably should be) implemented for bitmap heap
+scans.
+
+Just as we do for non-parallel access methods, we build Paths to
+represent access strategies that can be used in a parallel plan.  These
+are, in essence, the same strategies that are available in the
+non-parallel plan, but there is an important difference: a path that
+will run beneath a Gather node returns only a subset of the query
+results in each worker, not all of them.  To form a path that can
+actually be executed, the (rather large) cost of the Gather node must be
+accounted for.  For this reason among others, paths intended to run
+beneath a Gather node - which we call "partial" paths since they return
+only a subset of the results in each worker - must be kept separate from
+ordinary paths (see RelOptInfo's partial_pathlist and the function
+add_partial_path).
+
+One of the keys to making parallel query effective is to run as much of
+the query in parallel as possible.  Therefore, we expect it to generally
+be desirable to postpone the Gather stage until as near to the top of the
+plan as possible.  Expanding the range of cases in which more work can be
+pushed below the Gather (and costly them accurately) is likely to keep us
+busy for a long time to come.
diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index 1fdcae5..fdbe13f 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -72,6 +72,7 @@ static void set_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
 				 Index rti, RangeTblEntry *rte);
 static void set_plain_rel_size(PlannerInfo *root, RelOptInfo *rel,
 				   RangeTblEntry *rte);
+static void create_parallel_paths(PlannerInfo *root, RelOptInfo *rel);
 static void set_rel_consider_parallel(PlannerInfo *root, RelOptInfo *rel,
 						  RangeTblEntry *rte);
 static bool function_rte_parallel_ok(RangeTblEntry *rte);
@@ -107,6 +108,7 @@ static void set_cte_pathlist(PlannerInfo *root, RelOptInfo *rel,
 				 RangeTblEntry *rte);
 static void set_worktable_pathlist(PlannerInfo *root, RelOptInfo *rel,
 					   RangeTblEntry *rte);
+static void generate_gather_paths(PlannerInfo *root, RelOptInfo *rel);
 static RelOptInfo *make_rel_from_joinlist(PlannerInfo *root, List *joinlist);
 static bool subquery_is_pushdown_safe(Query *subquery, Query *topquery,
 						  pushdown_safety_info *safetyInfo);
@@ -612,7 +614,6 @@ static void
 set_plain_rel_pathlist(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
 {
 	Relids		required_outer;
-	int			parallel_threshold = 1000;
 
 	/*
 	 * We don't support pushing join clauses into the quals of a seqscan, but
@@ -624,39 +625,9 @@ set_plain_rel_pathlist(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
 	/* Consider sequential scan */
 	add_path(rel, create_seqscan_path(root, rel, required_outer, 0));
 
-	/* Consider parallel sequential scan */
-	if (rel->consider_parallel && rel->pages > parallel_threshold &&
-		required_outer == NULL)
-	{
-		Path *path;
-		int parallel_degree = 1;
-
-		/*
-		 * Limit the degree of parallelism logarithmically based on the size
-		 * of the relation.  This probably needs to be a good deal more
-		 * sophisticated, but we need something here for now.
-		 */
-		while (rel->pages > parallel_threshold * 3 &&
-			   parallel_degree < max_parallel_degree)
-		{
-			parallel_degree++;
-			parallel_threshold *= 3;
-			if (parallel_threshold >= PG_INT32_MAX / 3)
-				break;
-		}
-
-		/*
-		 * Ideally we should consider postponing the gather operation until
-		 * much later, after we've pushed joins and so on atop the parallel
-		 * sequential scan path.  But we don't have the infrastructure for
-		 * that yet, so just do this for now.
-		 */
-		path = create_seqscan_path(root, rel, required_outer, parallel_degree);
-		path = (Path *)
-			create_gather_path(root, rel, path, required_outer,
-							   parallel_degree);
-		add_path(rel, path);
-	}
+	/* If appropriate, consider parallel sequential scan */
+	if (rel->consider_parallel && required_outer == NULL)
+		create_parallel_paths(root, rel);
 
 	/* Consider index scans */
 	create_index_paths(root, rel);
@@ -666,6 +637,54 @@ set_plain_rel_pathlist(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
 }
 
 /*
+ * create_parallel_paths
+ *	  Build parallel access paths for a plain relation
+ */
+static void
+create_parallel_paths(PlannerInfo *root, RelOptInfo *rel)
+{
+	int		parallel_threshold = 1000;
+	int		parallel_degree = 1;
+
+	/*
+	 * If this relation is too small to be worth a parallel scan, just return
+	 * without doing anything ... unless it's an inheritance child.  In that case,
+	 * we want to generate a parallel path here anyway.  It might not be worthwhile
+	 * just for this relation, but when combined with all of its inheritance siblings
+	 * it may well pay off.
+	 */
+	if (rel->pages < parallel_threshold && rel->reloptkind == RELOPT_BASEREL)
+		return;
+
+	/*
+	 * Limit the degree of parallelism logarithmically based on the size of the
+	 * relation.  This probably needs to be a good deal more sophisticated, but we
+	 * need something here for now.
+	 */
+	while (rel->pages > parallel_threshold * 3 &&
+		   parallel_degree < max_parallel_degree)
+	{
+		parallel_degree++;
+		parallel_threshold *= 3;
+		if (parallel_threshold >= PG_INT32_MAX / 3)
+			break;
+	}
+
+	/* Add an unordered partial path based on a parallel sequential scan. */
+	add_partial_path(rel, create_seqscan_path(root, rel, NULL, parallel_degree));
+
+	/*
+	 * If this is a baserel, consider gathering any partial paths we may have
+	 * just created.  If we gathered an inheritance child, we could end up with
+	 * a very large number of gather nodes, each trying to grab its own pool of
+	 * workers, so don't do this in that case.  Instead, we'll consider gathering
+	 * partial paths for the appendrel.
+	 */
+	if (rel->reloptkind == RELOPT_BASEREL)
+		generate_gather_paths(root, rel);
+}
+
+/*
  * set_tablesample_rel_size
  *	  Set size estimates for a sampled relation
  */
@@ -1039,6 +1058,8 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
 	List	   *live_childrels = NIL;
 	List	   *subpaths = NIL;
 	bool		subpaths_valid = true;
+	List	   *partial_subpaths = NIL;
+	bool		partial_subpaths_valid = true;
 	List	   *all_child_pathkeys = NIL;
 	List	   *all_child_outers = NIL;
 	ListCell   *l;
@@ -1093,6 +1114,13 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
 		else
 			subpaths_valid = false;
 
+		/* Same idea, but for a partial plan. */
+		if (childrel->partial_pathlist != NIL)
+			partial_subpaths = accumulate_append_subpath(partial_subpaths,
+										linitial(childrel->partial_pathlist));
+		else
+			partial_subpaths_valid = false;
+
 		/*
 		 * Collect lists of all the available path orderings and
 		 * parameterizations for all the children.  We use these as a
@@ -1164,7 +1192,38 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
 	 * if we have zero or one live subpath due to constraint exclusion.)
 	 */
 	if (subpaths_valid)
-		add_path(rel, (Path *) create_append_path(rel, subpaths, NULL));
+		add_path(rel, (Path *) create_append_path(rel, subpaths, NULL, 0));
+
+	/*
+	 * Consider an append of partial unordered, unparameterized partial paths.
+	 */
+	if (partial_subpaths_valid)
+	{
+		AppendPath *appendpath;
+		ListCell *lc;
+		int parallel_degree = 0;
+
+		/*
+		 * Decide what parallel degree to request for this append path.  For
+		 * now, we just use the maximum parallel degree of any member.  It
+		 * might be useful to use a higher number if the Append node were smart
+		 * enough to spread out the workers, but it currently isn't.
+		 */
+		foreach (lc, partial_subpaths)
+		{
+			Path *path = lfirst(lc);
+			parallel_degree = Max(parallel_degree, path->parallel_degree);
+		}
+		Assert(parallel_degree > 0);
+
+		/* Generate a partial append path. */
+		appendpath = create_append_path(rel, partial_subpaths, NULL,
+										parallel_degree);
+		add_partial_path(rel, (Path *) appendpath);
+
+		/* Consider gathering it. */
+		generate_gather_paths(root, rel);
+	}
 
 	/*
 	 * Also build unparameterized MergeAppend paths based on the collected
@@ -1214,7 +1273,7 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
 
 		if (subpaths_valid)
 			add_path(rel, (Path *)
-					 create_append_path(rel, subpaths, required_outer));
+					 create_append_path(rel, subpaths, required_outer, 0));
 	}
 }
 
@@ -1440,8 +1499,9 @@ set_dummy_rel_pathlist(RelOptInfo *rel)
 
 	/* Discard any pre-existing paths; no further need for them */
 	rel->pathlist = NIL;
+	rel->partial_pathlist = NIL;
 
-	add_path(rel, (Path *) create_append_path(rel, NIL, NULL));
+	add_path(rel, (Path *) create_append_path(rel, NIL, NULL, 0));
 
 	/*
 	 * We set the cheapest path immediately, to ensure that IS_DUMMY_REL()
@@ -1844,6 +1904,35 @@ 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.
+ */
+static void
+generate_gather_paths(PlannerInfo *root, RelOptInfo *rel)
+{
+	Path   *cheapest_partial_path;
+	Path   *simple_gather_path;
+
+	/* 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.
+	 *
+	 * 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.
+	 */
+	cheapest_partial_path = linitial(rel->partial_pathlist);
+	simple_gather_path = (Path *)
+		create_gather_path(root, rel, cheapest_partial_path, NULL);
+	add_path(rel, simple_gather_path);
+}
+
+/*
  * make_rel_from_joinlist
  *	  Build access paths using a "joinlist" to guide the join path search.
  *
diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 990486c..9d65be9 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -186,8 +186,7 @@ clamp_row_est(double nrows)
  */
 void
 cost_seqscan(Path *path, PlannerInfo *root,
-			 RelOptInfo *baserel, ParamPathInfo *param_info,
-			 int nworkers)
+			 RelOptInfo *baserel, ParamPathInfo *param_info)
 {
 	Cost		startup_cost = 0;
 	Cost		run_cost = 0;
@@ -232,8 +231,8 @@ cost_seqscan(Path *path, PlannerInfo *root,
 	 * This is almost certainly not exactly the right way to model this, so
 	 * this will probably need to be changed at some point...
 	 */
-	if (nworkers > 0)
-		run_cost = run_cost / (nworkers + 0.5);
+	if (path->parallel_degree > 0)
+		run_cost = run_cost / (path->parallel_degree + 0.5);
 
 	path->startup_cost = startup_cost;
 	path->total_cost = startup_cost + run_cost;
diff --git a/src/backend/optimizer/path/joinrels.c b/src/backend/optimizer/path/joinrels.c
index b2cc9f0..9b2b0b4 100644
--- a/src/backend/optimizer/path/joinrels.c
+++ b/src/backend/optimizer/path/joinrels.c
@@ -1069,9 +1069,10 @@ mark_dummy_rel(RelOptInfo *rel)
 
 	/* Evict any previously chosen paths */
 	rel->pathlist = NIL;
+	rel->partial_pathlist = NIL;
 
 	/* Set up the dummy path */
-	add_path(rel, (Path *) create_append_path(rel, NIL, NULL));
+	add_path(rel, (Path *) create_append_path(rel, NIL, NULL, 0));
 
 	/* Set or update cheapest_total_path and related fields */
 	set_cheapest(rel);
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 411b36c..95d95f1 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -1125,7 +1125,7 @@ create_gather_plan(PlannerInfo *root, GatherPath *best_path)
 
 	gather_plan = make_gather(subplan->targetlist,
 							  NIL,
-							  best_path->num_workers,
+							  best_path->path.parallel_degree,
 							  best_path->single_copy,
 							  subplan);
 
diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index 09c3244..354a9e9 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -685,6 +685,151 @@ add_path_precheck(RelOptInfo *parent_rel,
 	return true;
 }
 
+/*
+ * add_partial_path
+ *	  Like add_path, our goal here is to consider whether a path is worthy
+ *	  of being kept around, but the considerations here are a bit different.
+ *	  A partial path is one which can be executed in any number of workers in
+ *	  parallel such that each worker will generate a subset of the path's
+ *	  overall result.
+ *
+ *	  We don't generate parameterized partial paths because they seem unlikely
+ *	  ever to be worthwhile.  The only way we could ever use such a path is
+ *	  by executing a nested loop with a complete path on the outer side - thus,
+ *	  each worker would scan the entire outer relation - and the partial path
+ *	  on the inner side - thus, each worker would scan only part of the inner
+ *	  relation.  This is silly: a parameterized path is generally going to be
+ *	  based on an index scan, and we can't generate a partial path for that.
+ *	  And it's generally only going to produce a few rows, so splitting them
+ *	  up between workers doesn't really make sense.  It would be better to
+ *	  use an unparameterized partial path on the outer side of the join with
+ *	  a parameterized complete path on the inner side in virtually every case.
+ *
+ *	  Because we don't need to consider parameterized paths here, we also don't
+ *	  need to consider the row counts as a measure of quality: every path will
+ *	  produce the same number of rows.  Neither do we need to consider startup
+ *	  costs: parallelism is only used for plans that will be run to completion.
+ *	  Therefore, this routine is much simpler than add_path: it needs to
+ *	  consider only pathkeys and total cost.
+ */
+void
+add_partial_path(RelOptInfo *parent_rel, Path *new_path)
+{
+	bool		accept_new = true;		/* unless we find a superior old path */
+	ListCell   *insert_after = NULL;	/* where to insert new item */
+	ListCell   *p1;
+	ListCell   *p1_prev;
+	ListCell   *p1_next;
+
+	/* Check for query cancel. */
+	CHECK_FOR_INTERRUPTS();
+
+	/*
+	 * As in add_path, throw out any paths which are dominated by the new path,
+	 * but throw out the new path if some existing path dominates it.
+	 */
+	p1_prev = NULL;
+	for (p1 = list_head(parent_rel->partial_pathlist); p1 != NULL;
+		 p1 = p1_next)
+	{
+		Path	   *old_path = (Path *) lfirst(p1);
+		bool		remove_old = false; /* unless new proves superior */
+		PathKeysComparison keyscmp;
+
+		p1_next = lnext(p1);
+
+		/* Compare pathkeys. */
+		keyscmp = compare_pathkeys(new_path->pathkeys, old_path->pathkeys);
+
+		/* Unless pathkeys are incompable, keep just one of the two paths. */
+		if (keyscmp != PATHKEYS_DIFFERENT)
+		{
+			if (new_path->total_cost > old_path->total_cost * STD_FUZZ_FACTOR)
+			{
+				/* New path costs more; keep it only if pathkeys are better. */
+				if (keyscmp != PATHKEYS_BETTER1)
+					accept_new = false;
+			}
+			else if (old_path->total_cost > new_path->total_cost
+						* STD_FUZZ_FACTOR)
+			{
+				/* Old path costs more; keep it only if pathkeys are better. */
+				if (keyscmp != PATHKEYS_BETTER2)
+					remove_old = true;
+			}
+			else if (keyscmp == PATHKEYS_BETTER1)
+			{
+				/* Costs are about the same, new path has better pathkeys. */
+				remove_old = true;
+			}
+			else if (keyscmp == PATHKEYS_BETTER2)
+			{
+				/* Costs are about the same, old path has better pathkeys. */
+				accept_new = false;
+			}
+			else if (old_path->total_cost > new_path->total_cost * 1.0000000001)
+			{
+				/* Pathkeys are the same, and the old path costs more. */
+				remove_old = true;
+			}
+			else
+			{
+				/*
+				 * Pathkeys are the same, and new path isn't materially
+				 * cheaper.
+				 */
+				accept_new = false;
+			}
+		}
+
+		/*
+		 * Remove current element from partial_pathlist if dominated by new.
+		 */
+		if (remove_old)
+		{
+			parent_rel->partial_pathlist =
+				list_delete_cell(parent_rel->partial_pathlist, p1, p1_prev);
+			/* add_path has a special case for IndexPath; we don't need it */
+			Assert(!IsA(old_path, IndexPath));
+			pfree(old_path);
+			/* p1_prev does not advance */
+		}
+		else
+		{
+			/* new belongs after this old path if it has cost >= old's */
+			if (new_path->total_cost >= old_path->total_cost)
+				insert_after = p1;
+			/* p1_prev advances */
+			p1_prev = p1;
+		}
+
+		/*
+		 * If we found an old path that dominates new_path, we can quit
+		 * scanning the partial_pathlist; we will not add new_path, and we
+		 * assume new_path cannot dominate any later path.
+		 */
+		if (!accept_new)
+			break;
+	}
+
+	if (accept_new)
+	{
+		/* Accept the new path: insert it at proper place */
+		if (insert_after)
+			lappend_cell(parent_rel->partial_pathlist, insert_after, new_path);
+		else
+			parent_rel->partial_pathlist =
+				lcons(new_path, parent_rel->partial_pathlist);
+	}
+	else
+	{
+		/* add_path has a special case for IndexPath; we don't need it */
+		Assert(!IsA(new_path, IndexPath));
+		/* Reject and recycle the new path */
+		pfree(new_path);
+	}
+}
+
 
 /*****************************************************************************
  *		PATH NODE CREATION ROUTINES
@@ -697,7 +842,7 @@ add_path_precheck(RelOptInfo *parent_rel,
  */
 Path *
 create_seqscan_path(PlannerInfo *root, RelOptInfo *rel,
-					Relids required_outer, int nworkers)
+					Relids required_outer, int parallel_degree)
 {
 	Path	   *pathnode = makeNode(Path);
 
@@ -705,10 +850,11 @@ create_seqscan_path(PlannerInfo *root, RelOptInfo *rel,
 	pathnode->parent = rel;
 	pathnode->param_info = get_baserel_parampathinfo(root, rel,
 													 required_outer);
-	pathnode->parallel_aware = nworkers > 0 ? true : false;
+	pathnode->parallel_aware = parallel_degree > 0 ? true : false;
+	pathnode->parallel_degree = parallel_degree;
 	pathnode->pathkeys = NIL;	/* seqscan has unordered result */
 
-	cost_seqscan(pathnode, root, rel, pathnode->param_info, nworkers);
+	cost_seqscan(pathnode, root, rel, pathnode->param_info);
 
 	return pathnode;
 }
@@ -727,6 +873,7 @@ create_samplescan_path(PlannerInfo *root, RelOptInfo *rel, Relids required_outer
 	pathnode->param_info = get_baserel_parampathinfo(root, rel,
 													 required_outer);
 	pathnode->parallel_aware = false;
+	pathnode->parallel_degree = 0;
 	pathnode->pathkeys = NIL;	/* samplescan has unordered result */
 
 	cost_samplescan(pathnode, root, rel, pathnode->param_info);
@@ -781,6 +928,7 @@ create_index_path(PlannerInfo *root,
 	pathnode->path.param_info = get_baserel_parampathinfo(root, rel,
 														  required_outer);
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.pathkeys = pathkeys;
 
 	/* Convert clauses to indexquals the executor can handle */
@@ -827,6 +975,7 @@ create_bitmap_heap_path(PlannerInfo *root,
 	pathnode->path.param_info = get_baserel_parampathinfo(root, rel,
 														  required_outer);
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.pathkeys = NIL;		/* always unordered */
 
 	pathnode->bitmapqual = bitmapqual;
@@ -853,6 +1002,7 @@ create_bitmap_and_path(PlannerInfo *root,
 	pathnode->path.parent = rel;
 	pathnode->path.param_info = NULL;	/* not used in bitmap trees */
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.pathkeys = NIL;		/* always unordered */
 
 	pathnode->bitmapquals = bitmapquals;
@@ -878,6 +1028,7 @@ create_bitmap_or_path(PlannerInfo *root,
 	pathnode->path.parent = rel;
 	pathnode->path.param_info = NULL;	/* not used in bitmap trees */
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.pathkeys = NIL;		/* always unordered */
 
 	pathnode->bitmapquals = bitmapquals;
@@ -903,6 +1054,7 @@ create_tidscan_path(PlannerInfo *root, RelOptInfo *rel, List *tidquals,
 	pathnode->path.param_info = get_baserel_parampathinfo(root, rel,
 														  required_outer);
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.pathkeys = NIL;		/* always unordered */
 
 	pathnode->tidquals = tidquals;
@@ -921,7 +1073,8 @@ create_tidscan_path(PlannerInfo *root, RelOptInfo *rel, List *tidquals,
  * Note that we must handle subpaths = NIL, representing a dummy access path.
  */
 AppendPath *
-create_append_path(RelOptInfo *rel, List *subpaths, Relids required_outer)
+create_append_path(RelOptInfo *rel, List *subpaths, Relids required_outer,
+				   int parallel_degree)
 {
 	AppendPath *pathnode = makeNode(AppendPath);
 	ListCell   *l;
@@ -931,6 +1084,7 @@ create_append_path(RelOptInfo *rel, List *subpaths, Relids required_outer)
 	pathnode->path.param_info = get_appendrel_parampathinfo(rel,
 															required_outer);
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_degree = parallel_degree;
 	pathnode->path.pathkeys = NIL;		/* result is always considered
 										 * unsorted */
 	pathnode->subpaths = subpaths;
@@ -985,6 +1139,7 @@ create_merge_append_path(PlannerInfo *root,
 	pathnode->path.param_info = get_appendrel_parampathinfo(rel,
 															required_outer);
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.pathkeys = pathkeys;
 	pathnode->subpaths = subpaths;
 
@@ -1060,6 +1215,7 @@ create_result_path(List *quals)
 	pathnode->path.parent = NULL;
 	pathnode->path.param_info = NULL;	/* there are no other rels... */
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.pathkeys = NIL;
 	pathnode->quals = quals;
 
@@ -1094,6 +1250,7 @@ create_material_path(RelOptInfo *rel, Path *subpath)
 	pathnode->path.parent = rel;
 	pathnode->path.param_info = subpath->param_info;
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.pathkeys = subpath->pathkeys;
 
 	pathnode->subpath = subpath;
@@ -1155,6 +1312,7 @@ create_unique_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
 	pathnode->path.parent = rel;
 	pathnode->path.param_info = subpath->param_info;
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_degree = 0;
 
 	/*
 	 * Assume the output is unsorted, since we don't necessarily have pathkeys
@@ -1328,7 +1486,7 @@ create_unique_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
  */
 GatherPath *
 create_gather_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
-				   Relids required_outer, int nworkers)
+				   Relids required_outer)
 {
 	GatherPath *pathnode = makeNode(GatherPath);
 
@@ -1336,11 +1494,18 @@ create_gather_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
 	pathnode->path.parent = rel;
 	pathnode->path.param_info = get_baserel_parampathinfo(root, rel,
 														  required_outer);
-	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_degree = subpath->parallel_degree;
 	pathnode->path.pathkeys = NIL;		/* Gather has unordered result */
 
 	pathnode->subpath = subpath;
-	pathnode->num_workers = nworkers;
+	pathnode->single_copy = false;
+
+	if (pathnode->path.parallel_degree == 0)
+	{
+		pathnode->path.parallel_degree = 1;
+		pathnode->path.pathkeys = subpath->pathkeys;
+		pathnode->single_copy = true;
+	}
 
 	cost_gather(pathnode, root, rel, pathnode->path.param_info);
 
@@ -1393,6 +1558,7 @@ create_subqueryscan_path(PlannerInfo *root, RelOptInfo *rel,
 	pathnode->param_info = get_baserel_parampathinfo(root, rel,
 													 required_outer);
 	pathnode->parallel_aware = false;
+	pathnode->parallel_degree = 0;
 	pathnode->pathkeys = pathkeys;
 
 	cost_subqueryscan(pathnode, root, rel, pathnode->param_info);
@@ -1416,6 +1582,7 @@ create_functionscan_path(PlannerInfo *root, RelOptInfo *rel,
 	pathnode->param_info = get_baserel_parampathinfo(root, rel,
 													 required_outer);
 	pathnode->parallel_aware = false;
+	pathnode->parallel_degree = 0;
 	pathnode->pathkeys = pathkeys;
 
 	cost_functionscan(pathnode, root, rel, pathnode->param_info);
@@ -1439,6 +1606,7 @@ create_valuesscan_path(PlannerInfo *root, RelOptInfo *rel,
 	pathnode->param_info = get_baserel_parampathinfo(root, rel,
 													 required_outer);
 	pathnode->parallel_aware = false;
+	pathnode->parallel_degree = 0;
 	pathnode->pathkeys = NIL;	/* result is always unordered */
 
 	cost_valuesscan(pathnode, root, rel, pathnode->param_info);
@@ -1461,6 +1629,7 @@ create_ctescan_path(PlannerInfo *root, RelOptInfo *rel, Relids required_outer)
 	pathnode->param_info = get_baserel_parampathinfo(root, rel,
 													 required_outer);
 	pathnode->parallel_aware = false;
+	pathnode->parallel_degree = 0;
 	pathnode->pathkeys = NIL;	/* XXX for now, result is always unordered */
 
 	cost_ctescan(pathnode, root, rel, pathnode->param_info);
@@ -1484,6 +1653,7 @@ create_worktablescan_path(PlannerInfo *root, RelOptInfo *rel,
 	pathnode->param_info = get_baserel_parampathinfo(root, rel,
 													 required_outer);
 	pathnode->parallel_aware = false;
+	pathnode->parallel_degree = 0;
 	pathnode->pathkeys = NIL;	/* result is always unordered */
 
 	/* Cost is the same as for a regular CTE scan */
@@ -1516,6 +1686,7 @@ create_foreignscan_path(PlannerInfo *root, RelOptInfo *rel,
 	pathnode->path.param_info = get_baserel_parampathinfo(root, rel,
 														  required_outer);
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.rows = rows;
 	pathnode->path.startup_cost = startup_cost;
 	pathnode->path.total_cost = total_cost;
@@ -1651,6 +1822,7 @@ create_nestloop_path(PlannerInfo *root,
 								  required_outer,
 								  &restrict_clauses);
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.pathkeys = pathkeys;
 	pathnode->jointype = jointype;
 	pathnode->outerjoinpath = outer_path;
@@ -1709,6 +1881,7 @@ create_mergejoin_path(PlannerInfo *root,
 								  required_outer,
 								  &restrict_clauses);
 	pathnode->jpath.path.parallel_aware = false;
+	pathnode->jpath.path.parallel_degree = 0;
 	pathnode->jpath.path.pathkeys = pathkeys;
 	pathnode->jpath.jointype = jointype;
 	pathnode->jpath.outerjoinpath = outer_path;
@@ -1766,6 +1939,7 @@ create_hashjoin_path(PlannerInfo *root,
 								  required_outer,
 								  &restrict_clauses);
 	pathnode->jpath.path.parallel_aware = false;
+	pathnode->jpath.path.parallel_degree = 0;
 
 	/*
 	 * A hashjoin never has pathkeys, since its output ordering is
diff --git a/src/backend/optimizer/util/relnode.c b/src/backend/optimizer/util/relnode.c
index 996b7fe..8d7ac48 100644
--- a/src/backend/optimizer/util/relnode.c
+++ b/src/backend/optimizer/util/relnode.c
@@ -107,6 +107,7 @@ build_simple_rel(PlannerInfo *root, int relid, RelOptKind reloptkind)
 	rel->reltargetlist = NIL;
 	rel->pathlist = NIL;
 	rel->ppilist = NIL;
+	rel->partial_pathlist = NIL;
 	rel->cheapest_startup_path = NULL;
 	rel->cheapest_total_path = NULL;
 	rel->cheapest_unique_path = NULL;
@@ -369,6 +370,7 @@ build_join_rel(PlannerInfo *root,
 	joinrel->reltargetlist = NIL;
 	joinrel->pathlist = NIL;
 	joinrel->ppilist = NIL;
+	joinrel->partial_pathlist = NIL;
 	joinrel->cheapest_startup_path = NULL;
 	joinrel->cheapest_total_path = NULL;
 	joinrel->cheapest_unique_path = NULL;
diff --git a/src/include/nodes/relation.h b/src/include/nodes/relation.h
index 9a0dd28..bdf4c53 100644
--- a/src/include/nodes/relation.h
+++ b/src/include/nodes/relation.h
@@ -458,6 +458,7 @@ typedef struct RelOptInfo
 	List	   *reltargetlist;	/* Vars to be output by scan of relation */
 	List	   *pathlist;		/* Path structures */
 	List	   *ppilist;		/* ParamPathInfos used in pathlist */
+	List	   *partial_pathlist;	/* partial Paths */
 	struct Path *cheapest_startup_path;
 	struct Path *cheapest_total_path;
 	struct Path *cheapest_unique_path;
@@ -755,6 +756,7 @@ typedef struct Path
 	RelOptInfo *parent;			/* the relation this path can build */
 	ParamPathInfo *param_info;	/* parameterization info, or NULL if none */
 	bool		parallel_aware; /* engage parallel-aware logic? */
+	int			parallel_degree; /* desired parallel degree; 0 = not parallel */
 
 	/* estimated size/costs for path (see costsize.c for more info) */
 	double		rows;			/* estimated number of result tuples */
@@ -1057,7 +1059,6 @@ typedef struct GatherPath
 {
 	Path		path;
 	Path	   *subpath;		/* path for each worker */
-	int			num_workers;	/* number of workers sought to help */
 	bool		single_copy;	/* path must not be executed >1x */
 } GatherPath;
 
diff --git a/src/include/optimizer/cost.h b/src/include/optimizer/cost.h
index ac21a3a..25a7303 100644
--- a/src/include/optimizer/cost.h
+++ b/src/include/optimizer/cost.h
@@ -72,7 +72,7 @@ extern double clamp_row_est(double nrows);
 extern double index_pages_fetched(double tuples_fetched, BlockNumber pages,
 					double index_pages, PlannerInfo *root);
 extern void cost_seqscan(Path *path, PlannerInfo *root, RelOptInfo *baserel,
-			 ParamPathInfo *param_info, int nworkers);
+			 ParamPathInfo *param_info);
 extern void cost_samplescan(Path *path, PlannerInfo *root, RelOptInfo *baserel,
 				ParamPathInfo *param_info);
 extern void cost_index(IndexPath *path, PlannerInfo *root,
diff --git a/src/include/optimizer/pathnode.h b/src/include/optimizer/pathnode.h
index f28b4e2..38d4859 100644
--- a/src/include/optimizer/pathnode.h
+++ b/src/include/optimizer/pathnode.h
@@ -29,9 +29,10 @@ extern void add_path(RelOptInfo *parent_rel, Path *new_path);
 extern bool add_path_precheck(RelOptInfo *parent_rel,
 				  Cost startup_cost, Cost total_cost,
 				  List *pathkeys, Relids required_outer);
+extern void add_partial_path(RelOptInfo *parent_rel, Path *new_path);
 
 extern Path *create_seqscan_path(PlannerInfo *root, RelOptInfo *rel,
-					Relids required_outer, int nworkers);
+					Relids required_outer, int parallel_degree);
 extern Path *create_samplescan_path(PlannerInfo *root, RelOptInfo *rel,
 					   Relids required_outer);
 extern IndexPath *create_index_path(PlannerInfo *root,
@@ -59,7 +60,7 @@ extern BitmapOrPath *create_bitmap_or_path(PlannerInfo *root,
 extern TidPath *create_tidscan_path(PlannerInfo *root, RelOptInfo *rel,
 					List *tidquals, Relids required_outer);
 extern AppendPath *create_append_path(RelOptInfo *rel, List *subpaths,
-				   Relids required_outer);
+				   Relids required_outer, int parallel_degree);
 extern MergeAppendPath *create_merge_append_path(PlannerInfo *root,
 						 RelOptInfo *rel,
 						 List *subpaths,
@@ -70,8 +71,7 @@ extern MaterialPath *create_material_path(RelOptInfo *rel, Path *subpath);
 extern UniquePath *create_unique_path(PlannerInfo *root, RelOptInfo *rel,
 				   Path *subpath, SpecialJoinInfo *sjinfo);
 extern GatherPath *create_gather_path(PlannerInfo *root,
-				   RelOptInfo *rel, Path *subpath, Relids required_outer,
-				   int nworkers);
+				   RelOptInfo *rel, Path *subpath, Relids required_outer);
 extern Path *create_subqueryscan_path(PlannerInfo *root, RelOptInfo *rel,
 						 List *pathkeys, Relids required_outer);
 extern Path *create_functionscan_path(PlannerInfo *root, RelOptInfo *rel,
-- 
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