Support parallel joins, and make related improvements.

The core innovation of this patch is the introduction of the concept
of a partial path; that is, a path which if executed in parallel will
generate a subset of the output rows in each process.  Gathering a
partial path produces an ordinary (complete) path.  This allows us to
generate paths for parallel joins by joining a partial path for one
side (which at the baserel level is currently always a Partial Seq
Scan) to an ordinary path on the other side.  This is subject to
various restrictions at present, especially that this strategy seems
unlikely to be sensible for merge joins, so only nested loops and
hash joins paths are generated.

This also allows an Append node to be pushed below a Gather node in
the case of a partitioned table.

Testing revealed that early versions of this patch made poor decisions
in some cases, which turned out to be caused by the fact that the
original cost model for Parallel Seq Scan wasn't very good.  So this
patch tries to make some modest improvements in that area.

There is much more to be done in the area of generating good parallel
plans in all cases, but this seems like a useful step forward.

Patch by me, reviewed by Dilip Kumar and Amit Kapila.

src/backend/executor/execParallel.c

@@ -167,25 +167,25 @@ ExecParallelEstimate(PlanState *planstate, ExecParallelEstimateContext *e)
 	e->nnodes++;
 
 	/* Call estimators for parallel-aware nodes. */
-	switch (nodeTag(planstate))
+	if (planstate->plan->parallel_aware)
 	{
-		case T_SeqScanState:
-			ExecSeqScanEstimate((SeqScanState *) planstate,
-								e->pcxt);
-			break;
-		default:
-			break;
+		switch (nodeTag(planstate))
+		{
+			case T_SeqScanState:
+				ExecSeqScanEstimate((SeqScanState *) planstate,
+									e->pcxt);
+				break;
+			default:
+				break;
+		}
 	}
 
 	return planstate_tree_walker(planstate, ExecParallelEstimate, e);
 }
 
 /*
- * Ordinary plan nodes won't do anything here, but parallel-aware plan nodes
- * may need to initialize shared state in the DSM before parallel workers
- * are available.  They can allocate the space they previous estimated using
- * shm_toc_allocate, and add the keys they previously estimated using
- * shm_toc_insert, in each case targeting pcxt->toc.
+ * Initialize the dynamic shared memory segment that will be used to control
+ * parallel execution.
  */
 static bool
 ExecParallelInitializeDSM(PlanState *planstate,
@@ -202,15 +202,26 @@ ExecParallelInitializeDSM(PlanState *planstate,
 	/* Count this node. */
 	d->nnodes++;
 
-	/* Call initializers for parallel-aware plan nodes. */
-	switch (nodeTag(planstate))
+	/*
+	 * Call initializers for parallel-aware plan nodes.
+	 *
+	 * Ordinary plan nodes won't do anything here, but parallel-aware plan
+	 * nodes may need to initialize shared state in the DSM before parallel
+	 * workers are available.  They can allocate the space they previously
+	 * estimated using shm_toc_allocate, and add the keys they previously
+	 * estimated using shm_toc_insert, in each case targeting pcxt->toc.
+	 */
+	if (planstate->plan->parallel_aware)
 	{
-		case T_SeqScanState:
-			ExecSeqScanInitializeDSM((SeqScanState *) planstate,
-									 d->pcxt);
-			break;
-		default:
-			break;
+		switch (nodeTag(planstate))
+		{
+			case T_SeqScanState:
+				ExecSeqScanInitializeDSM((SeqScanState *) planstate,
+										 d->pcxt);
+				break;
+			default:
+				break;
+		}
 	}
 
 	return planstate_tree_walker(planstate, ExecParallelInitializeDSM, d);
@@ -623,13 +634,16 @@ ExecParallelInitializeWorker(PlanState *planstate, shm_toc *toc)
 		return false;
 
 	/* Call initializers for parallel-aware plan nodes. */
-	switch (nodeTag(planstate))
+	if (planstate->plan->parallel_aware)
 	{
-		case T_SeqScanState:
-			ExecSeqScanInitializeWorker((SeqScanState *) planstate, toc);
-			break;
-		default:
-			break;
+		switch (nodeTag(planstate))
+		{
+			case T_SeqScanState:
+				ExecSeqScanInitializeWorker((SeqScanState *) planstate, toc);
+				break;
+			default:
+				break;
+		}
 	}
 
 	return planstate_tree_walker(planstate, ExecParallelInitializeWorker, toc);

src/backend/nodes/outfuncs.c

@@ -1591,6 +1591,8 @@ _outPathInfo(StringInfo str, const Path *node)
 	else
 		_outBitmapset(str, NULL);
 	WRITE_BOOL_FIELD(parallel_aware);
+	WRITE_BOOL_FIELD(parallel_safe);
+	WRITE_INT_FIELD(parallel_degree);
 	WRITE_FLOAT_FIELD(rows, "%.0f");
 	WRITE_FLOAT_FIELD(startup_cost, "%.2f");
 	WRITE_FLOAT_FIELD(total_cost, "%.2f");
@@ -1768,7 +1770,6 @@ _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);
 }
 
@@ -1890,6 +1891,7 @@ _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);

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.

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);
@@ -612,7 +613,6 @@ 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 +624,9 @@ 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);
@@ -665,6 +635,54 @@ set_plain_rel_pathlist(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
 	create_tidscan_paths(root, rel);
 }
 
+/*
+ * 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 +1057,8 @@ 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 +1113,13 @@ 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 +1191,39 @@ 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()
@@ -1843,6 +1903,36 @@ set_worktable_pathlist(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
 	add_path(rel, create_worktablescan_path(root, rel, required_outer));
 }
 
+/*
+ * generate_gather_paths
+ *		Generate parallel access paths for a relation by pushing a Gather on
+ *		top of a partial path.
+ */
+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.

src/backend/optimizer/path/costsize.c

@@ -187,11 +187,11 @@ 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;
+	Cost		cpu_run_cost;
+	Cost		disk_run_cost;
 	double		spc_seq_page_cost;
 	QualCost	qpqual_cost;
 	Cost		cpu_per_tuple;
@@ -217,27 +217,58 @@ cost_seqscan(Path *path, PlannerInfo *root,
 	/*
 	 * disk costs
 	 */
-	run_cost += spc_seq_page_cost * baserel->pages;
+	disk_run_cost = spc_seq_page_cost * baserel->pages;
 
 	/* CPU costs */
 	get_restriction_qual_cost(root, baserel, param_info, &qpqual_cost);
 
 	startup_cost += qpqual_cost.startup;
 	cpu_per_tuple = cpu_tuple_cost + qpqual_cost.per_tuple;
-	run_cost += cpu_per_tuple * baserel->tuples;
+	cpu_run_cost = cpu_per_tuple * baserel->tuples;
 
-	/*
-	 * Primitive parallel cost model.  Assume the leader will do half as much
-	 * work as a regular worker, because it will also need to read the tuples
-	 * returned by the workers when they percolate up to the gather node. 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);
+	/* Adjust costing for parallelism, if used. */
+	if (path->parallel_degree > 0)
+	{
+		double	parallel_divisor = path->parallel_degree;
+		double	leader_contribution;
+
+		/*
+		 * Early experience with parallel query suggests that when there is
+		 * only one worker, the leader often makes a very substantial
+		 * contribution to executing the parallel portion of the plan, but as
+		 * more workers are added, it does less and less, because it's busy
+		 * reading tuples from the workers and doing whatever non-paralell
+		 * post-processing is needed.  By the time we reach 4 workers, the
+		 * leader no longer makes a meaningful contribution.  Thus, for now,
+		 * estimate that the leader spends 30% of its time servicing each
+		 * worker, and the remainder executing the parallel plan.
+		 */
+		leader_contribution = 1.0 - (0.3 * path->parallel_degree);
+		if (leader_contribution > 0)
+			parallel_divisor += leader_contribution;
+
+		/*
+		 * In the case of a parallel plan, the row count needs to represent
+		 * the number of tuples processed per worker.  Otherwise, higher-level
+		 * plan nodes that appear below the gather will be costed incorrectly,
+		 * because they'll anticipate receiving more rows than any given copy
+		 * will actually get.
+		 */
+		path->rows /= parallel_divisor;
+
+		/* The CPU cost is divided among all the workers. */
+		cpu_run_cost /= parallel_divisor;
+
+		/*
+		 * It may be possible to amortize some of the I/O cost, but probably
+		 * not very much, because most operating systems already do aggressive
+		 * prefetching.  For now, we assume that the disk run cost can't be
+		 * amortized at all.
+		 */
+	}
 
 	path->startup_cost = startup_cost;
-	path->total_cost = startup_cost + run_cost;
+	path->total_cost = startup_cost + cpu_run_cost + disk_run_cost;
 }
 
 /*

src/backend/optimizer/path/joinpath.c

@@ -34,6 +34,12 @@ static void sort_inner_and_outer(PlannerInfo *root, RelOptInfo *joinrel,
 static void match_unsorted_outer(PlannerInfo *root, RelOptInfo *joinrel,
 					 RelOptInfo *outerrel, RelOptInfo *innerrel,
 					 JoinType jointype, JoinPathExtraData *extra);
+static void consider_parallel_nestloop(PlannerInfo *root,
+						   RelOptInfo *joinrel,
+						   RelOptInfo *outerrel,
+						   RelOptInfo *innerrel,
+						   JoinType jointype,
+						   JoinPathExtraData *extra);
 static void hash_inner_and_outer(PlannerInfo *root, RelOptInfo *joinrel,
 					 RelOptInfo *outerrel, RelOptInfo *innerrel,
 					 JoinType jointype, JoinPathExtraData *extra);
@@ -216,7 +222,12 @@ add_paths_to_joinrel(PlannerInfo *root,
 												 jointype, &extra);
 
 	/*
-	 * 6. Finally, give extensions a chance to manipulate the path list.
+	 * 6. Consider gathering partial paths.
+	 */
+	generate_gather_paths(root, joinrel);
+
+	/*
+	 * 7. Finally, give extensions a chance to manipulate the path list.
 	 */
 	if (set_join_pathlist_hook)
 		set_join_pathlist_hook(root, joinrel, outerrel, innerrel,
@@ -329,6 +340,62 @@ try_nestloop_path(PlannerInfo *root,
 	}
 }
 
+/*
+ * try_partial_nestloop_path
+ *	  Consider a partial nestloop join path; if it appears useful, push it into
+ *	  the joinrel's partial_pathlist via add_partial_path().
+ */
+static void
+try_partial_nestloop_path(PlannerInfo *root,
+				  RelOptInfo *joinrel,
+				  Path *outer_path,
+				  Path *inner_path,
+				  List *pathkeys,
+				  JoinType jointype,
+				  JoinPathExtraData *extra)
+{
+	JoinCostWorkspace workspace;
+
+	/*
+	 * If the inner path is parameterized, the parameterization must be fully
+	 * satisfied by the proposed outer path.  Parameterized partial paths are
+	 * not supported.  The caller should already have verified that no
+	 * extra_lateral_rels are required here.
+	 */
+	Assert(bms_is_empty(joinrel->lateral_relids));
+	if (inner_path->param_info != NULL)
+	{
+		Relids		inner_paramrels = inner_path->param_info->ppi_req_outer;
+
+		if (!bms_is_subset(inner_paramrels, outer_path->parent->relids))
+			return;
+	}
+
+	/*
+	 * Before creating a path, get a quick lower bound on what it is likely
+	 * to cost.  Bail out right away if it looks terrible.
+	 */
+	initial_cost_nestloop(root, &workspace, jointype,
+						  outer_path, inner_path,
+						  extra->sjinfo, &extra->semifactors);
+	if (!add_partial_path_precheck(joinrel, workspace.total_cost, pathkeys))
+		return;
+
+	/* Might be good enough to be worth trying, so let's try it. */
+	add_partial_path(joinrel, (Path *)
+			 create_nestloop_path(root,
+								  joinrel,
+								  jointype,
+								  &workspace,
+								  extra->sjinfo,
+								  &extra->semifactors,
+								  outer_path,
+								  inner_path,
+								  extra->restrictlist,
+								  pathkeys,
+								  NULL));
+}
+
 /*
  * try_mergejoin_path
  *	  Consider a merge join path; if it appears useful, push it into
@@ -471,6 +538,62 @@ try_hashjoin_path(PlannerInfo *root,
 	}
 }
 
+/*
+ * try_partial_hashjoin_path
+ *	  Consider a partial hashjoin join path; if it appears useful, push it into
+ *	  the joinrel's partial_pathlist via add_partial_path().
+ */
+static void
+try_partial_hashjoin_path(PlannerInfo *root,
+						  RelOptInfo *joinrel,
+						  Path *outer_path,
+						  Path *inner_path,
+						  List *hashclauses,
+						  JoinType jointype,
+						  JoinPathExtraData *extra)
+{
+	JoinCostWorkspace workspace;
+
+	/*
+	 * If the inner path is parameterized, the parameterization must be fully
+	 * satisfied by the proposed outer path.  Parameterized partial paths are
+	 * not supported.  The caller should already have verified that no
+	 * extra_lateral_rels are required here.
+	 */
+	Assert(bms_is_empty(joinrel->lateral_relids));
+	if (inner_path->param_info != NULL)
+	{
+		Relids		inner_paramrels = inner_path->param_info->ppi_req_outer;
+
+		if (!bms_is_empty(inner_paramrels))
+			return;
+	}
+
+	/*
+	 * Before creating a path, get a quick lower bound on what it is likely
+	 * to cost.  Bail out right away if it looks terrible.
+	 */
+	initial_cost_hashjoin(root, &workspace, jointype, hashclauses,
+						  outer_path, inner_path,
+						  extra->sjinfo, &extra->semifactors);
+	if (!add_partial_path_precheck(joinrel, workspace.total_cost, NIL))
+		return;
+
+	/* Might be good enough to be worth trying, so let's try it. */
+	add_partial_path(joinrel, (Path *)
+			 create_hashjoin_path(root,
+								  joinrel,
+								  jointype,
+								  &workspace,
+								  extra->sjinfo,
+								  &extra->semifactors,
+								  outer_path,
+								  inner_path,
+								  extra->restrictlist,
+								  NULL,
+								  hashclauses));
+}
+
 /*
  * clause_sides_match_join
  *	  Determine whether a join clause is of the right form to use in this join.
@@ -1063,6 +1186,85 @@ match_unsorted_outer(PlannerInfo *root,
 				break;
 		}
 	}
+
+	/*
+	 * If the joinrel is parallel-safe and the join type supports nested loops,
+	 * we may be able to consider a partial nestloop plan.  However, we can't
+	 * handle JOIN_UNIQUE_OUTER, because the outer path will be partial, and
+	 * therefore we won't be able to properly guarantee uniqueness.  Nor can
+	 * we handle extra_lateral_rels, since partial paths must not be
+	 * parameterized.
+	 */
+	if (joinrel->consider_parallel && nestjoinOK &&
+		save_jointype != JOIN_UNIQUE_OUTER &&
+		bms_is_empty(joinrel->lateral_relids))
+		consider_parallel_nestloop(root, joinrel, outerrel, innerrel,
+								   save_jointype, extra);
+}
+
+/*
+ * consider_parallel_nestloop
+ *	  Try to build partial paths for a joinrel by joining a partial path for the
+ *	  outer relation to a complete path for the inner relation.
+ *
+ * 'joinrel' is the join relation
+ * 'outerrel' is the outer join relation
+ * 'innerrel' is the inner join relation
+ * 'jointype' is the type of join to do
+ * 'extra' contains additional input values
+ */
+static void
+consider_parallel_nestloop(PlannerInfo *root,
+						   RelOptInfo *joinrel,
+						   RelOptInfo *outerrel,
+						   RelOptInfo *innerrel,
+						   JoinType jointype,
+						   JoinPathExtraData *extra)
+{
+	ListCell   *lc1;
+
+	foreach(lc1, outerrel->partial_pathlist)
+	{
+		Path	   *outerpath = (Path *) lfirst(lc1);
+		List	   *pathkeys;
+		ListCell   *lc2;
+
+		/* Figure out what useful ordering any paths we create will have. */
+		pathkeys = build_join_pathkeys(root, joinrel, jointype,
+									   outerpath->pathkeys);
+
+		/*
+		 * Try the cheapest parameterized paths; only those which will
+		 * produce an unparameterized path when joined to this outerrel
+		 * will survive try_partial_nestloop_path.  The cheapest
+		 * unparameterized path is also in this list.
+		 */
+		foreach(lc2, innerrel->cheapest_parameterized_paths)
+		{
+			Path	   *innerpath = (Path *) lfirst(lc2);
+
+			/* Can't join to an inner path that is not parallel-safe */
+			if (!innerpath->parallel_safe)
+				continue;
+
+			/*
+			 * Like match_unsorted_outer, we only consider a single nestloop
+			 * path when the jointype is JOIN_UNIQUE_INNER.  But we have to scan
+			 * cheapest_parameterized_paths to find the one we want to consider,
+			 * because cheapest_total_path might not be parallel-safe.
+			 */
+			if (jointype == JOIN_UNIQUE_INNER)
+			{
+				if (!bms_is_empty(PATH_REQ_OUTER(innerpath)))
+					continue;
+				innerpath = (Path *) create_unique_path(root, innerrel,
+											   innerpath, extra->sjinfo);
+			}
+
+			try_partial_nestloop_path(root, joinrel, outerpath, innerpath,
+									  pathkeys, jointype, extra);
+		}
+	}
 }
 
 /*
@@ -1240,6 +1442,55 @@ hash_inner_and_outer(PlannerInfo *root,
 				}
 			}
 		}
+
+		/*
+		 * If the joinrel is parallel-safe, we may be able to consider a
+		 * partial hash join.  However, we can't handle JOIN_UNIQUE_OUTER,
+		 * because the outer path will be partial, and therefore we won't be
+		 * able to properly guarantee uniqueness.  Also, the resulting path
+		 * must not be parameterized.
+		 */
+		if (joinrel->consider_parallel && jointype != JOIN_UNIQUE_OUTER &&
+			outerrel->partial_pathlist != NIL &&
+			bms_is_empty(joinrel->lateral_relids))
+		{
+			Path   *cheapest_partial_outer;
+			Path   *cheapest_safe_inner = NULL;
+
+			cheapest_partial_outer =
+				(Path *) linitial(outerrel->partial_pathlist);
+
+			/*
+			 * Normally, given that the joinrel is parallel-safe, the cheapest
+			 * total inner path will also be parallel-safe, but if not, we'll
+			 * have to search cheapest_parameterized_paths for the cheapest
+			 * unparameterized inner path.
+			 */
+			if (cheapest_total_inner->parallel_safe)
+				cheapest_safe_inner = cheapest_total_inner;
+			else
+			{
+				ListCell   *lc;
+
+				foreach(lc, innerrel->cheapest_parameterized_paths)
+				{
+					Path	   *innerpath = (Path *) lfirst(lc);
+
+					if (innerpath->parallel_safe &&
+						bms_is_empty(PATH_REQ_OUTER(innerpath)))
+					{
+						cheapest_safe_inner = innerpath;
+						break;
+					}
+				}
+			}
+
+			if (cheapest_safe_inner != NULL)
+				try_partial_hashjoin_path(root, joinrel,
+										  cheapest_partial_outer,
+										  cheapest_safe_inner,
+										  hashclauses, jointype, extra);
+		}
 	}
 }
 

src/backend/optimizer/path/joinrels.c

@@ -1194,9 +1194,10 @@ 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);

src/backend/optimizer/plan/createplan.c

@@ -1130,7 +1130,7 @@ 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);
 

src/backend/optimizer/plan/planmain.c

@@ -84,7 +84,8 @@ query_planner(PlannerInfo *root, List *tlist,
 
 		/* The only path for it is a trivial Result path */
 		add_path(final_rel, (Path *)
-				 create_result_path((List *) parse->jointree->quals));
+				 create_result_path(final_rel,
+									(List *) parse->jointree->quals));
 
 		/* Select cheapest path (pretty easy in this case...) */
 		set_cheapest(final_rel);

src/backend/optimizer/util/pathnode.c

@@ -217,7 +217,12 @@ compare_path_costs_fuzzily(Path *path1, Path *path2, double fuzz_factor)
  * The cheapest_parameterized_paths list collects all parameterized paths
  * that have survived the add_path() tournament for this relation.  (Since
  * add_path ignores pathkeys for a parameterized path, these will be paths
- * that have best cost or best row count for their parameterization.)
+ * that have best cost or best row count for their parameterization.  We
+ * may also have both a parallel-safe and a non-parallel-safe path in some
+ * cases for the same parameterization in some cases, but this should be
+ * relatively rare since, most typically, all paths for the same relation
+ * will be parallel-safe or none of them will.)
+ *
  * cheapest_parameterized_paths always includes the cheapest-total
  * unparameterized path, too, if there is one; the users of that list find
  * it more convenient if that's included.
@@ -352,11 +357,12 @@ set_cheapest(RelOptInfo *parent_rel)
  *	  A path is worthy if it has a better sort order (better pathkeys) or
  *	  cheaper cost (on either dimension), or generates fewer rows, than any
  *	  existing path that has the same or superset parameterization rels.
+ *	  We also consider parallel-safe paths more worthy than others.
  *
  *	  We also remove from the rel's pathlist any old paths that are dominated
  *	  by new_path --- that is, new_path is cheaper, at least as well ordered,
- *	  generates no more rows, and requires no outer rels not required by the
- *	  old path.
+ *	  generates no more rows, requires no outer rels not required by the old
+ *	  path, and is no less parallel-safe.
  *
  *	  In most cases, a path with a superset parameterization will generate
  *	  fewer rows (since it has more join clauses to apply), so that those two
@@ -470,14 +476,16 @@ add_path(RelOptInfo *parent_rel, Path *new_path)
 						{
 							if ((outercmp == BMS_EQUAL ||
 								 outercmp == BMS_SUBSET1) &&
-								new_path->rows <= old_path->rows)
+								new_path->rows <= old_path->rows &&
+								new_path->parallel_safe >= old_path->parallel_safe)
 								remove_old = true;		/* new dominates old */
 						}
 						else if (keyscmp == PATHKEYS_BETTER2)
 						{
 							if ((outercmp == BMS_EQUAL ||
 								 outercmp == BMS_SUBSET2) &&
-								new_path->rows >= old_path->rows)
+								new_path->rows >= old_path->rows &&
+								new_path->parallel_safe <= old_path->parallel_safe)
 								accept_new = false;		/* old dominates new */
 						}
 						else	/* keyscmp == PATHKEYS_EQUAL */
@@ -487,19 +495,25 @@ add_path(RelOptInfo *parent_rel, Path *new_path)
 								/*
 								 * Same pathkeys and outer rels, and fuzzily
 								 * the same cost, so keep just one; to decide
-								 * which, first check rows and then do a fuzzy
-								 * cost comparison with very small fuzz limit.
-								 * (We used to do an exact cost comparison,
-								 * but that results in annoying
-								 * platform-specific plan variations due to
-								 * roundoff in the cost estimates.)  If things
-								 * are still tied, arbitrarily keep only the
-								 * old path.  Notice that we will keep only
-								 * the old path even if the less-fuzzy
-								 * comparison decides the startup and total
-								 * costs compare differently.
+								 * which, first check parallel-safety, then
+								 * rows, then do a fuzzy cost comparison with
+								 * very small fuzz limit.  (We used to do an
+								 * exact cost comparison, but that results in
+								 * annoying platform-specific plan variations
+								 * due to roundoff in the cost estimates.)	If
+								 * things are still tied, arbitrarily keep
+								 * only the old path.  Notice that we will
+								 * keep only the old path even if the
+								 * less-fuzzy comparison decides the startup
+								 * and total costs compare differently.
 								 */
-								if (new_path->rows < old_path->rows)
+								if (new_path->parallel_safe >
+									old_path->parallel_safe)
+									remove_old = true;	/* new dominates old */
+								else if (new_path->parallel_safe <
+										 old_path->parallel_safe)
+									accept_new = false; /* old dominates new */
+								else if (new_path->rows < old_path->rows)
 									remove_old = true;	/* new dominates old */
 								else if (new_path->rows > old_path->rows)
 									accept_new = false; /* old dominates new */
@@ -512,10 +526,12 @@ add_path(RelOptInfo *parent_rel, Path *new_path)
 														 * dominates new */
 							}
 							else if (outercmp == BMS_SUBSET1 &&
-									 new_path->rows <= old_path->rows)
+									 new_path->rows <= old_path->rows &&
+									 new_path->parallel_safe >= old_path->parallel_safe)
 								remove_old = true;		/* new dominates old */
 							else if (outercmp == BMS_SUBSET2 &&
-									 new_path->rows >= old_path->rows)
+									 new_path->rows >= old_path->rows &&
+									 new_path->parallel_safe <= old_path->parallel_safe)
 								accept_new = false;		/* old dominates new */
 							/* else different parameterizations, keep both */
 						}
@@ -527,7 +543,8 @@ add_path(RelOptInfo *parent_rel, Path *new_path)
 												   PATH_REQ_OUTER(old_path));
 							if ((outercmp == BMS_EQUAL ||
 								 outercmp == BMS_SUBSET1) &&
-								new_path->rows <= old_path->rows)
+								new_path->rows <= old_path->rows &&
+								new_path->parallel_safe >= old_path->parallel_safe)
 								remove_old = true;		/* new dominates old */
 						}
 						break;
@@ -538,7 +555,8 @@ add_path(RelOptInfo *parent_rel, Path *new_path)
 												   PATH_REQ_OUTER(old_path));
 							if ((outercmp == BMS_EQUAL ||
 								 outercmp == BMS_SUBSET2) &&
-								new_path->rows >= old_path->rows)
+								new_path->rows >= old_path->rows &&
+								new_path->parallel_safe <= old_path->parallel_safe)
 								accept_new = false;		/* old dominates new */
 						}
 						break;
@@ -685,6 +703,214 @@ 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 for several reasons.  Most
+ *	  importantly, they're not safe to execute, because there's nothing to
+ *	  make sure that a parallel scan within the parameterized portion of the
+ *	  plan is running with the same value in every worker at the same time.
+ *	  Fortunately, it seems unlikely to be worthwhile anyway, because having
+ *	  each worker scan the entire outer relation and a subset of the inner
+ *	  relation will generally be a terrible plan.  The inner (parameterized)
+ *	  side of the plan will be small anyway.  There could be rare cases where
+ *	  this wins big - e.g. if join order constraints put a 1-row relation on
+ *	  the outer side of the topmost join with a parameterized plan on the inner
+ *	  side - but we'll have to be content not to handle such cases until somebody
+ *	  builds an executor infrastructure that can cope with them.
+ *
+ *	  Because we don't 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);
+	}
+}
+
+/*
+ * add_partial_path_precheck
+ *	  Check whether a proposed new partial path could possibly get accepted.
+ *
+ * Unlike add_path_precheck, we can ignore startup cost and parameterization,
+ * since they don't matter for partial paths (see add_partial_path).  But
+ * we do want to make sure we don't add a partial path if there's already
+ * a complete path that dominates it, since in that case the proposed path
+ * is surely a loser.
+ */
+bool
+add_partial_path_precheck(RelOptInfo *parent_rel, Cost total_cost,
+						  List *pathkeys)
+{
+	ListCell   *p1;
+
+	/*
+	 * Our goal here is twofold.  First, we want to find out whether this path
+	 * is clearly inferior to some existing partial path.  If so, we want to
+	 * reject it immediately.  Second, we want to find out whether this path
+	 * is clearly superior to some existing partial path -- at least, modulo
+	 * final cost computations.  If so, we definitely want to consider it.
+	 *
+	 * Unlike add_path(), we always compare pathkeys here.  This is because we
+	 * expect partial_pathlist to be very short, and getting a definitive
+	 * answer at this stage avoids the need to call add_path_precheck.
+	 */
+	foreach(p1, parent_rel->partial_pathlist)
+	{
+		Path	   *old_path = (Path *) lfirst(p1);
+		PathKeysComparison keyscmp;
+
+		keyscmp = compare_pathkeys(pathkeys, old_path->pathkeys);
+		if (keyscmp != PATHKEYS_DIFFERENT)
+		{
+			if (total_cost > old_path->total_cost * STD_FUZZ_FACTOR &&
+				keyscmp != PATHKEYS_BETTER1)
+				return false;
+			if (old_path->total_cost > total_cost * STD_FUZZ_FACTOR &&
+				keyscmp != PATHKEYS_BETTER2)
+				return true;
+		}
+	}
+
+	/*
+	 * This path is neither clearly inferior to an existing partial path nor
+	 * clearly good enough that it might replace one.  Compare it to
+	 * non-parallel plans.  If it loses even before accounting for the cost of
+	 * the Gather node, we should definitely reject it.
+	 *
+	 * Note that we pass the total_cost to add_path_precheck twice.  This is
+	 * because it's never advantageous to consider the startup cost of a
+	 * partial path; the resulting plans, if run in parallel, will be run to
+	 * completion.
+	 */
+	if (!add_path_precheck(parent_rel, total_cost, total_cost, pathkeys,
+						   NULL))
+		return false;
+
+	return true;
+}
+
 
 /*****************************************************************************
  *		PATH NODE CREATION ROUTINES
@@ -697,7 +923,7 @@ 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 +931,12 @@ 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_safe = rel->consider_parallel;
+	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 +955,8 @@ 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_safe = rel->consider_parallel;
+	pathnode->parallel_degree = 0;
 	pathnode->pathkeys = NIL;	/* samplescan has unordered result */
 
 	cost_samplescan(pathnode, root, rel, pathnode->param_info);
@@ -781,6 +1011,8 @@ create_index_path(PlannerInfo *root,
 	pathnode->path.param_info = get_baserel_parampathinfo(root, rel,
 														  required_outer);
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_safe = rel->consider_parallel;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.pathkeys = pathkeys;
 
 	/* Convert clauses to indexquals the executor can handle */
@@ -827,6 +1059,8 @@ 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_safe = bitmapqual->parallel_safe;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.pathkeys = NIL;		/* always unordered */
 
 	pathnode->bitmapqual = bitmapqual;
@@ -852,7 +1086,17 @@ create_bitmap_and_path(PlannerInfo *root,
 	pathnode->path.pathtype = T_BitmapAnd;
 	pathnode->path.parent = rel;
 	pathnode->path.param_info = NULL;	/* not used in bitmap trees */
+
+	/*
+	 * Currently, a BitmapHeapPath, BitmapAndPath, or BitmapOrPath will be
+	 * parallel-safe if and only if rel->consider_parallel is set.  So, we can
+	 * set the flag for this path based only on the relation-level flag,
+	 * without actually iterating over the list of children.
+	 */
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_safe = rel->consider_parallel;
+	pathnode->path.parallel_degree = 0;
+
 	pathnode->path.pathkeys = NIL;		/* always unordered */
 
 	pathnode->bitmapquals = bitmapquals;
@@ -877,7 +1121,17 @@ create_bitmap_or_path(PlannerInfo *root,
 	pathnode->path.pathtype = T_BitmapOr;
 	pathnode->path.parent = rel;
 	pathnode->path.param_info = NULL;	/* not used in bitmap trees */
+
+	/*
+	 * Currently, a BitmapHeapPath, BitmapAndPath, or BitmapOrPath will be
+	 * parallel-safe if and only if rel->consider_parallel is set.  So, we can
+	 * set the flag for this path based only on the relation-level flag,
+	 * without actually iterating over the list of children.
+	 */
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_safe = rel->consider_parallel;
+	pathnode->path.parallel_degree = 0;
+
 	pathnode->path.pathkeys = NIL;		/* always unordered */
 
 	pathnode->bitmapquals = bitmapquals;
@@ -903,6 +1157,8 @@ 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_safe = rel->consider_parallel;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.pathkeys = NIL;		/* always unordered */
 
 	pathnode->tidquals = tidquals;
@@ -921,7 +1177,8 @@ 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 +1188,8 @@ 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_safe = rel->consider_parallel;
+	pathnode->path.parallel_degree = parallel_degree;
 	pathnode->path.pathkeys = NIL;		/* result is always considered
 										 * unsorted */
 	pathnode->subpaths = subpaths;
@@ -955,6 +1214,8 @@ create_append_path(RelOptInfo *rel, List *subpaths, Relids required_outer)
 		if (l == list_head(subpaths))	/* first node? */
 			pathnode->path.startup_cost = subpath->startup_cost;
 		pathnode->path.total_cost += subpath->total_cost;
+		pathnode->path.parallel_safe = pathnode->path.parallel_safe &&
+			subpath->parallel_safe;
 
 		/* All child paths must have same parameterization */
 		Assert(bms_equal(PATH_REQ_OUTER(subpath), required_outer));
@@ -985,6 +1246,8 @@ create_merge_append_path(PlannerInfo *root,
 	pathnode->path.param_info = get_appendrel_parampathinfo(rel,
 															required_outer);
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_safe = rel->consider_parallel;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.pathkeys = pathkeys;
 	pathnode->subpaths = subpaths;
 
@@ -1008,6 +1271,8 @@ create_merge_append_path(PlannerInfo *root,
 		Path	   *subpath = (Path *) lfirst(l);
 
 		pathnode->path.rows += subpath->rows;
+		pathnode->path.parallel_safe = pathnode->path.parallel_safe &&
+			subpath->parallel_safe;
 
 		if (pathkeys_contained_in(pathkeys, subpath->pathkeys))
 		{
@@ -1052,7 +1317,7 @@ create_merge_append_path(PlannerInfo *root,
  *	  This is only used for the case of a query with an empty jointree.
  */
 ResultPath *
-create_result_path(List *quals)
+create_result_path(RelOptInfo *rel, List *quals)
 {
 	ResultPath *pathnode = makeNode(ResultPath);
 
@@ -1060,6 +1325,8 @@ 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_safe = rel->consider_parallel;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.pathkeys = NIL;
 	pathnode->quals = quals;
 
@@ -1094,6 +1361,8 @@ 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_safe = subpath->parallel_safe;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.pathkeys = subpath->pathkeys;
 
 	pathnode->subpath = subpath;
@@ -1155,6 +1424,8 @@ 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_safe = subpath->parallel_safe;
+	pathnode->path.parallel_degree = 0;
 
 	/*
 	 * Assume the output is unsorted, since we don't necessarily have pathkeys
@@ -1328,19 +1599,30 @@ 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);
 
+	Assert(subpath->parallel_safe);
+
 	pathnode->path.pathtype = T_Gather;
 	pathnode->path.parent = rel;
 	pathnode->path.param_info = get_baserel_parampathinfo(root, rel,
 														  required_outer);
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_safe = 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 +1675,8 @@ create_subqueryscan_path(PlannerInfo *root, RelOptInfo *rel,
 	pathnode->param_info = get_baserel_parampathinfo(root, rel,
 													 required_outer);
 	pathnode->parallel_aware = false;
+	pathnode->parallel_safe = rel->consider_parallel;
+	pathnode->parallel_degree = 0;
 	pathnode->pathkeys = pathkeys;
 
 	cost_subqueryscan(pathnode, root, rel, pathnode->param_info);
@@ -1416,6 +1700,8 @@ create_functionscan_path(PlannerInfo *root, RelOptInfo *rel,
 	pathnode->param_info = get_baserel_parampathinfo(root, rel,
 													 required_outer);
 	pathnode->parallel_aware = false;
+	pathnode->parallel_safe = rel->consider_parallel;
+	pathnode->parallel_degree = 0;
 	pathnode->pathkeys = pathkeys;
 
 	cost_functionscan(pathnode, root, rel, pathnode->param_info);
@@ -1439,6 +1725,8 @@ create_valuesscan_path(PlannerInfo *root, RelOptInfo *rel,
 	pathnode->param_info = get_baserel_parampathinfo(root, rel,
 													 required_outer);
 	pathnode->parallel_aware = false;
+	pathnode->parallel_safe = rel->consider_parallel;
+	pathnode->parallel_degree = 0;
 	pathnode->pathkeys = NIL;	/* result is always unordered */
 
 	cost_valuesscan(pathnode, root, rel, pathnode->param_info);
@@ -1461,6 +1749,8 @@ 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_safe = rel->consider_parallel;
+	pathnode->parallel_degree = 0;
 	pathnode->pathkeys = NIL;	/* XXX for now, result is always unordered */
 
 	cost_ctescan(pathnode, root, rel, pathnode->param_info);
@@ -1484,6 +1774,8 @@ create_worktablescan_path(PlannerInfo *root, RelOptInfo *rel,
 	pathnode->param_info = get_baserel_parampathinfo(root, rel,
 													 required_outer);
 	pathnode->parallel_aware = false;
+	pathnode->parallel_safe = rel->consider_parallel;
+	pathnode->parallel_degree = 0;
 	pathnode->pathkeys = NIL;	/* result is always unordered */
 
 	/* Cost is the same as for a regular CTE scan */
@@ -1517,6 +1809,8 @@ 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_safe = rel->consider_parallel;
+	pathnode->path.parallel_degree = 0;
 	pathnode->path.rows = rows;
 	pathnode->path.startup_cost = startup_cost;
 	pathnode->path.total_cost = total_cost;
@@ -1653,6 +1947,10 @@ create_nestloop_path(PlannerInfo *root,
 								  required_outer,
 								  &restrict_clauses);
 	pathnode->path.parallel_aware = false;
+	pathnode->path.parallel_safe = joinrel->consider_parallel &&
+		outer_path->parallel_safe && inner_path->parallel_safe;
+	/* This is a foolish way to estimate parallel_degree, but for now... */
+	pathnode->path.parallel_degree = outer_path->parallel_degree;
 	pathnode->path.pathkeys = pathkeys;
 	pathnode->jointype = jointype;
 	pathnode->outerjoinpath = outer_path;
@@ -1711,6 +2009,9 @@ create_mergejoin_path(PlannerInfo *root,
 								  required_outer,
 								  &restrict_clauses);
 	pathnode->jpath.path.parallel_aware = false;
+	pathnode->jpath.path.parallel_safe = joinrel->consider_parallel &&
+		outer_path->parallel_safe && inner_path->parallel_safe;
+	pathnode->jpath.path.parallel_degree = 0;
 	pathnode->jpath.path.pathkeys = pathkeys;
 	pathnode->jpath.jointype = jointype;
 	pathnode->jpath.outerjoinpath = outer_path;
@@ -1768,6 +2069,10 @@ create_hashjoin_path(PlannerInfo *root,
 								  required_outer,
 								  &restrict_clauses);
 	pathnode->jpath.path.parallel_aware = false;
+	pathnode->jpath.path.parallel_safe = joinrel->consider_parallel &&
+		outer_path->parallel_safe && inner_path->parallel_safe;
+	/* This is a foolish way to estimate parallel_degree, but for now... */
+	pathnode->jpath.path.parallel_degree = outer_path->parallel_degree;
 
 	/*
 	 * A hashjoin never has pathkeys, since its output ordering is

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;
@@ -370,6 +371,7 @@ 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;

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;
@@ -761,6 +762,8 @@ 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? */
+	bool		parallel_safe;	/* OK to use as part of parallel plan? */
+	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 */
@@ -1064,7 +1067,6 @@ 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;
 

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,

src/include/optimizer/pathnode.h

@@ -29,9 +29,12 @@ 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 bool add_partial_path_precheck(RelOptInfo *parent_rel,
+						  Cost total_cost, List *pathkeys);
 
 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,
@@ -59,19 +62,18 @@ 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,
 						 List *pathkeys,
 						 Relids required_outer);
-extern ResultPath *create_result_path(List *quals);
+extern ResultPath *create_result_path(RelOptInfo *rel, List *quals);
 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,

src/include/optimizer/paths.h

@@ -50,6 +50,8 @@ extern RelOptInfo *make_one_rel(PlannerInfo *root, List *joinlist);
 extern RelOptInfo *standard_join_search(PlannerInfo *root, int levels_needed,
 					 List *initial_rels);
 
+extern void generate_gather_paths(PlannerInfo *root, RelOptInfo *rel);
+
 #ifdef OPTIMIZER_DEBUG
 extern void debug_print_rel(PlannerInfo *root, RelOptInfo *rel);
 #endif