Force rescanning of parallel-aware scan nodes below a Gather[Merge].

The ExecReScan machinery contains various optimizations for postponing
or skipping rescans of plan subtrees; for example a HashAgg node may
conclude that it can re-use the table it built before, instead of
re-reading its input subtree.  But that is wrong if the input contains
a parallel-aware table scan node, since the portion of the table scanned
by the leader process is likely to vary from one rescan to the next.
This explains the timing-dependent buildfarm failures we saw after
commit a2b70c89.

The established mechanism for showing that a plan node's output is
potentially variable is to mark it as depending on some runtime Param.
Hence, to fix this, invent a dummy Param (one that has a PARAM_EXEC
parameter number, but carries no actual value) associated with each Gather
or GatherMerge node, mark parallel-aware nodes below that node as dependent
on that Param, and arrange for ExecReScanGather[Merge] to flag that Param
as changed whenever the Gather[Merge] node is rescanned.

This solution breaks an undocumented assumption made by the parallel
executor logic, namely that all rescans of nodes below a Gather[Merge]
will happen synchronously during the ReScan of the top node itself.
But that's fundamentally contrary to the design of the ExecReScan code,
and so was doomed to fail someday anyway (even if you want to argue
that the bug being fixed here wasn't a failure of that assumption).
A follow-on patch will address that issue.  In the meantime, the worst
that's expected to happen is that given very bad timing luck, the leader
might have to do all the work during a rescan, because workers think
they have nothing to do, if they are able to start up before the eventual
ReScan of the leader's parallel-aware table scan node has reset the
shared scan state.

Although this problem exists in 9.6, there does not seem to be any way
for it to manifest there.  Without GatherMerge, it seems that a plan tree
that has a rescan-short-circuiting node below Gather will always also
have one above it that will short-circuit in the same cases, preventing
the Gather from being rescanned.  Hence we won't take the risk of
back-patching this change into 9.6.  But v10 needs it.

Discussion: https://postgr.es/m/CAA4eK1JkByysFJNh9M349u_nNjqETuEnY_y1VUc_kJiU0bxtaQ@mail.gmail.com

src/backend/executor/nodeGather.c

@@ -432,6 +432,9 @@ ExecShutdownGather(GatherState *node)
 void
 ExecReScanGather(GatherState *node)
 {
+	Gather	   *gather = (Gather *) node->ps.plan;
+	PlanState  *outerPlan = outerPlanState(node);
+
 	/*
 	 * Re-initialize the parallel workers to perform rescan of relation. We
 	 * want to gracefully shutdown all the workers so that they should be able
@@ -445,5 +448,22 @@ ExecReScanGather(GatherState *node)
 	if (node->pei)
 		ExecParallelReinitialize(node->pei);
 
-	ExecReScan(node->ps.lefttree);
+	/*
+	 * Set child node's chgParam to tell it that the next scan might deliver a
+	 * different set of rows within the leader process.  (The overall rowset
+	 * shouldn't change, but the leader process's subset might; hence nodes
+	 * between here and the parallel table scan node mustn't optimize on the
+	 * assumption of an unchanging rowset.)
+	 */
+	if (gather->rescan_param >= 0)
+		outerPlan->chgParam = bms_add_member(outerPlan->chgParam,
+											 gather->rescan_param);
+
+
+	/*
+	 * if chgParam of subnode is not null then plan will be re-scanned by
+	 * first ExecProcNode.
+	 */
+	if (outerPlan->chgParam == NULL)
+		ExecReScan(outerPlan);
 }

src/backend/executor/nodeGatherMerge.c

@@ -327,6 +327,9 @@ ExecShutdownGatherMergeWorkers(GatherMergeState *node)
 void
 ExecReScanGatherMerge(GatherMergeState *node)
 {
+	GatherMerge *gm = (GatherMerge *) node->ps.plan;
+	PlanState  *outerPlan = outerPlanState(node);
+
 	/*
 	 * Re-initialize the parallel workers to perform rescan of relation. We
 	 * want to gracefully shutdown all the workers so that they should be able
@@ -341,7 +344,24 @@ ExecReScanGatherMerge(GatherMergeState *node)
 	if (node->pei)
 		ExecParallelReinitialize(node->pei);
 
-	ExecReScan(node->ps.lefttree);
+	/*
+	 * Set child node's chgParam to tell it that the next scan might deliver a
+	 * different set of rows within the leader process.  (The overall rowset
+	 * shouldn't change, but the leader process's subset might; hence nodes
+	 * between here and the parallel table scan node mustn't optimize on the
+	 * assumption of an unchanging rowset.)
+	 */
+	if (gm->rescan_param >= 0)
+		outerPlan->chgParam = bms_add_member(outerPlan->chgParam,
+											 gm->rescan_param);
+
+
+	/*
+	 * if chgParam of subnode is not null then plan will be re-scanned by
+	 * first ExecProcNode.
+	 */
+	if (outerPlan->chgParam == NULL)
+		ExecReScan(outerPlan);
 }
 
 /*

src/backend/nodes/copyfuncs.c

@@ -361,6 +361,7 @@ _copyGather(const Gather *from)
 	 * copy remainder of node
 	 */
 	COPY_SCALAR_FIELD(num_workers);
+	COPY_SCALAR_FIELD(rescan_param);
 	COPY_SCALAR_FIELD(single_copy);
 	COPY_SCALAR_FIELD(invisible);
 
@@ -384,6 +385,7 @@ _copyGatherMerge(const GatherMerge *from)
 	 * copy remainder of node
 	 */
 	COPY_SCALAR_FIELD(num_workers);
+	COPY_SCALAR_FIELD(rescan_param);
 	COPY_SCALAR_FIELD(numCols);
 	COPY_POINTER_FIELD(sortColIdx, from->numCols * sizeof(AttrNumber));
 	COPY_POINTER_FIELD(sortOperators, from->numCols * sizeof(Oid));

src/backend/nodes/outfuncs.c

@@ -479,6 +479,7 @@ _outGather(StringInfo str, const Gather *node)
 	_outPlanInfo(str, (const Plan *) node);
 
 	WRITE_INT_FIELD(num_workers);
+	WRITE_INT_FIELD(rescan_param);
 	WRITE_BOOL_FIELD(single_copy);
 	WRITE_BOOL_FIELD(invisible);
 }
@@ -493,6 +494,7 @@ _outGatherMerge(StringInfo str, const GatherMerge *node)
 	_outPlanInfo(str, (const Plan *) node);
 
 	WRITE_INT_FIELD(num_workers);
+	WRITE_INT_FIELD(rescan_param);
 	WRITE_INT_FIELD(numCols);
 
 	appendStringInfoString(str, " :sortColIdx");

src/backend/nodes/readfuncs.c

@@ -2163,6 +2163,7 @@ _readGather(void)
 	ReadCommonPlan(&local_node->plan);
 
 	READ_INT_FIELD(num_workers);
+	READ_INT_FIELD(rescan_param);
 	READ_BOOL_FIELD(single_copy);
 	READ_BOOL_FIELD(invisible);
 
@@ -2180,6 +2181,7 @@ _readGatherMerge(void)
 	ReadCommonPlan(&local_node->plan);
 
 	READ_INT_FIELD(num_workers);
+	READ_INT_FIELD(rescan_param);
 	READ_INT_FIELD(numCols);
 	READ_ATTRNUMBER_ARRAY(sortColIdx, local_node->numCols);
 	READ_OID_ARRAY(sortOperators, local_node->numCols);

src/backend/optimizer/README

@@ -374,6 +374,7 @@ RelOptInfo      - a relation or joined relations
   MaterialPath  - a Material plan node
   UniquePath    - remove duplicate rows (either by hashing or sorting)
   GatherPath    - collect the results of parallel workers
+  GatherMergePath - collect parallel results, preserving their common sort order
   ProjectionPath - a Result plan node with child (used for projection)
   ProjectSetPath - a ProjectSet plan node applied to some sub-path
   SortPath      - a Sort plan node applied to some sub-path
@@ -1030,7 +1031,7 @@ 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
+fuller description.  The academic literature on parallel query suggests
 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
@@ -1046,16 +1047,14 @@ 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.
+collecting the results.  Requirements (2) and (3) are satisfied by the
+executor node Gather (or GatherMerge), 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 table scan 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 scan node is executed.
 
 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

src/backend/optimizer/plan/createplan.c

@@ -267,7 +267,7 @@ static Unique *make_unique_from_sortclauses(Plan *lefttree, List *distinctList);
 static Unique *make_unique_from_pathkeys(Plan *lefttree,
 						  List *pathkeys, int numCols);
 static Gather *make_gather(List *qptlist, List *qpqual,
-			int nworkers, bool single_copy, Plan *subplan);
+			int nworkers, int rescan_param, bool single_copy, Plan *subplan);
 static SetOp *make_setop(SetOpCmd cmd, SetOpStrategy strategy, Plan *lefttree,
 		   List *distinctList, AttrNumber flagColIdx, int firstFlag,
 		   long numGroups);
@@ -1471,6 +1471,7 @@ create_gather_plan(PlannerInfo *root, GatherPath *best_path)
 	gather_plan = make_gather(tlist,
 							  NIL,
 							  best_path->num_workers,
+							  SS_assign_special_param(root),
 							  best_path->single_copy,
 							  subplan);
 
@@ -1505,6 +1506,9 @@ create_gather_merge_plan(PlannerInfo *root, GatherMergePath *best_path)
 	gm_plan->num_workers = best_path->num_workers;
 	copy_generic_path_info(&gm_plan->plan, &best_path->path);
 
+	/* Assign the rescan Param. */
+	gm_plan->rescan_param = SS_assign_special_param(root);
+
 	/* Gather Merge is pointless with no pathkeys; use Gather instead. */
 	Assert(pathkeys != NIL);
 
@@ -6238,6 +6242,7 @@ static Gather *
 make_gather(List *qptlist,
 			List *qpqual,
 			int nworkers,
+			int rescan_param,
 			bool single_copy,
 			Plan *subplan)
 {
@@ -6249,6 +6254,7 @@ make_gather(List *qptlist,
 	plan->lefttree = subplan;
 	plan->righttree = NULL;
 	node->num_workers = nworkers;
+	node->rescan_param = rescan_param;
 	node->single_copy = single_copy;
 	node->invisible = false;
 

src/backend/optimizer/plan/planner.c

@@ -374,6 +374,12 @@ standard_planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
 		gather->single_copy = true;
 		gather->invisible = (force_parallel_mode == FORCE_PARALLEL_REGRESS);
 
+		/*
+		 * Since this Gather has no parallel-aware descendants to signal to,
+		 * we don't need a rescan Param.
+		 */
+		gather->rescan_param = -1;
+
 		/*
 		 * Ideally we'd use cost_gather here, but setting up dummy path data
 		 * to satisfy it doesn't seem much cleaner than knowing what it does.

src/backend/optimizer/plan/subselect.c

@@ -79,6 +79,7 @@ static Node *process_sublinks_mutator(Node *node,
 						 process_sublinks_context *context);
 static Bitmapset *finalize_plan(PlannerInfo *root,
 			  Plan *plan,
+			  int gather_param,
 			  Bitmapset *valid_params,
 			  Bitmapset *scan_params);
 static bool finalize_primnode(Node *node, finalize_primnode_context *context);
@@ -2217,12 +2218,15 @@ void
 SS_finalize_plan(PlannerInfo *root, Plan *plan)
 {
 	/* No setup needed, just recurse through plan tree. */
-	(void) finalize_plan(root, plan, root->outer_params, NULL);
+	(void) finalize_plan(root, plan, -1, root->outer_params, NULL);
 }
 
 /*
  * Recursive processing of all nodes in the plan tree
  *
+ * gather_param is the rescan_param of an ancestral Gather/GatherMerge,
+ * or -1 if there is none.
+ *
  * valid_params is the set of param IDs supplied by outer plan levels
  * that are valid to reference in this plan node or its children.
  *
@@ -2249,7 +2253,9 @@ SS_finalize_plan(PlannerInfo *root, Plan *plan)
  * can be handled more cleanly.
  */
 static Bitmapset *
-finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
+finalize_plan(PlannerInfo *root, Plan *plan,
+			  int gather_param,
+			  Bitmapset *valid_params,
 			  Bitmapset *scan_params)
 {
 	finalize_primnode_context context;
@@ -2302,6 +2308,18 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 	finalize_primnode((Node *) plan->targetlist, &context);
 	finalize_primnode((Node *) plan->qual, &context);
 
+	/*
+	 * If it's a parallel-aware scan node, mark it as dependent on the parent
+	 * Gather/GatherMerge's rescan Param.
+	 */
+	if (plan->parallel_aware)
+	{
+		if (gather_param < 0)
+			elog(ERROR, "parallel-aware plan node is not below a Gather");
+		context.paramids =
+			bms_add_member(context.paramids, gather_param);
+	}
+
 	/* Check additional node-type-specific fields */
 	switch (nodeTag(plan))
 	{
@@ -2512,6 +2530,7 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 						bms_add_members(context.paramids,
 										finalize_plan(root,
 													  (Plan *) lfirst(lc),
+													  gather_param,
 													  valid_params,
 													  scan_params));
 				}
@@ -2542,6 +2561,7 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 						bms_add_members(context.paramids,
 										finalize_plan(root,
 													  (Plan *) lfirst(l),
+													  gather_param,
 													  valid_params,
 													  scan_params));
 				}
@@ -2558,6 +2578,7 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 						bms_add_members(context.paramids,
 										finalize_plan(root,
 													  (Plan *) lfirst(l),
+													  gather_param,
 													  valid_params,
 													  scan_params));
 				}
@@ -2574,6 +2595,7 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 						bms_add_members(context.paramids,
 										finalize_plan(root,
 													  (Plan *) lfirst(l),
+													  gather_param,
 													  valid_params,
 													  scan_params));
 				}
@@ -2590,6 +2612,7 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 						bms_add_members(context.paramids,
 										finalize_plan(root,
 													  (Plan *) lfirst(l),
+													  gather_param,
 													  valid_params,
 													  scan_params));
 				}
@@ -2606,6 +2629,7 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 						bms_add_members(context.paramids,
 										finalize_plan(root,
 													  (Plan *) lfirst(l),
+													  gather_param,
 													  valid_params,
 													  scan_params));
 				}
@@ -2697,13 +2721,51 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 							  &context);
 			break;
 
+		case T_Gather:
+			/* child nodes are allowed to reference rescan_param, if any */
+			locally_added_param = ((Gather *) plan)->rescan_param;
+			if (locally_added_param >= 0)
+			{
+				valid_params = bms_add_member(bms_copy(valid_params),
+											  locally_added_param);
+
+				/*
+				 * We currently don't support nested Gathers.  The issue so
+				 * far as this function is concerned would be how to identify
+				 * which child nodes depend on which Gather.
+				 */
+				Assert(gather_param < 0);
+				/* Pass down rescan_param to child parallel-aware nodes */
+				gather_param = locally_added_param;
+			}
+			/* rescan_param does *not* get added to scan_params */
+			break;
+
+		case T_GatherMerge:
+			/* child nodes are allowed to reference rescan_param, if any */
+			locally_added_param = ((GatherMerge *) plan)->rescan_param;
+			if (locally_added_param >= 0)
+			{
+				valid_params = bms_add_member(bms_copy(valid_params),
+											  locally_added_param);
+
+				/*
+				 * We currently don't support nested Gathers.  The issue so
+				 * far as this function is concerned would be how to identify
+				 * which child nodes depend on which Gather.
+				 */
+				Assert(gather_param < 0);
+				/* Pass down rescan_param to child parallel-aware nodes */
+				gather_param = locally_added_param;
+			}
+			/* rescan_param does *not* get added to scan_params */
+			break;
+
 		case T_ProjectSet:
 		case T_Hash:
 		case T_Material:
 		case T_Sort:
 		case T_Unique:
-		case T_Gather:
-		case T_GatherMerge:
 		case T_SetOp:
 		case T_Group:
 			/* no node-type-specific fields need fixing */
@@ -2717,6 +2779,7 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 	/* Process left and right child plans, if any */
 	child_params = finalize_plan(root,
 								 plan->lefttree,
+								 gather_param,
 								 valid_params,
 								 scan_params);
 	context.paramids = bms_add_members(context.paramids, child_params);
@@ -2726,6 +2789,7 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 		/* right child can reference nestloop_params as well as valid_params */
 		child_params = finalize_plan(root,
 									 plan->righttree,
+									 gather_param,
 									 bms_union(nestloop_params, valid_params),
 									 scan_params);
 		/* ... and they don't count as parameters used at my level */
@@ -2737,6 +2801,7 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 		/* easy case */
 		child_params = finalize_plan(root,
 									 plan->righttree,
+									 gather_param,
 									 valid_params,
 									 scan_params);
 	}

src/include/nodes/plannodes.h

@@ -825,13 +825,21 @@ typedef struct Unique
 
 /* ------------
  *		gather node
+ *
+ * Note: rescan_param is the ID of a PARAM_EXEC parameter slot.  That slot
+ * will never actually contain a value, but the Gather node must flag it as
+ * having changed whenever it is rescanned.  The child parallel-aware scan
+ * nodes are marked as depending on that parameter, so that the rescan
+ * machinery is aware that their output is likely to change across rescans.
+ * In some cases we don't need a rescan Param, so rescan_param is set to -1.
  * ------------
  */
 typedef struct Gather
 {
 	Plan		plan;
-	int			num_workers;
-	bool		single_copy;
+	int			num_workers;	/* planned number of worker processes */
+	int			rescan_param;	/* ID of Param that signals a rescan, or -1 */
+	bool		single_copy;	/* don't execute plan more than once */
 	bool		invisible;		/* suppress EXPLAIN display (for testing)? */
 } Gather;
 
@@ -842,7 +850,8 @@ typedef struct Gather
 typedef struct GatherMerge
 {
 	Plan		plan;
-	int			num_workers;
+	int			num_workers;	/* planned number of worker processes */
+	int			rescan_param;	/* ID of Param that signals a rescan, or -1 */
 	/* remaining fields are just like the sort-key info in struct Sort */
 	int			numCols;		/* number of sort-key columns */
 	AttrNumber *sortColIdx;		/* their indexes in the target list */

src/include/nodes/relation.h

@@ -1268,9 +1268,9 @@ typedef struct GatherPath
 } GatherPath;
 
 /*
- * GatherMergePath runs several copies of a plan in parallel and
- * collects the results. For gather merge parallel leader always execute the
- * plan.
+ * GatherMergePath runs several copies of a plan in parallel and collects
+ * the results, preserving their common sort order.  For gather merge, the
+ * parallel leader always executes the plan too, so we don't need single_copy.
  */
 typedef struct GatherMergePath
 {