Pass InitPlan values to workers via Gather (Merge).

If a PARAM_EXEC parameter is used below a Gather (Merge) but the InitPlan
that computes it is attached to or above the Gather (Merge), force the
value to be computed before starting parallelism and pass it down to all
workers.  This allows us to use parallelism in cases where it previously
would have had to be rejected as unsafe.  We do - in this case - lose the
optimization that the value is only computed if it's actually used.  An
alternative strategy would be to have the first worker that needs the value
compute it, but one downside of that approach is that we'd then need to
select a parallel-safe path to compute the parameter value; it couldn't for
example contain a Gather (Merge) node.  At some point in the future, we
might want to consider both approaches.

Independent of that consideration, there is a great deal more work that
could be done to make more kinds of PARAM_EXEC parameters parallel-safe.
This infrastructure could be used to allow a Gather (Merge) on the inner
side of a nested loop (although that's not a very appealing plan) and
cases where the InitPlan is attached below the Gather (Merge) could be
addressed as well using various techniques.  But this is a good start.

Amit Kapila, reviewed and revised by me.  Reviewing and testing from
Kuntal Ghosh, Haribabu Kommi, and Tushar Ahuja.

Discussion: http://postgr.es/m/CAA4eK1LV0Y1AUV4cUCdC+sYOx0Z0-8NAJ2Pd9=UKsbQ5Sr7+JQ@mail.gmail.com

src/backend/commands/explain.c

@@ -107,6 +107,7 @@ static void show_tidbitmap_info(BitmapHeapScanState *planstate,
 static void show_instrumentation_count(const char *qlabel, int which,
 						   PlanState *planstate, ExplainState *es);
 static void show_foreignscan_info(ForeignScanState *fsstate, ExplainState *es);
+static void show_eval_params(Bitmapset *bms_params, ExplainState *es);
 static const char *explain_get_index_name(Oid indexId);
 static void show_buffer_usage(ExplainState *es, const BufferUsage *usage);
 static void ExplainIndexScanDetails(Oid indexid, ScanDirection indexorderdir,
@@ -1441,6 +1442,11 @@ ExplainNode(PlanState *planstate, List *ancestors,
 											   planstate, es);
 				ExplainPropertyInteger("Workers Planned",
 									   gather->num_workers, es);
+
+				/* Show params evaluated at gather node */
+				if (gather->initParam)
+					show_eval_params(gather->initParam, es);
+
 				if (es->analyze)
 				{
 					int			nworkers;
@@ -1463,6 +1469,11 @@ ExplainNode(PlanState *planstate, List *ancestors,
 											   planstate, es);
 				ExplainPropertyInteger("Workers Planned",
 									   gm->num_workers, es);
+
+				/* Show params evaluated at gather-merge node */
+				if (gm->initParam)
+					show_eval_params(gm->initParam, es);
+
 				if (es->analyze)
 				{
 					int			nworkers;
@@ -2487,6 +2498,29 @@ show_foreignscan_info(ForeignScanState *fsstate, ExplainState *es)
 	}
 }
 
+/*
+ * Show initplan params evaluated at Gather or Gather Merge node.
+ */
+static void
+show_eval_params(Bitmapset *bms_params, ExplainState *es)
+{
+	int			paramid = -1;
+	List	   *params = NIL;
+
+	Assert(bms_params);
+
+	while ((paramid = bms_next_member(bms_params, paramid)) >= 0)
+	{
+		char		param[32];
+
+		snprintf(param, sizeof(param), "$%d", paramid);
+		params = lappend(params, pstrdup(param));
+	}
+
+	if (params)
+		ExplainPropertyList("Params Evaluated", params, es);
+}
+
 /*
  * Fetch the name of an index in an EXPLAIN
  *

src/backend/executor/execExprInterp.c

@@ -1926,6 +1926,33 @@ ExecEvalParamExec(ExprState *state, ExprEvalStep *op, ExprContext *econtext)
 	*op->resnull = prm->isnull;
 }
 
+/*
+ * ExecEvalParamExecParams
+ *
+ * Execute the subplan stored in PARAM_EXEC initplans params, if not executed
+ * till now.
+ */
+void
+ExecEvalParamExecParams(Bitmapset *params, EState *estate)
+{
+	ParamExecData *prm;
+	int			paramid;
+
+	paramid = -1;
+	while ((paramid = bms_next_member(params, paramid)) >= 0)
+	{
+		prm = &(estate->es_param_exec_vals[paramid]);
+
+		if (prm->execPlan != NULL)
+		{
+			/* Parameter not evaluated yet, so go do it */
+			ExecSetParamPlan(prm->execPlan, GetPerTupleExprContext(estate));
+			/* ExecSetParamPlan should have processed this param... */
+			Assert(prm->execPlan == NULL);
+		}
+	}
+}
+
 /*
  * Evaluate a PARAM_EXTERN parameter.
  *

src/backend/executor/execParallel.c

@@ -23,6 +23,7 @@
 
 #include "postgres.h"
 
+#include "executor/execExpr.h"
 #include "executor/execParallel.h"
 #include "executor/executor.h"
 #include "executor/nodeBitmapHeapscan.h"
@@ -38,7 +39,9 @@
 #include "optimizer/planner.h"
 #include "storage/spin.h"
 #include "tcop/tcopprot.h"
+#include "utils/datum.h"
 #include "utils/dsa.h"
+#include "utils/lsyscache.h"
 #include "utils/memutils.h"
 #include "utils/snapmgr.h"
 #include "pgstat.h"
@@ -50,7 +53,7 @@
  */
 #define PARALLEL_KEY_EXECUTOR_FIXED		UINT64CONST(0xE000000000000001)
 #define PARALLEL_KEY_PLANNEDSTMT		UINT64CONST(0xE000000000000002)
-#define PARALLEL_KEY_PARAMS				UINT64CONST(0xE000000000000003)
+#define PARALLEL_KEY_PARAMLISTINFO		UINT64CONST(0xE000000000000003)
 #define PARALLEL_KEY_BUFFER_USAGE		UINT64CONST(0xE000000000000004)
 #define PARALLEL_KEY_TUPLE_QUEUE		UINT64CONST(0xE000000000000005)
 #define PARALLEL_KEY_INSTRUMENTATION	UINT64CONST(0xE000000000000006)
@@ -65,6 +68,7 @@
 typedef struct FixedParallelExecutorState
 {
 	int64		tuples_needed;	/* tuple bound, see ExecSetTupleBound */
+	dsa_pointer param_exec;
 } FixedParallelExecutorState;
 
 /*
@@ -266,6 +270,133 @@ ExecParallelEstimate(PlanState *planstate, ExecParallelEstimateContext *e)
 	return planstate_tree_walker(planstate, ExecParallelEstimate, e);
 }
 
+/*
+ * Estimate the amount of space required to serialize the indicated parameters.
+ */
+static Size
+EstimateParamExecSpace(EState *estate, Bitmapset *params)
+{
+	int			paramid;
+	Size		sz = sizeof(int);
+
+	paramid = -1;
+	while ((paramid = bms_next_member(params, paramid)) >= 0)
+	{
+		Oid			typeOid;
+		int16		typLen;
+		bool		typByVal;
+		ParamExecData *prm;
+
+		prm = &(estate->es_param_exec_vals[paramid]);
+		typeOid = list_nth_oid(estate->es_plannedstmt->paramExecTypes,
+							   paramid);
+
+		sz = add_size(sz, sizeof(int)); /* space for paramid */
+
+		/* space for datum/isnull */
+		if (OidIsValid(typeOid))
+			get_typlenbyval(typeOid, &typLen, &typByVal);
+		else
+		{
+			/* If no type OID, assume by-value, like copyParamList does. */
+			typLen = sizeof(Datum);
+			typByVal = true;
+		}
+		sz = add_size(sz,
+					  datumEstimateSpace(prm->value, prm->isnull,
+										 typByVal, typLen));
+	}
+	return sz;
+}
+
+/*
+ * Serialize specified PARAM_EXEC parameters.
+ *
+ * We write the number of parameters first, as a 4-byte integer, and then
+ * write details for each parameter in turn.  The details for each parameter
+ * consist of a 4-byte paramid (location of param in execution time internal
+ * parameter array) and then the datum as serialized by datumSerialize().
+ */
+static dsa_pointer
+SerializeParamExecParams(EState *estate, Bitmapset *params)
+{
+	Size		size;
+	int			nparams;
+	int			paramid;
+	ParamExecData *prm;
+	dsa_pointer handle;
+	char	   *start_address;
+
+	/* Allocate enough space for the current parameter values. */
+	size = EstimateParamExecSpace(estate, params);
+	handle = dsa_allocate(estate->es_query_dsa, size);
+	start_address = dsa_get_address(estate->es_query_dsa, handle);
+
+	/* First write the number of parameters as a 4-byte integer. */
+	nparams = bms_num_members(params);
+	memcpy(start_address, &nparams, sizeof(int));
+	start_address += sizeof(int);
+
+	/* Write details for each parameter in turn. */
+	paramid = -1;
+	while ((paramid = bms_next_member(params, paramid)) >= 0)
+	{
+		Oid			typeOid;
+		int16		typLen;
+		bool		typByVal;
+
+		prm = &(estate->es_param_exec_vals[paramid]);
+		typeOid = list_nth_oid(estate->es_plannedstmt->paramExecTypes,
+							   paramid);
+
+		/* Write paramid. */
+		memcpy(start_address, &paramid, sizeof(int));
+		start_address += sizeof(int);
+
+		/* Write datum/isnull */
+		if (OidIsValid(typeOid))
+			get_typlenbyval(typeOid, &typLen, &typByVal);
+		else
+		{
+			/* If no type OID, assume by-value, like copyParamList does. */
+			typLen = sizeof(Datum);
+			typByVal = true;
+		}
+		datumSerialize(prm->value, prm->isnull, typByVal, typLen,
+					   &start_address);
+	}
+
+	return handle;
+}
+
+/*
+ * Restore specified PARAM_EXEC parameters.
+ */
+static void
+RestoreParamExecParams(char *start_address, EState *estate)
+{
+	int			nparams;
+	int			i;
+	int			paramid;
+
+	memcpy(&nparams, start_address, sizeof(int));
+	start_address += sizeof(int);
+
+	for (i = 0; i < nparams; i++)
+	{
+		ParamExecData *prm;
+
+		/* Read paramid */
+		memcpy(&paramid, start_address, sizeof(int));
+		start_address += sizeof(int);
+		prm = &(estate->es_param_exec_vals[paramid]);
+
+		/* Read datum/isnull. */
+		prm->value = datumRestore(&start_address, &prm->isnull);
+		prm->execPlan = NULL;
+	}
+}
+
 /*
  * Initialize the dynamic shared memory segment that will be used to control
  * parallel execution.
@@ -395,7 +526,8 @@ ExecParallelSetupTupleQueues(ParallelContext *pcxt, bool reinitialize)
  * execution and return results to the main backend.
  */
 ParallelExecutorInfo *
-ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers,
+ExecInitParallelPlan(PlanState *planstate, EState *estate,
+					 Bitmapset *sendParams, int nworkers,
 					 int64 tuples_needed)
 {
 	ParallelExecutorInfo *pei;
@@ -405,17 +537,20 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers,
 	FixedParallelExecutorState *fpes;
 	char	   *pstmt_data;
 	char	   *pstmt_space;
-	char	   *param_space;
+	char	   *paramlistinfo_space;
 	BufferUsage *bufusage_space;
 	SharedExecutorInstrumentation *instrumentation = NULL;
 	int			pstmt_len;
-	int			param_len;
+	int			paramlistinfo_len;
 	int			instrumentation_len = 0;
 	int			instrument_offset = 0;
 	Size		dsa_minsize = dsa_minimum_size();
 	char	   *query_string;
 	int			query_len;
 
+	/* Force parameters we're going to pass to workers to be evaluated. */
+	ExecEvalParamExecParams(sendParams, estate);
+
 	/* Allocate object for return value. */
 	pei = palloc0(sizeof(ParallelExecutorInfo));
 	pei->finished = false;
@@ -450,8 +585,8 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers,
 	shm_toc_estimate_keys(&pcxt->estimator, 1);
 
 	/* Estimate space for serialized ParamListInfo. */
-	param_len = EstimateParamListSpace(estate->es_param_list_info);
-	shm_toc_estimate_chunk(&pcxt->estimator, param_len);
+	paramlistinfo_len = EstimateParamListSpace(estate->es_param_list_info);
+	shm_toc_estimate_chunk(&pcxt->estimator, paramlistinfo_len);
 	shm_toc_estimate_keys(&pcxt->estimator, 1);
 
 	/*
@@ -511,6 +646,7 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers,
 	/* Store fixed-size state. */
 	fpes = shm_toc_allocate(pcxt->toc, sizeof(FixedParallelExecutorState));
 	fpes->tuples_needed = tuples_needed;
+	fpes->param_exec = InvalidDsaPointer;
 	shm_toc_insert(pcxt->toc, PARALLEL_KEY_EXECUTOR_FIXED, fpes);
 
 	/* Store query string */
@@ -524,9 +660,9 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers,
 	shm_toc_insert(pcxt->toc, PARALLEL_KEY_PLANNEDSTMT, pstmt_space);
 
 	/* Store serialized ParamListInfo. */
-	param_space = shm_toc_allocate(pcxt->toc, param_len);
-	shm_toc_insert(pcxt->toc, PARALLEL_KEY_PARAMS, param_space);
-	SerializeParamList(estate->es_param_list_info, &param_space);
+	paramlistinfo_space = shm_toc_allocate(pcxt->toc, paramlistinfo_len);
+	shm_toc_insert(pcxt->toc, PARALLEL_KEY_PARAMLISTINFO, paramlistinfo_space);
+	SerializeParamList(estate->es_param_list_info, &paramlistinfo_space);
 
 	/* Allocate space for each worker's BufferUsage; no need to initialize. */
 	bufusage_space = shm_toc_allocate(pcxt->toc,
@@ -577,13 +713,25 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers,
 		pei->area = dsa_create_in_place(area_space, dsa_minsize,
 										LWTRANCHE_PARALLEL_QUERY_DSA,
 										pcxt->seg);
-	}
 
-	/*
-	 * Make the area available to executor nodes running in the leader.  See
-	 * also ParallelQueryMain which makes it available to workers.
-	 */
-	estate->es_query_dsa = pei->area;
+		/*
+		 * Make the area available to executor nodes running in the leader.
+		 * See also ParallelQueryMain which makes it available to workers.
+		 */
+		estate->es_query_dsa = pei->area;
+
+		/*
+		 * Serialize parameters, if any, using DSA storage.  We don't dare use
+		 * the main parallel query DSM for this because we might relaunch
+		 * workers after the values have changed (and thus the amount of
+		 * storage required has changed).
+		 */
+		if (!bms_is_empty(sendParams))
+		{
+			pei->param_exec = SerializeParamExecParams(estate, sendParams);
+			fpes->param_exec = pei->param_exec;
+		}
+	}
 
 	/*
 	 * Give parallel-aware nodes a chance to initialize their shared data.
@@ -640,16 +788,39 @@ ExecParallelCreateReaders(ParallelExecutorInfo *pei)
  */
 void
 ExecParallelReinitialize(PlanState *planstate,
-						 ParallelExecutorInfo *pei)
+						 ParallelExecutorInfo *pei,
+						 Bitmapset *sendParams)
 {
+	EState	   *estate = planstate->state;
+	FixedParallelExecutorState *fpes;
+
 	/* Old workers must already be shut down */
 	Assert(pei->finished);
 
+	/* Force parameters we're going to pass to workers to be evaluated. */
+	ExecEvalParamExecParams(sendParams, estate);
+
 	ReinitializeParallelDSM(pei->pcxt);
 	pei->tqueue = ExecParallelSetupTupleQueues(pei->pcxt, true);
 	pei->reader = NULL;
 	pei->finished = false;
 
+	fpes = shm_toc_lookup(pei->pcxt->toc, PARALLEL_KEY_EXECUTOR_FIXED, false);
+
+	/* Free any serialized parameters from the last round. */
+	if (DsaPointerIsValid(fpes->param_exec))
+	{
+		dsa_free(estate->es_query_dsa, fpes->param_exec);
+		fpes->param_exec = InvalidDsaPointer;
+	}
+
+	/* Serialize current parameter values if required. */
+	if (!bms_is_empty(sendParams))
+	{
+		pei->param_exec = SerializeParamExecParams(estate, sendParams);
+		fpes->param_exec = pei->param_exec;
+	}
+
 	/* Traverse plan tree and let each child node reset associated state. */
 	ExecParallelReInitializeDSM(planstate, pei->pcxt);
 }
@@ -831,6 +1002,12 @@ ExecParallelFinish(ParallelExecutorInfo *pei)
 void
 ExecParallelCleanup(ParallelExecutorInfo *pei)
 {
+	/* Free any serialized parameters. */
+	if (DsaPointerIsValid(pei->param_exec))
+	{
+		dsa_free(pei->area, pei->param_exec);
+		pei->param_exec = InvalidDsaPointer;
+	}
 	if (pei->area != NULL)
 	{
 		dsa_detach(pei->area);
@@ -882,7 +1059,7 @@ ExecParallelGetQueryDesc(shm_toc *toc, DestReceiver *receiver,
 	pstmt = (PlannedStmt *) stringToNode(pstmtspace);
 
 	/* Reconstruct ParamListInfo. */
-	paramspace = shm_toc_lookup(toc, PARALLEL_KEY_PARAMS, false);
+	paramspace = shm_toc_lookup(toc, PARALLEL_KEY_PARAMLISTINFO, false);
 	paramLI = RestoreParamList(&paramspace);
 
 	/*
@@ -1046,6 +1223,14 @@ ParallelQueryMain(dsm_segment *seg, shm_toc *toc)
 
 	/* Special executor initialization steps for parallel workers */
 	queryDesc->planstate->state->es_query_dsa = area;
+	if (DsaPointerIsValid(fpes->param_exec))
+	{
+		char	   *paramexec_space;
+
+		paramexec_space = dsa_get_address(area, fpes->param_exec);
+		RestoreParamExecParams(paramexec_space, queryDesc->estate);
+
+	}
 	ExecParallelInitializeWorker(queryDesc->planstate, toc);
 
 	/* Pass down any tuple bound */

src/backend/executor/nodeGather.c

@@ -160,11 +160,13 @@ ExecGather(PlanState *pstate)
 			if (!node->pei)
 				node->pei = ExecInitParallelPlan(node->ps.lefttree,
 												 estate,
+												 gather->initParam,
 												 gather->num_workers,
 												 node->tuples_needed);
 			else
 				ExecParallelReinitialize(node->ps.lefttree,
-										 node->pei);
+										 node->pei,
+										 gather->initParam);
 
 			/*
 			 * Register backend workers. We might not get as many as we

src/backend/executor/nodeGatherMerge.c

@@ -203,11 +203,13 @@ ExecGatherMerge(PlanState *pstate)
 			if (!node->pei)
 				node->pei = ExecInitParallelPlan(node->ps.lefttree,
 												 estate,
+												 gm->initParam,
 												 gm->num_workers,
 												 node->tuples_needed);
 			else
 				ExecParallelReinitialize(node->ps.lefttree,
-										 node->pei);
+										 node->pei,
+										 gm->initParam);
 
 			/* Try to launch workers. */
 			pcxt = node->pei->pcxt;

src/backend/nodes/copyfuncs.c

@@ -364,6 +364,7 @@ _copyGather(const Gather *from)
 	COPY_SCALAR_FIELD(rescan_param);
 	COPY_SCALAR_FIELD(single_copy);
 	COPY_SCALAR_FIELD(invisible);
+	COPY_BITMAPSET_FIELD(initParam);
 
 	return newnode;
 }
@@ -391,6 +392,7 @@ _copyGatherMerge(const GatherMerge *from)
 	COPY_POINTER_FIELD(sortOperators, from->numCols * sizeof(Oid));
 	COPY_POINTER_FIELD(collations, from->numCols * sizeof(Oid));
 	COPY_POINTER_FIELD(nullsFirst, from->numCols * sizeof(bool));
+	COPY_BITMAPSET_FIELD(initParam);
 
 	return newnode;
 }

src/backend/nodes/outfuncs.c

@@ -487,6 +487,7 @@ _outGather(StringInfo str, const Gather *node)
 	WRITE_INT_FIELD(rescan_param);
 	WRITE_BOOL_FIELD(single_copy);
 	WRITE_BOOL_FIELD(invisible);
+	WRITE_BITMAPSET_FIELD(initParam);
 }
 
 static void
@@ -517,6 +518,8 @@ _outGatherMerge(StringInfo str, const GatherMerge *node)
 	appendStringInfoString(str, " :nullsFirst");
 	for (i = 0; i < node->numCols; i++)
 		appendStringInfo(str, " %s", booltostr(node->nullsFirst[i]));
+
+	WRITE_BITMAPSET_FIELD(initParam);
 }
 
 static void

src/backend/nodes/readfuncs.c

@@ -2172,6 +2172,7 @@ _readGather(void)
 	READ_INT_FIELD(rescan_param);
 	READ_BOOL_FIELD(single_copy);
 	READ_BOOL_FIELD(invisible);
+	READ_BITMAPSET_FIELD(initParam);
 
 	READ_DONE();
 }
@@ -2193,6 +2194,7 @@ _readGatherMerge(void)
 	READ_OID_ARRAY(sortOperators, local_node->numCols);
 	READ_OID_ARRAY(collations, local_node->numCols);
 	READ_BOOL_ARRAY(nullsFirst, local_node->numCols);
+	READ_BITMAPSET_FIELD(initParam);
 
 	READ_DONE();
 }

src/backend/optimizer/plan/createplan.c

@@ -6279,6 +6279,7 @@ make_gather(List *qptlist,
 	node->rescan_param = rescan_param;
 	node->single_copy = single_copy;
 	node->invisible = false;
+	node->initParam = NULL;
 
 	return node;
 }

src/backend/optimizer/plan/planner.c

@@ -377,6 +377,14 @@ standard_planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
 	{
 		Gather	   *gather = makeNode(Gather);
 
+		/*
+		 * If there are any initPlans attached to the formerly-top plan node,
+		 * move them up to the Gather node; same as we do for Material node in
+		 * materialize_finished_plan.
+		 */
+		gather->plan.initPlan = top_plan->initPlan;
+		top_plan->initPlan = NIL;
+
 		gather->plan.targetlist = top_plan->targetlist;
 		gather->plan.qual = NIL;
 		gather->plan.lefttree = top_plan;

src/backend/optimizer/plan/setrefs.c

@@ -107,6 +107,7 @@ static Node *fix_scan_expr_mutator(Node *node, fix_scan_expr_context *context);
 static bool fix_scan_expr_walker(Node *node, fix_scan_expr_context *context);
 static void set_join_references(PlannerInfo *root, Join *join, int rtoffset);
 static void set_upper_references(PlannerInfo *root, Plan *plan, int rtoffset);
+static void set_param_references(PlannerInfo *root, Plan *plan);
 static Node *convert_combining_aggrefs(Node *node, void *context);
 static void set_dummy_tlist_references(Plan *plan, int rtoffset);
 static indexed_tlist *build_tlist_index(List *tlist);
@@ -632,7 +633,10 @@ set_plan_refs(PlannerInfo *root, Plan *plan, int rtoffset)
 
 		case T_Gather:
 		case T_GatherMerge:
-			set_upper_references(root, plan, rtoffset);
+			{
+				set_upper_references(root, plan, rtoffset);
+				set_param_references(root, plan);
+			}
 			break;
 
 		case T_Hash:
@@ -1781,6 +1785,51 @@ set_upper_references(PlannerInfo *root, Plan *plan, int rtoffset)
 	pfree(subplan_itlist);
 }
 
+/*
+ * set_param_references
+ *	  Initialize the initParam list in Gather or Gather merge node such that
+ *	  it contains reference of all the params that needs to be evaluated
+ *	  before execution of the node.  It contains the initplan params that are
+ *	  being passed to the plan nodes below it.
+ */
+static void
+set_param_references(PlannerInfo *root, Plan *plan)
+{
+	Assert(IsA(plan, Gather) || IsA(plan, GatherMerge));
+
+	if (plan->lefttree->extParam)
+	{
+		PlannerInfo *proot;
+		Bitmapset  *initSetParam = NULL;
+		ListCell   *l;
+
+		for (proot = root; proot != NULL; proot = proot->parent_root)
+		{
+			foreach(l, proot->init_plans)
+			{
+				SubPlan    *initsubplan = (SubPlan *) lfirst(l);
+				ListCell   *l2;
+
+				foreach(l2, initsubplan->setParam)
+				{
+					initSetParam = bms_add_member(initSetParam, lfirst_int(l2));
+				}
+			}
+		}
+
+		/*
+		 * Remember the list of all external initplan params that are used by
+		 * the children of Gather or Gather merge node.
+		 */
+		if (IsA(plan, Gather))
+			((Gather *) plan)->initParam =
+				bms_intersect(plan->lefttree->extParam, initSetParam);
+		else
+			((GatherMerge *) plan)->initParam =
+				bms_intersect(plan->lefttree->extParam, initSetParam);
+	}
+}
+
 /*
  * Recursively scan an expression tree and convert Aggrefs to the proper
  * intermediate form for combining aggregates.  This means (1) replacing each

src/backend/optimizer/util/clauses.c

@@ -1087,6 +1087,8 @@ bool
 is_parallel_safe(PlannerInfo *root, Node *node)
 {
 	max_parallel_hazard_context context;
+	PlannerInfo *proot;
+	ListCell   *l;
 
 	/*
 	 * Even if the original querytree contained nothing unsafe, we need to
@@ -1101,6 +1103,25 @@ is_parallel_safe(PlannerInfo *root, Node *node)
 	context.max_hazard = PROPARALLEL_SAFE;
 	context.max_interesting = PROPARALLEL_RESTRICTED;
 	context.safe_param_ids = NIL;
+
+	/*
+	 * The params that refer to the same or parent query level are considered
+	 * parallel-safe.  The idea is that we compute such params at Gather or
+	 * Gather Merge node and pass their value to workers.
+	 */
+	for (proot = root; proot != NULL; proot = proot->parent_root)
+	{
+		foreach(l, proot->init_plans)
+		{
+			SubPlan    *initsubplan = (SubPlan *) lfirst(l);
+			ListCell   *l2;
+
+			foreach(l2, initsubplan->setParam)
+				context.safe_param_ids = lcons_int(lfirst_int(l2),
+											context.safe_param_ids);
+		}
+	}
+
 	return !max_parallel_hazard_walker(node, &context);
 }
 
@@ -1225,7 +1246,8 @@ max_parallel_hazard_walker(Node *node, max_parallel_hazard_context *context)
 	 * We can't pass Params to workers at the moment either, so they are also
 	 * parallel-restricted, unless they are PARAM_EXTERN Params or are
 	 * PARAM_EXEC Params listed in safe_param_ids, meaning they could be
-	 * generated within the worker.
+	 * either generated within the worker or can be computed in master and
+	 * then their value can be passed to the worker.
 	 */
 	else if (IsA(node, Param))
 	{

src/include/executor/execExpr.h

@@ -609,6 +609,7 @@ extern ExprEvalOp ExecEvalStepOp(ExprState *state, ExprEvalStep *op);
  */
 extern void ExecEvalParamExec(ExprState *state, ExprEvalStep *op,
 				  ExprContext *econtext);
+extern void ExecEvalParamExecParams(Bitmapset *params, EState *estate);
 extern void ExecEvalParamExtern(ExprState *state, ExprEvalStep *op,
 					ExprContext *econtext);
 extern void ExecEvalSQLValueFunction(ExprState *state, ExprEvalStep *op);

src/include/executor/execParallel.h

@@ -28,6 +28,7 @@ typedef struct ParallelExecutorInfo
 	BufferUsage *buffer_usage;	/* points to bufusage area in DSM */
 	SharedExecutorInstrumentation *instrumentation; /* optional */
 	dsa_area   *area;			/* points to DSA area in DSM */
+	dsa_pointer	param_exec;		/* serialized PARAM_EXEC parameters */
 	bool		finished;		/* set true by ExecParallelFinish */
 	/* These two arrays have pcxt->nworkers_launched entries: */
 	shm_mq_handle **tqueue;		/* tuple queues for worker output */
@@ -35,12 +36,13 @@ typedef struct ParallelExecutorInfo
 } ParallelExecutorInfo;
 
 extern ParallelExecutorInfo *ExecInitParallelPlan(PlanState *planstate,
-					 EState *estate, int nworkers, int64 tuples_needed);
+					 EState *estate, Bitmapset *sendParam, int nworkers,
+					 int64 tuples_needed);
 extern void ExecParallelCreateReaders(ParallelExecutorInfo *pei);
 extern void ExecParallelFinish(ParallelExecutorInfo *pei);
 extern void ExecParallelCleanup(ParallelExecutorInfo *pei);
 extern void ExecParallelReinitialize(PlanState *planstate,
-						 ParallelExecutorInfo *pei);
+						 ParallelExecutorInfo *pei, Bitmapset *sendParam);
 
 extern void ParallelQueryMain(dsm_segment *seg, shm_toc *toc);
 

src/include/nodes/plannodes.h

@@ -841,6 +841,8 @@ typedef struct Gather
 	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)? */
+	Bitmapset  *initParam;		/* param id's of initplans which are referred
+								 * at gather or one of it's child node */
 } Gather;
 
 /* ------------
@@ -858,6 +860,8 @@ typedef struct GatherMerge
 	Oid		   *sortOperators;	/* OIDs of operators to sort them by */
 	Oid		   *collations;		/* OIDs of collations */
 	bool	   *nullsFirst;		/* NULLS FIRST/LAST directions */
+	Bitmapset  *initParam;		/* param id's of initplans which are referred
+								 * at gather merge or one of it's child node */
 } GatherMerge;
 
 /* ----------------

src/test/regress/expected/select_parallel.out

@@ -201,6 +201,41 @@ explain (costs off)
      ->  Seq Scan on tenk2
 (4 rows)
 
+alter table tenk2 reset (parallel_workers);
+-- test parallel plan for a query containing initplan.
+set enable_indexscan = off;
+set enable_indexonlyscan = off;
+set enable_bitmapscan = off;
+alter table tenk2 set (parallel_workers = 2);
+explain (costs off)
+	select count(*) from tenk1
+        where tenk1.unique1 = (Select max(tenk2.unique1) from tenk2);
+                      QUERY PLAN                      
+------------------------------------------------------
+ Aggregate
+   InitPlan 1 (returns $2)
+     ->  Finalize Aggregate
+           ->  Gather
+                 Workers Planned: 2
+                 ->  Partial Aggregate
+                       ->  Parallel Seq Scan on tenk2
+   ->  Gather
+         Workers Planned: 4
+         Params Evaluated: $2
+         ->  Parallel Seq Scan on tenk1
+               Filter: (unique1 = $2)
+(12 rows)
+
+select count(*) from tenk1
+    where tenk1.unique1 = (Select max(tenk2.unique1) from tenk2);
+ count 
+-------
+     1
+(1 row)
+
+reset enable_indexscan;
+reset enable_indexonlyscan;
+reset enable_bitmapscan;
 alter table tenk2 reset (parallel_workers);
 -- test parallel index scans.
 set enable_seqscan to off;

src/test/regress/sql/select_parallel.sql

@@ -74,6 +74,23 @@ explain (costs off)
 	(select ten from tenk2);
 alter table tenk2 reset (parallel_workers);
 
+-- test parallel plan for a query containing initplan.
+set enable_indexscan = off;
+set enable_indexonlyscan = off;
+set enable_bitmapscan = off;
+alter table tenk2 set (parallel_workers = 2);
+
+explain (costs off)
+	select count(*) from tenk1
+        where tenk1.unique1 = (Select max(tenk2.unique1) from tenk2);
+select count(*) from tenk1
+    where tenk1.unique1 = (Select max(tenk2.unique1) from tenk2);
+
+reset enable_indexscan;
+reset enable_indexonlyscan;
+reset enable_bitmapscan;
+alter table tenk2 reset (parallel_workers);
+
 -- test parallel index scans.
 set enable_seqscan to off;
 set enable_bitmapscan to off;