Make sequential scans parallel-aware.

In addition, this path fills in a number of missing bits and pieces in
the parallel infrastructure.  Paths and plans now have a parallel_aware
flag indicating whether whatever parallel-aware logic they have should
be engaged.  It is believed that we will need this flag for a number of
path/plan types, not just sequential scans, which is why the flag is
generic rather than part of the SeqScan structures specifically.
Also, execParallel.c now gives parallel nodes a chance to initialize
their PlanState nodes from the DSM during parallel worker startup.

Amit Kapila, with a fair amount of adjustment by me.  Review of previous
patch versions by Haribabu Kommi and others.

src/backend/executor/execAmi.c

@@ -439,6 +439,15 @@ ExecSupportsBackwardScan(Plan *node)
 	if (node == NULL)
 		return false;
 
+	/*
+	 * Parallel-aware nodes return a subset of the tuples in each worker,
+	 * and in general we can't expect to have enough bookkeeping state to
+	 * know which ones we returned in this worker as opposed to some other
+	 * worker.
+	 */
+	if (node->parallel_aware)
+		return false;
+
 	switch (nodeTag(node))
 	{
 		case T_Result:

src/backend/executor/execParallel.c

@@ -25,6 +25,7 @@
 
 #include "executor/execParallel.h"
 #include "executor/executor.h"
+#include "executor/nodeSeqscan.h"
 #include "executor/tqueue.h"
 #include "nodes/nodeFuncs.h"
 #include "optimizer/planmain.h"
@@ -167,10 +168,16 @@ ExecParallelEstimate(PlanState *planstate, ExecParallelEstimateContext *e)
 	/* Count this node. */
 	e->nnodes++;
 
-	/*
-	 * XXX. Call estimators for parallel-aware nodes here, when we have
-	 * some.
-	 */
+	/* Call estimators for parallel-aware nodes. */
+	switch (nodeTag(planstate))
+	{
+		case T_SeqScanState:
+			ExecSeqScanEstimate((SeqScanState *) planstate,
+								e->pcxt);
+			break;
+		default:
+			break;
+	}
 
 	return planstate_tree_walker(planstate, ExecParallelEstimate, e);
 }
@@ -205,10 +212,16 @@ ExecParallelInitializeDSM(PlanState *planstate,
 	/* Count this node. */
 	d->nnodes++;
 
-	/*
-	 * XXX. Call initializers for parallel-aware plan nodes, when we have
-	 * some.
-	 */
+	/* Call initializers for parallel-aware plan nodes. */
+	switch (nodeTag(planstate))
+	{
+		case T_SeqScanState:
+			ExecSeqScanInitializeDSM((SeqScanState *) planstate,
+									 d->pcxt);
+			break;
+		default:
+			break;
+	}
 
 	return planstate_tree_walker(planstate, ExecParallelInitializeDSM, d);
 }
@@ -574,6 +587,30 @@ ExecParallelReportInstrumentation(PlanState *planstate,
 								 instrumentation);
 }
 
+/*
+ * Initialize the PlanState and its descendents with the information
+ * retrieved from shared memory.  This has to be done once the PlanState
+ * is allocated and initialized by executor; that is, after ExecutorStart().
+ */
+static bool
+ExecParallelInitializeWorker(PlanState *planstate, shm_toc *toc)
+{
+	if (planstate == NULL)
+		return false;
+
+	/* Call initializers for parallel-aware plan nodes. */
+	switch (nodeTag(planstate))
+	{
+		case T_SeqScanState:
+			ExecSeqScanInitializeWorker((SeqScanState *) planstate, toc);
+			break;
+		default:
+			break;
+	}
+
+	return planstate_tree_walker(planstate, ExecParallelInitializeWorker, toc);
+}
+
 /*
  * Main entrypoint for parallel query worker processes.
  *
@@ -610,6 +647,7 @@ ParallelQueryMain(dsm_segment *seg, shm_toc *toc)
 
 	/* Start up the executor, have it run the plan, and then shut it down. */
 	ExecutorStart(queryDesc, 0);
+	ExecParallelInitializeWorker(queryDesc->planstate, toc);
 	ExecutorRun(queryDesc, ForwardScanDirection, 0L);
 	ExecutorFinish(queryDesc);
 

src/backend/executor/nodeSeqscan.c

@@ -19,6 +19,10 @@
  *		ExecInitSeqScan			creates and initializes a seqscan node.
  *		ExecEndSeqScan			releases any storage allocated.
  *		ExecReScanSeqScan		rescans the relation
+ *
+ *		ExecSeqScanEstimate		estimates DSM space needed for parallel scan
+ *		ExecSeqScanInitializeDSM initialize DSM for parallel scan
+ *		ExecSeqScanInitializeWorker attach to DSM info in parallel worker
  */
 #include "postgres.h"
 
@@ -53,10 +57,22 @@ SeqNext(SeqScanState *node)
 	/*
 	 * get information from the estate and scan state
 	 */
-	scandesc = node->ss_currentScanDesc;
-	estate = node->ps.state;
+	scandesc = node->ss.ss_currentScanDesc;
+	estate = node->ss.ps.state;
 	direction = estate->es_direction;
-	slot = node->ss_ScanTupleSlot;
+	slot = node->ss.ss_ScanTupleSlot;
+
+	if (scandesc == NULL)
+	{
+		/*
+		 * We reach here if the scan is not parallel, or if we're executing
+		 * a scan that was intended to be parallel serially.
+		 */
+		scandesc = heap_beginscan(node->ss.ss_currentRelation,
+								  estate->es_snapshot,
+								  0, NULL);
+		node->ss.ss_currentScanDesc = scandesc;
+	}
 
 	/*
 	 * get the next tuple from the table
@@ -123,27 +139,19 @@ static void
 InitScanRelation(SeqScanState *node, EState *estate, int eflags)
 {
 	Relation	currentRelation;
-	HeapScanDesc currentScanDesc;
 
 	/*
 	 * get the relation object id from the relid'th entry in the range table,
 	 * open that relation and acquire appropriate lock on it.
 	 */
 	currentRelation = ExecOpenScanRelation(estate,
-									  ((SeqScan *) node->ps.plan)->scanrelid,
+									  ((SeqScan *) node->ss.ps.plan)->scanrelid,
 										   eflags);
 
-	/* initialize a heapscan */
-	currentScanDesc = heap_beginscan(currentRelation,
-									 estate->es_snapshot,
-									 0,
-									 NULL);
-
-	node->ss_currentRelation = currentRelation;
-	node->ss_currentScanDesc = currentScanDesc;
+	node->ss.ss_currentRelation = currentRelation;
 
 	/* and report the scan tuple slot's rowtype */
-	ExecAssignScanType(node, RelationGetDescr(currentRelation));
+	ExecAssignScanType(&node->ss, RelationGetDescr(currentRelation));
 }
 
 
@@ -167,44 +175,44 @@ ExecInitSeqScan(SeqScan *node, EState *estate, int eflags)
 	 * create state structure
 	 */
 	scanstate = makeNode(SeqScanState);
-	scanstate->ps.plan = (Plan *) node;
-	scanstate->ps.state = estate;
+	scanstate->ss.ps.plan = (Plan *) node;
+	scanstate->ss.ps.state = estate;
 
 	/*
 	 * Miscellaneous initialization
 	 *
 	 * create expression context for node
 	 */
-	ExecAssignExprContext(estate, &scanstate->ps);
+	ExecAssignExprContext(estate, &scanstate->ss.ps);
 
 	/*
 	 * initialize child expressions
 	 */
-	scanstate->ps.targetlist = (List *)
+	scanstate->ss.ps.targetlist = (List *)
 		ExecInitExpr((Expr *) node->plan.targetlist,
 					 (PlanState *) scanstate);
-	scanstate->ps.qual = (List *)
+	scanstate->ss.ps.qual = (List *)
 		ExecInitExpr((Expr *) node->plan.qual,
 					 (PlanState *) scanstate);
 
 	/*
 	 * tuple table initialization
 	 */
-	ExecInitResultTupleSlot(estate, &scanstate->ps);
-	ExecInitScanTupleSlot(estate, scanstate);
+	ExecInitResultTupleSlot(estate, &scanstate->ss.ps);
+	ExecInitScanTupleSlot(estate, &scanstate->ss);
 
 	/*
 	 * initialize scan relation
 	 */
 	InitScanRelation(scanstate, estate, eflags);
 
-	scanstate->ps.ps_TupFromTlist = false;
+	scanstate->ss.ps.ps_TupFromTlist = false;
 
 	/*
 	 * Initialize result tuple type and projection info.
 	 */
-	ExecAssignResultTypeFromTL(&scanstate->ps);
-	ExecAssignScanProjectionInfo(scanstate);
+	ExecAssignResultTypeFromTL(&scanstate->ss.ps);
+	ExecAssignScanProjectionInfo(&scanstate->ss);
 
 	return scanstate;
 }
@@ -224,24 +232,25 @@ ExecEndSeqScan(SeqScanState *node)
 	/*
 	 * get information from node
 	 */
-	relation = node->ss_currentRelation;
-	scanDesc = node->ss_currentScanDesc;
+	relation = node->ss.ss_currentRelation;
+	scanDesc = node->ss.ss_currentScanDesc;
 
 	/*
 	 * Free the exprcontext
 	 */
-	ExecFreeExprContext(&node->ps);
+	ExecFreeExprContext(&node->ss.ps);
 
 	/*
 	 * clean out the tuple table
 	 */
-	ExecClearTuple(node->ps.ps_ResultTupleSlot);
-	ExecClearTuple(node->ss_ScanTupleSlot);
+	ExecClearTuple(node->ss.ps.ps_ResultTupleSlot);
+	ExecClearTuple(node->ss.ss_ScanTupleSlot);
 
 	/*
 	 * close heap scan
 	 */
-	heap_endscan(scanDesc);
+	if (scanDesc != NULL)
+		heap_endscan(scanDesc);
 
 	/*
 	 * close the heap relation.
@@ -265,10 +274,71 @@ ExecReScanSeqScan(SeqScanState *node)
 {
 	HeapScanDesc scan;
 
-	scan = node->ss_currentScanDesc;
+	scan = node->ss.ss_currentScanDesc;
 
-	heap_rescan(scan,			/* scan desc */
-				NULL);			/* new scan keys */
+	if (scan != NULL)
+		heap_rescan(scan,			/* scan desc */
+					NULL);			/* new scan keys */
 
 	ExecScanReScan((ScanState *) node);
 }
+
+/* ----------------------------------------------------------------
+ *						Parallel Scan Support
+ * ----------------------------------------------------------------
+ */
+
+/* ----------------------------------------------------------------
+ *		ExecSeqScanEstimate
+ *
+ *		estimates the space required to serialize seqscan node.
+ * ----------------------------------------------------------------
+ */
+void
+ExecSeqScanEstimate(SeqScanState *node,
+					ParallelContext *pcxt)
+{
+	EState	   *estate = node->ss.ps.state;
+
+	node->pscan_len = heap_parallelscan_estimate(estate->es_snapshot);
+	shm_toc_estimate_chunk(&pcxt->estimator, node->pscan_len);
+	shm_toc_estimate_keys(&pcxt->estimator, 1);
+}
+
+/* ----------------------------------------------------------------
+ *		ExecSeqScanInitializeDSM
+ *
+ *		Set up a parallel heap scan descriptor.
+ * ----------------------------------------------------------------
+ */
+void
+ExecSeqScanInitializeDSM(SeqScanState *node,
+						 ParallelContext *pcxt)
+{
+	EState	   *estate = node->ss.ps.state;
+	ParallelHeapScanDesc	pscan;
+
+	pscan = shm_toc_allocate(pcxt->toc, node->pscan_len);
+	heap_parallelscan_initialize(pscan,
+								 node->ss.ss_currentRelation,
+								 estate->es_snapshot);
+	shm_toc_insert(pcxt->toc, node->ss.ps.plan->plan_node_id, pscan);
+	node->ss.ss_currentScanDesc =
+		heap_beginscan_parallel(node->ss.ss_currentRelation, pscan);
+}
+
+/* ----------------------------------------------------------------
+ *		ExecSeqScanInitializeWorker
+ *
+ *		Copy relevant information from TOC into planstate.
+ * ----------------------------------------------------------------
+ */
+void
+ExecSeqScanInitializeWorker(SeqScanState *node, shm_toc *toc)
+{
+	ParallelHeapScanDesc	pscan;
+
+	pscan = shm_toc_lookup(toc, node->ss.ps.plan->plan_node_id);
+	node->ss.ss_currentScanDesc =
+		heap_beginscan_parallel(node->ss.ss_currentRelation, pscan);
+}