Add a Gather executor node.

A Gather executor node runs any number of copies of a plan in an equal
number of workers and merges all of the results into a single tuple
stream.  It can also run the plan itself, if the workers are
unavailable or haven't started up yet.  It is intended to work with
the Partial Seq Scan node which will be added in future commits.

It could also be used to implement parallel query of a different sort
by itself, without help from Partial Seq Scan, if the single_copy mode
is used.  In that mode, a worker executes the plan, and the parallel
leader does not, merely collecting the worker's results.  So, a Gather
node could be inserted into a plan to split the execution of that plan
across two processes.  Nested Gather nodes aren't currently supported,
but we might want to add support for that in the future.

There's nothing in the planner to actually generate Gather nodes yet,
so it's not quite time to break out the champagne.  But we're getting
close.

Amit Kapila.  Some designs suggestions were provided by me, and I also
reviewed the patch.  Single-copy mode, documentation, and other minor
changes also by me.

1 files changed, 299 insertions, 0 deletions

diff --git a/src/backend/executor/nodeGather.c b/src/backend/executor/nodeGather.c
new file mode 100644
index 00000000000..735dbaa2226
--- /dev/null
+++ b/src/backend/executor/nodeGather.c
@@ -0,0 +1,299 @@
+/*-------------------------------------------------------------------------
+ *
+ * nodeGather.c
+ *	  Support routines for scanning a plan via multiple workers.
+ *
+ * Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/executor/nodeGather.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "access/relscan.h"
+#include "executor/execdebug.h"
+#include "executor/execParallel.h"
+#include "executor/nodeGather.h"
+#include "executor/nodeSubplan.h"
+#include "executor/tqueue.h"
+#include "utils/rel.h"
+
+
+static TupleTableSlot *gather_getnext(GatherState *gatherstate);
+
+
+/* ----------------------------------------------------------------
+ *		ExecInitGather
+ * ----------------------------------------------------------------
+ */
+GatherState *
+ExecInitGather(Gather *node, EState *estate, int eflags)
+{
+	GatherState *gatherstate;
+
+	/* Gather node doesn't have innerPlan node. */
+	Assert(innerPlan(node) == NULL);
+
+	/*
+	 * create state structure
+	 */
+	gatherstate = makeNode(GatherState);
+	gatherstate->ps.plan = (Plan *) node;
+	gatherstate->ps.state = estate;
+	gatherstate->need_to_scan_workers = false;
+	gatherstate->need_to_scan_locally = !node->single_copy;
+
+	/*
+	 * Miscellaneous initialization
+	 *
+	 * create expression context for node
+	 */
+	ExecAssignExprContext(estate, &gatherstate->ps);
+
+	/*
+	 * initialize child expressions
+	 */
+	gatherstate->ps.targetlist = (List *)
+		ExecInitExpr((Expr *) node->plan.targetlist,
+					 (PlanState *) gatherstate);
+	gatherstate->ps.qual = (List *)
+		ExecInitExpr((Expr *) node->plan.qual,
+					 (PlanState *) gatherstate);
+
+	/*
+	 * tuple table initialization
+	 */
+	ExecInitResultTupleSlot(estate, &gatherstate->ps);
+
+	/*
+	 * now initialize outer plan
+	 */
+	outerPlanState(gatherstate) = ExecInitNode(outerPlan(node), estate, eflags);
+
+
+	gatherstate->ps.ps_TupFromTlist = false;
+
+	/*
+	 * Initialize result tuple type and projection info.
+	 */
+	ExecAssignResultTypeFromTL(&gatherstate->ps);
+	ExecAssignProjectionInfo(&gatherstate->ps, NULL);
+
+	return gatherstate;
+}
+
+/* ----------------------------------------------------------------
+ *		ExecGather(node)
+ *
+ *		Scans the relation via multiple workers and returns
+ *		the next qualifying tuple.
+ * ----------------------------------------------------------------
+ */
+TupleTableSlot *
+ExecGather(GatherState *node)
+{
+	int			i;
+	TupleTableSlot *slot;
+
+	/*
+	 * Initialize the parallel context and workers on first execution. We do
+	 * this on first execution rather than during node initialization, as it
+	 * needs to allocate large dynamic segement, so it is better to do if it
+	 * is really needed.
+	 */
+	if (!node->pei)
+	{
+		EState	   *estate = node->ps.state;
+
+		/* Initialize the workers required to execute Gather node. */
+		node->pei = ExecInitParallelPlan(node->ps.lefttree,
+										 estate,
+								  ((Gather *) (node->ps.plan))->num_workers);
+
+		/*
+		 * Register backend workers. If the required number of workers are not
+		 * available then we perform the scan with available workers and if
+		 * there are no more workers available, then the Gather node will just
+		 * scan locally.
+		 */
+		LaunchParallelWorkers(node->pei->pcxt);
+
+		node->funnel = CreateTupleQueueFunnel();
+
+		for (i = 0; i < node->pei->pcxt->nworkers; ++i)
+		{
+			if (node->pei->pcxt->worker[i].bgwhandle)
+			{
+				shm_mq_set_handle(node->pei->tqueue[i],
+								  node->pei->pcxt->worker[i].bgwhandle);
+				RegisterTupleQueueOnFunnel(node->funnel, node->pei->tqueue[i]);
+				node->need_to_scan_workers = true;
+			}
+		}
+
+		/* If no workers are available, we must always scan locally. */
+		if (!node->need_to_scan_workers)
+			node->need_to_scan_locally = true;
+	}
+
+	slot = gather_getnext(node);
+
+	if (TupIsNull(slot))
+	{
+		/*
+		 * Destroy the parallel context once we complete fetching all the
+		 * tuples.  Otherwise, the DSM and workers will stick around for the
+		 * lifetime of the entire statement.
+		 */
+		ExecShutdownGather(node);
+	}
+	return slot;
+}
+
+/* ----------------------------------------------------------------
+ *		ExecEndGather
+ *
+ *		frees any storage allocated through C routines.
+ * ----------------------------------------------------------------
+ */
+void
+ExecEndGather(GatherState *node)
+{
+	ExecShutdownGather(node);
+	ExecFreeExprContext(&node->ps);
+	ExecClearTuple(node->ps.ps_ResultTupleSlot);
+	ExecEndNode(outerPlanState(node));
+}
+
+/*
+ * gather_getnext
+ *
+ * Get the next tuple from shared memory queue.  This function
+ * is reponsible for fetching tuples from all the queues associated
+ * with worker backends used in Gather node execution and if there is
+ * no data available from queues or no worker is available, it does
+ * fetch the data from local node.
+ */
+TupleTableSlot *
+gather_getnext(GatherState *gatherstate)
+{
+	PlanState  *outerPlan;
+	TupleTableSlot *outerTupleSlot;
+	TupleTableSlot *slot;
+	HeapTuple	tup;
+
+	/*
+	 * We can use projection info of Gather for the tuples received from
+	 * worker backends as currently for all cases worker backends sends the
+	 * projected tuple as required by Gather node.
+	 */
+	slot = gatherstate->ps.ps_ProjInfo->pi_slot;
+
+	while (gatherstate->need_to_scan_workers ||
+		   gatherstate->need_to_scan_locally)
+	{
+		if (gatherstate->need_to_scan_workers)
+		{
+			bool		done = false;
+
+			/* wait only if local scan is done */
+			tup = TupleQueueFunnelNext(gatherstate->funnel,
+									   gatherstate->need_to_scan_locally,
+									   &done);
+			if (done)
+				gatherstate->need_to_scan_workers = false;
+
+			if (HeapTupleIsValid(tup))
+			{
+				ExecStoreTuple(tup,		/* tuple to store */
+							   slot,	/* slot to store in */
+							   InvalidBuffer,	/* buffer associated with this
+												 * tuple */
+							   true);	/* pfree this pointer if not from heap */
+
+				return slot;
+			}
+		}
+
+		if (gatherstate->need_to_scan_locally)
+		{
+			outerPlan = outerPlanState(gatherstate);
+
+			outerTupleSlot = ExecProcNode(outerPlan);
+
+			if (!TupIsNull(outerTupleSlot))
+				return outerTupleSlot;
+
+			gatherstate->need_to_scan_locally = false;
+		}
+	}
+
+	return ExecClearTuple(slot);
+}
+
+/* ----------------------------------------------------------------
+ *		ExecShutdownGather
+ *
+ *		Destroy the setup for parallel workers.  Collect all the
+ *		stats after workers are stopped, else some work done by
+ *		workers won't be accounted.
+ * ----------------------------------------------------------------
+ */
+void
+ExecShutdownGather(GatherState *node)
+{
+	Gather *gather;
+
+	if (node->pei == NULL || node->pei->pcxt == NULL)
+		return;
+
+	/*
+	 * Ensure all workers have finished before destroying the parallel context
+	 * to ensure a clean exit.
+	 */
+	if (node->funnel)
+	{
+		DestroyTupleQueueFunnel(node->funnel);
+		node->funnel = NULL;
+	}
+
+	ExecParallelFinish(node->pei);
+
+	/* destroy parallel context. */
+	DestroyParallelContext(node->pei->pcxt);
+	node->pei->pcxt = NULL;
+
+	gather = (Gather *) node->ps.plan;
+	node->need_to_scan_locally = !gather->single_copy;
+	node->need_to_scan_workers = false;
+}
+
+/* ----------------------------------------------------------------
+ *						Join Support
+ * ----------------------------------------------------------------
+ */
+
+/* ----------------------------------------------------------------
+ *		ExecReScanGather
+ *
+ *		Re-initialize the workers and rescans a relation via them.
+ * ----------------------------------------------------------------
+ */
+void
+ExecReScanGather(GatherState *node)
+{
+	/*
+	 * Re-initialize the parallel context and workers to perform rescan of
+	 * relation.  We want to gracefully shutdown all the workers so that they
+	 * should be able to propagate any error or other information to master
+	 * backend before dying.
+	 */
+	ExecShutdownGather(node);
+
+	ExecReScan(node->ps.lefttree);
+}