Add a Gather Merge executor node.

Like Gather, we spawn multiple workers and run the same plan in each
one; however, Gather Merge is used when each worker produces the same
output ordering and we want to preserve that output ordering while
merging together the streams of tuples from various workers.  (In a
way, Gather Merge is like a hybrid of Gather and MergeAppend.)

This works out to a win if it saves us from having to perform an
expensive Sort.  In cases where only a small amount of data would need
to be sorted, it may actually be faster to use a regular Gather node
and then sort the results afterward, because Gather Merge sometimes
needs to wait synchronously for tuples whereas a pure Gather generally
doesn't.  But if this avoids an expensive sort then it's a win.

Rushabh Lathia, reviewed and tested by Amit Kapila, Thomas Munro,
and Neha Sharma, and reviewed and revised by me.

Discussion: http://postgr.es/m/CAGPqQf09oPX-cQRpBKS0Gq49Z+m6KBxgxd_p9gX8CKk_d75HoQ@mail.gmail.com

1 files changed, 687 insertions, 0 deletions

diff --git a/src/backend/executor/nodeGatherMerge.c b/src/backend/executor/nodeGatherMerge.c
new file mode 100644
index 00000000000..62a6b1866dc
--- /dev/null
+++ b/src/backend/executor/nodeGatherMerge.c
@@ -0,0 +1,687 @@
+/*-------------------------------------------------------------------------
+ *
+ * nodeGatherMerge.c
+ *		Scan a plan in multiple workers, and do order-preserving merge.
+ *
+ * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/backend/executor/nodeGatherMerge.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "access/relscan.h"
+#include "access/xact.h"
+#include "executor/execdebug.h"
+#include "executor/execParallel.h"
+#include "executor/nodeGatherMerge.h"
+#include "executor/nodeSubplan.h"
+#include "executor/tqueue.h"
+#include "lib/binaryheap.h"
+#include "miscadmin.h"
+#include "utils/memutils.h"
+#include "utils/rel.h"
+
+/*
+ * Tuple array for each worker
+ */
+typedef struct GMReaderTupleBuffer
+{
+	HeapTuple  *tuple;
+	int			readCounter;
+	int			nTuples;
+	bool		done;
+}	GMReaderTupleBuffer;
+
+/*
+ * When we read tuples from workers, it's a good idea to read several at once
+ * for efficiency when possible: this minimizes context-switching overhead.
+ * But reading too many at a time wastes memory without improving performance.
+ */
+#define MAX_TUPLE_STORE 10
+
+static int32 heap_compare_slots(Datum a, Datum b, void *arg);
+static TupleTableSlot *gather_merge_getnext(GatherMergeState *gm_state);
+static HeapTuple gm_readnext_tuple(GatherMergeState *gm_state, int nreader,
+				  bool nowait, bool *done);
+static void gather_merge_init(GatherMergeState *gm_state);
+static void ExecShutdownGatherMergeWorkers(GatherMergeState *node);
+static bool gather_merge_readnext(GatherMergeState *gm_state, int reader,
+					  bool nowait);
+static void form_tuple_array(GatherMergeState *gm_state, int reader);
+
+/* ----------------------------------------------------------------
+ *		ExecInitGather
+ * ----------------------------------------------------------------
+ */
+GatherMergeState *
+ExecInitGatherMerge(GatherMerge *node, EState *estate, int eflags)
+{
+	GatherMergeState *gm_state;
+	Plan	   *outerNode;
+	bool		hasoid;
+	TupleDesc	tupDesc;
+
+	/* Gather merge node doesn't have innerPlan node. */
+	Assert(innerPlan(node) == NULL);
+
+	/*
+	 * create state structure
+	 */
+	gm_state = makeNode(GatherMergeState);
+	gm_state->ps.plan = (Plan *) node;
+	gm_state->ps.state = estate;
+
+	/*
+	 * Miscellaneous initialization
+	 *
+	 * create expression context for node
+	 */
+	ExecAssignExprContext(estate, &gm_state->ps);
+
+	/*
+	 * initialize child expressions
+	 */
+	gm_state->ps.targetlist = (List *)
+		ExecInitExpr((Expr *) node->plan.targetlist,
+					 (PlanState *) gm_state);
+	gm_state->ps.qual = (List *)
+		ExecInitExpr((Expr *) node->plan.qual,
+					 (PlanState *) gm_state);
+
+	/*
+	 * tuple table initialization
+	 */
+	ExecInitResultTupleSlot(estate, &gm_state->ps);
+
+	/*
+	 * now initialize outer plan
+	 */
+	outerNode = outerPlan(node);
+	outerPlanState(gm_state) = ExecInitNode(outerNode, estate, eflags);
+
+	/*
+	 * Initialize result tuple type and projection info.
+	 */
+	ExecAssignResultTypeFromTL(&gm_state->ps);
+	ExecAssignProjectionInfo(&gm_state->ps, NULL);
+
+	gm_state->gm_initialized = false;
+
+	/*
+	 * initialize sort-key information
+	 */
+	if (node->numCols)
+	{
+		int			i;
+
+		gm_state->gm_nkeys = node->numCols;
+		gm_state->gm_sortkeys =
+			palloc0(sizeof(SortSupportData) * node->numCols);
+
+		for (i = 0; i < node->numCols; i++)
+		{
+			SortSupport sortKey = gm_state->gm_sortkeys + i;
+
+			sortKey->ssup_cxt = CurrentMemoryContext;
+			sortKey->ssup_collation = node->collations[i];
+			sortKey->ssup_nulls_first = node->nullsFirst[i];
+			sortKey->ssup_attno = node->sortColIdx[i];
+
+			/*
+			 * We don't perform abbreviated key conversion here, for the same
+			 * reasons that it isn't used in MergeAppend
+			 */
+			sortKey->abbreviate = false;
+
+			PrepareSortSupportFromOrderingOp(node->sortOperators[i], sortKey);
+		}
+	}
+
+	/*
+	 * store the tuple descriptor into gather merge state, so we can use it
+	 * later while initializing the gather merge slots.
+	 */
+	if (!ExecContextForcesOids(&gm_state->ps, &hasoid))
+		hasoid = false;
+	tupDesc = ExecTypeFromTL(outerNode->targetlist, hasoid);
+	gm_state->tupDesc = tupDesc;
+
+	return gm_state;
+}
+
+/* ----------------------------------------------------------------
+ *		ExecGatherMerge(node)
+ *
+ *		Scans the relation via multiple workers and returns
+ *		the next qualifying tuple.
+ * ----------------------------------------------------------------
+ */
+TupleTableSlot *
+ExecGatherMerge(GatherMergeState *node)
+{
+	TupleTableSlot *slot;
+	ExprContext *econtext;
+	int			i;
+
+	/*
+	 * As with Gather, we don't launch workers until this node is actually
+	 * executed.
+	 */
+	if (!node->initialized)
+	{
+		EState	   *estate = node->ps.state;
+		GatherMerge *gm = (GatherMerge *) node->ps.plan;
+
+		/*
+		 * Sometimes we might have to run without parallelism; but if parallel
+		 * mode is active then we can try to fire up some workers.
+		 */
+		if (gm->num_workers > 0 && IsInParallelMode())
+		{
+			ParallelContext *pcxt;
+
+			/* Initialize data structures for workers. */
+			if (!node->pei)
+				node->pei = ExecInitParallelPlan(node->ps.lefttree,
+												 estate,
+												 gm->num_workers);
+
+			/* Try to launch workers. */
+			pcxt = node->pei->pcxt;
+			LaunchParallelWorkers(pcxt);
+			node->nworkers_launched = pcxt->nworkers_launched;
+
+			/* Set up tuple queue readers to read the results. */
+			if (pcxt->nworkers_launched > 0)
+			{
+				node->nreaders = 0;
+				node->reader = palloc(pcxt->nworkers_launched *
+									  sizeof(TupleQueueReader *));
+
+				Assert(gm->numCols);
+
+				for (i = 0; i < pcxt->nworkers_launched; ++i)
+				{
+					shm_mq_set_handle(node->pei->tqueue[i],
+									  pcxt->worker[i].bgwhandle);
+					node->reader[node->nreaders++] =
+						CreateTupleQueueReader(node->pei->tqueue[i],
+											   node->tupDesc);
+				}
+			}
+			else
+			{
+				/* No workers?	Then never mind. */
+				ExecShutdownGatherMergeWorkers(node);
+			}
+		}
+
+		/* always allow leader to participate */
+		node->need_to_scan_locally = true;
+		node->initialized = true;
+	}
+
+	/*
+	 * Reset per-tuple memory context to free any expression evaluation
+	 * storage allocated in the previous tuple cycle.
+	 */
+	econtext = node->ps.ps_ExprContext;
+	ResetExprContext(econtext);
+
+	/*
+	 * Get next tuple, either from one of our workers, or by running the
+	 * plan ourselves.
+	 */
+	slot = gather_merge_getnext(node);
+	if (TupIsNull(slot))
+		return NULL;
+
+	/*
+	 * form the result tuple using ExecProject(), and return it --- unless
+	 * the projection produces an empty set, in which case we must loop
+	 * back around for another tuple
+	 */
+	econtext->ecxt_outertuple = slot;
+	return ExecProject(node->ps.ps_ProjInfo);
+}
+
+/* ----------------------------------------------------------------
+ *		ExecEndGatherMerge
+ *
+ *		frees any storage allocated through C routines.
+ * ----------------------------------------------------------------
+ */
+void
+ExecEndGatherMerge(GatherMergeState *node)
+{
+	ExecEndNode(outerPlanState(node));      /* let children clean up first */
+	ExecShutdownGatherMerge(node);
+	ExecFreeExprContext(&node->ps);
+	ExecClearTuple(node->ps.ps_ResultTupleSlot);
+}
+
+/* ----------------------------------------------------------------
+ *		ExecShutdownGatherMerge
+ *
+ *		Destroy the setup for parallel workers including parallel context.
+ *		Collect all the stats after workers are stopped, else some work
+ *		done by workers won't be accounted.
+ * ----------------------------------------------------------------
+ */
+void
+ExecShutdownGatherMerge(GatherMergeState *node)
+{
+	ExecShutdownGatherMergeWorkers(node);
+
+	/* Now destroy the parallel context. */
+	if (node->pei != NULL)
+	{
+		ExecParallelCleanup(node->pei);
+		node->pei = NULL;
+	}
+}
+
+/* ----------------------------------------------------------------
+ *		ExecShutdownGatherMergeWorkers
+ *
+ *		Destroy the parallel workers.  Collect all the stats after
+ *		workers are stopped, else some work done by workers won't be
+ *		accounted.
+ * ----------------------------------------------------------------
+ */
+static void
+ExecShutdownGatherMergeWorkers(GatherMergeState *node)
+{
+	/* Shut down tuple queue readers before shutting down workers. */
+	if (node->reader != NULL)
+	{
+		int			i;
+
+		for (i = 0; i < node->nreaders; ++i)
+			if (node->reader[i])
+				DestroyTupleQueueReader(node->reader[i]);
+
+		pfree(node->reader);
+		node->reader = NULL;
+	}
+
+	/* Now shut down the workers. */
+	if (node->pei != NULL)
+		ExecParallelFinish(node->pei);
+}
+
+/* ----------------------------------------------------------------
+ *		ExecReScanGatherMerge
+ *
+ *		Re-initialize the workers and rescans a relation via them.
+ * ----------------------------------------------------------------
+ */
+void
+ExecReScanGatherMerge(GatherMergeState *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
+	 * to propagate any error or other information to master backend before
+	 * dying.  Parallel context will be reused for rescan.
+	 */
+	ExecShutdownGatherMergeWorkers(node);
+
+	node->initialized = false;
+
+	if (node->pei)
+		ExecParallelReinitialize(node->pei);
+
+	ExecReScan(node->ps.lefttree);
+}
+
+/*
+ * Initialize the Gather merge tuple read.
+ *
+ * Pull at least a single tuple from each worker + leader and set up the heap.
+ */
+static void
+gather_merge_init(GatherMergeState *gm_state)
+{
+	int			nreaders = gm_state->nreaders;
+	bool		initialize = true;
+	int			i;
+
+	/*
+	 * Allocate gm_slots for the number of worker + one more slot for leader.
+	 * Last slot is always for leader. Leader always calls ExecProcNode() to
+	 * read the tuple which will return the TupleTableSlot. Later it will
+	 * directly get assigned to gm_slot. So just initialize leader gm_slot
+	 * with NULL. For other slots below code will call
+	 * ExecInitExtraTupleSlot() which will do the initialization of worker
+	 * slots.
+	 */
+	gm_state->gm_slots =
+		palloc((gm_state->nreaders + 1) * sizeof(TupleTableSlot *));
+	gm_state->gm_slots[gm_state->nreaders] = NULL;
+
+	/* Initialize the tuple slot and tuple array for each worker */
+	gm_state->gm_tuple_buffers =
+		(GMReaderTupleBuffer *) palloc0(sizeof(GMReaderTupleBuffer) *
+										(gm_state->nreaders + 1));
+	for (i = 0; i < gm_state->nreaders; i++)
+	{
+		/* Allocate the tuple array with MAX_TUPLE_STORE size */
+		gm_state->gm_tuple_buffers[i].tuple =
+			(HeapTuple *) palloc0(sizeof(HeapTuple) * MAX_TUPLE_STORE);
+
+		/* Initialize slot for worker */
+		gm_state->gm_slots[i] = ExecInitExtraTupleSlot(gm_state->ps.state);
+		ExecSetSlotDescriptor(gm_state->gm_slots[i],
+							  gm_state->tupDesc);
+	}
+
+	/* Allocate the resources for the merge */
+	gm_state->gm_heap = binaryheap_allocate(gm_state->nreaders + 1,
+											heap_compare_slots,
+											gm_state);
+
+	/*
+	 * First, try to read a tuple from each worker (including leader) in
+	 * nowait mode, so that we initialize read from each worker as well as
+	 * leader. After this, if all active workers are unable to produce a
+	 * tuple, then re-read and this time use wait mode. For workers that were
+	 * able to produce a tuple in the earlier loop and are still active, just
+	 * try to fill the tuple array if more tuples are avaiable.
+	 */
+reread:
+	for (i = 0; i < nreaders + 1; i++)
+	{
+		if (!gm_state->gm_tuple_buffers[i].done &&
+			(TupIsNull(gm_state->gm_slots[i]) ||
+			 gm_state->gm_slots[i]->tts_isempty))
+		{
+			if (gather_merge_readnext(gm_state, i, initialize))
+			{
+				binaryheap_add_unordered(gm_state->gm_heap,
+										 Int32GetDatum(i));
+			}
+		}
+		else
+			form_tuple_array(gm_state, i);
+	}
+	initialize = false;
+
+	for (i = 0; i < nreaders; i++)
+		if (!gm_state->gm_tuple_buffers[i].done &&
+			(TupIsNull(gm_state->gm_slots[i]) ||
+			 gm_state->gm_slots[i]->tts_isempty))
+			goto reread;
+
+	binaryheap_build(gm_state->gm_heap);
+	gm_state->gm_initialized = true;
+}
+
+/*
+ * Clear out a slot in the tuple table for each gather merge
+ * slot and return the clear cleared slot.
+ */
+static TupleTableSlot *
+gather_merge_clear_slots(GatherMergeState *gm_state)
+{
+	int			i;
+
+	for (i = 0; i < gm_state->nreaders; i++)
+	{
+		pfree(gm_state->gm_tuple_buffers[i].tuple);
+		gm_state->gm_slots[i] = ExecClearTuple(gm_state->gm_slots[i]);
+	}
+
+	/* Free tuple array as we don't need it any more */
+	pfree(gm_state->gm_tuple_buffers);
+	/* Free the binaryheap, which was created for sort */
+	binaryheap_free(gm_state->gm_heap);
+
+	/* return any clear slot */
+	return gm_state->gm_slots[0];
+}
+
+/*
+ * Read the next tuple for gather merge.
+ *
+ * Fetch the sorted tuple out of the heap.
+ */
+static TupleTableSlot *
+gather_merge_getnext(GatherMergeState *gm_state)
+{
+	int			i;
+
+	/*
+	 * First time through: pull the first tuple from each participate, and set
+	 * up the heap.
+	 */
+	if (gm_state->gm_initialized == false)
+		gather_merge_init(gm_state);
+	else
+	{
+		/*
+		 * Otherwise, pull the next tuple from whichever participant we
+		 * returned from last time, and reinsert the index into the heap,
+		 * because it might now compare differently against the existing
+		 * elements of the heap.
+		 */
+		i = DatumGetInt32(binaryheap_first(gm_state->gm_heap));
+
+		if (gather_merge_readnext(gm_state, i, false))
+			binaryheap_replace_first(gm_state->gm_heap, Int32GetDatum(i));
+		else
+			(void) binaryheap_remove_first(gm_state->gm_heap);
+	}
+
+	if (binaryheap_empty(gm_state->gm_heap))
+	{
+		/* All the queues are exhausted, and so is the heap */
+		return gather_merge_clear_slots(gm_state);
+	}
+	else
+	{
+		i = DatumGetInt32(binaryheap_first(gm_state->gm_heap));
+		return gm_state->gm_slots[i];
+	}
+
+	return gather_merge_clear_slots(gm_state);
+}
+
+/*
+ * Read the tuple for given reader in nowait mode, and form the tuple array.
+ */
+static void
+form_tuple_array(GatherMergeState *gm_state, int reader)
+{
+	GMReaderTupleBuffer *tuple_buffer = &gm_state->gm_tuple_buffers[reader];
+	int			i;
+
+	/* Last slot is for leader and we don't build tuple array for leader */
+	if (reader == gm_state->nreaders)
+		return;
+
+	/*
+	 * We here because we already read all the tuples from the tuple array, so
+	 * initialize the counter to zero.
+	 */
+	if (tuple_buffer->nTuples == tuple_buffer->readCounter)
+		tuple_buffer->nTuples = tuple_buffer->readCounter = 0;
+
+	/* Tuple array is already full? */
+	if (tuple_buffer->nTuples == MAX_TUPLE_STORE)
+		return;
+
+	for (i = tuple_buffer->nTuples; i < MAX_TUPLE_STORE; i++)
+	{
+		tuple_buffer->tuple[i] = heap_copytuple(gm_readnext_tuple(gm_state,
+																  reader,
+																  false,
+													   &tuple_buffer->done));
+		if (!HeapTupleIsValid(tuple_buffer->tuple[i]))
+			break;
+		tuple_buffer->nTuples++;
+	}
+}
+
+/*
+ * Store the next tuple for a given reader into the appropriate slot.
+ *
+ * Returns false if the reader is exhausted, and true otherwise.
+ */
+static bool
+gather_merge_readnext(GatherMergeState *gm_state, int reader, bool nowait)
+{
+	GMReaderTupleBuffer *tuple_buffer;
+	HeapTuple	tup = NULL;
+
+	/*
+	 * If we're being asked to generate a tuple from the leader, then we
+	 * just call ExecProcNode as normal to produce one.
+	 */
+	if (gm_state->nreaders == reader)
+	{
+		if (gm_state->need_to_scan_locally)
+		{
+			PlanState  *outerPlan = outerPlanState(gm_state);
+			TupleTableSlot *outerTupleSlot;
+
+			outerTupleSlot = ExecProcNode(outerPlan);
+
+			if (!TupIsNull(outerTupleSlot))
+			{
+				gm_state->gm_slots[reader] = outerTupleSlot;
+				return true;
+			}
+			gm_state->gm_tuple_buffers[reader].done = true;
+			gm_state->need_to_scan_locally = false;
+		}
+		return false;
+	}
+
+	/* Otherwise, check the state of the relevant tuple buffer. */
+	tuple_buffer = &gm_state->gm_tuple_buffers[reader];
+
+	if (tuple_buffer->nTuples > tuple_buffer->readCounter)
+	{
+		/* Return any tuple previously read that is still buffered. */
+		tuple_buffer = &gm_state->gm_tuple_buffers[reader];
+		tup = tuple_buffer->tuple[tuple_buffer->readCounter++];
+	}
+	else if (tuple_buffer->done)
+	{
+		/* Reader is known to be exhausted. */
+		DestroyTupleQueueReader(gm_state->reader[reader]);
+		gm_state->reader[reader] = NULL;
+		return false;
+	}
+	else
+	{
+		/* Read and buffer next tuple. */
+		tup = heap_copytuple(gm_readnext_tuple(gm_state,
+											   reader,
+											   nowait,
+											   &tuple_buffer->done));
+
+		/*
+		 * Attempt to read more tuples in nowait mode and store them in
+		 * the tuple array.
+		 */
+		if (HeapTupleIsValid(tup))
+			form_tuple_array(gm_state, reader);
+		else
+			return false;
+	}
+
+	Assert(HeapTupleIsValid(tup));
+
+	/* Build the TupleTableSlot for the given tuple */
+	ExecStoreTuple(tup,			/* tuple to store */
+				   gm_state->gm_slots[reader],	/* slot in which to store the
+												 * tuple */
+				   InvalidBuffer,		/* buffer associated with this tuple */
+				   true);		/* pfree this pointer if not from heap */
+
+	return true;
+}
+
+/*
+ * Attempt to read a tuple from given reader.
+ */
+static HeapTuple
+gm_readnext_tuple(GatherMergeState *gm_state, int nreader, bool nowait,
+				  bool *done)
+{
+	TupleQueueReader *reader;
+	HeapTuple	tup = NULL;
+	MemoryContext oldContext;
+	MemoryContext tupleContext;
+
+	tupleContext = gm_state->ps.ps_ExprContext->ecxt_per_tuple_memory;
+
+	if (done != NULL)
+		*done = false;
+
+	/* Check for async events, particularly messages from workers. */
+	CHECK_FOR_INTERRUPTS();
+
+	/* Attempt to read a tuple. */
+	reader = gm_state->reader[nreader];
+
+	/* Run TupleQueueReaders in per-tuple context */
+	oldContext = MemoryContextSwitchTo(tupleContext);
+	tup = TupleQueueReaderNext(reader, nowait, done);
+	MemoryContextSwitchTo(oldContext);
+
+	return tup;
+}
+
+/*
+ * We have one slot for each item in the heap array.  We use SlotNumber
+ * to store slot indexes.  This doesn't actually provide any formal
+ * type-safety, but it makes the code more self-documenting.
+ */
+typedef int32 SlotNumber;
+
+/*
+ * Compare the tuples in the two given slots.
+ */
+static int32
+heap_compare_slots(Datum a, Datum b, void *arg)
+{
+	GatherMergeState *node = (GatherMergeState *) arg;
+	SlotNumber	slot1 = DatumGetInt32(a);
+	SlotNumber	slot2 = DatumGetInt32(b);
+
+	TupleTableSlot *s1 = node->gm_slots[slot1];
+	TupleTableSlot *s2 = node->gm_slots[slot2];
+	int			nkey;
+
+	Assert(!TupIsNull(s1));
+	Assert(!TupIsNull(s2));
+
+	for (nkey = 0; nkey < node->gm_nkeys; nkey++)
+	{
+		SortSupport sortKey = node->gm_sortkeys + nkey;
+		AttrNumber	attno = sortKey->ssup_attno;
+		Datum		datum1,
+					datum2;
+		bool		isNull1,
+					isNull2;
+		int			compare;
+
+		datum1 = slot_getattr(s1, attno, &isNull1);
+		datum2 = slot_getattr(s2, attno, &isNull2);
+
+		compare = ApplySortComparator(datum1, isNull1,
+									  datum2, isNull2,
+									  sortKey);
+		if (compare != 0)
+			return -compare;
+	}
+	return 0;
+}