Fix EXPLAIN ANALYZE for parallel HashAgg plans

Since 1f39bce02, HashAgg nodes have had the ability to spill to disk when
memory consumption exceeds work_mem. That commit added new properties to
EXPLAIN ANALYZE to show the maximum memory usage and disk usage, however,
it didn't quite go as far as showing that information for parallel
workers.  Since workers may have experienced something very different from
the main process, we should show this information per worker, as is done
in Sort.

Reviewed-by: Justin Pryzby
Reviewed-by: Jeff Davis
Discussion: https://postgr.es/m/CAApHDvpEKbfZa18mM1TD7qV6PG+w97pwCWq5tVD0dX7e11gRJw@mail.gmail.com
Backpatch-through: 13, where the hashagg spilling code was added.

src/backend/commands/explain.c

@@ -3051,12 +3051,13 @@ show_hashagg_info(AggState *aggstate, ExplainState *es)
 	Agg		   *agg = (Agg *) aggstate->ss.ps.plan;
 	int64		memPeakKb = (aggstate->hash_mem_peak + 1023) / 1024;
 
-	Assert(IsA(aggstate, AggState));
-
 	if (agg->aggstrategy != AGG_HASHED &&
 		agg->aggstrategy != AGG_MIXED)
 		return;
 
+	if (es->format != EXPLAIN_FORMAT_TEXT)
+	{
+
 		if (es->costs && aggstate->hash_planned_partitions > 0)
 		{
 			ExplainPropertyInteger("Planned Partitions", NULL,
@@ -3076,6 +3077,87 @@ show_hashagg_info(AggState *aggstate, ExplainState *es)
 								   aggstate->hash_batches_used, es);
 		}
 	}
+	else
+	{
+		bool		gotone = false;
+
+		if (es->costs && aggstate->hash_planned_partitions > 0)
+		{
+			ExplainIndentText(es);
+			appendStringInfo(es->str, "Planned Partitions: %d",
+							 aggstate->hash_planned_partitions);
+			gotone = true;
+		}
+
+		if (!es->analyze)
+		{
+			if (gotone)
+				appendStringInfoChar(es->str, '\n');
+			return;
+		}
+
+		if (!gotone)
+			ExplainIndentText(es);
+		else
+			appendStringInfoString(es->str, "  ");
+
+		appendStringInfo(es->str, "Peak Memory Usage: " INT64_FORMAT " kB",
+						 memPeakKb);
+
+		if (aggstate->hash_batches_used > 0)
+			appendStringInfo(es->str, "  Disk Usage: " UINT64_FORMAT " kB  HashAgg Batches: %d",
+							 aggstate->hash_disk_used,
+							 aggstate->hash_batches_used);
+		appendStringInfoChar(es->str, '\n');
+	}
+
+	/* Display stats for each parallel worker */
+	if (es->analyze && aggstate->shared_info != NULL)
+	{
+		for (int n = 0; n < aggstate->shared_info->num_workers; n++)
+		{
+			AggregateInstrumentation *sinstrument;
+			uint64		hash_disk_used;
+			int			hash_batches_used;
+
+			sinstrument = &aggstate->shared_info->sinstrument[n];
+			hash_disk_used = sinstrument->hash_disk_used;
+			hash_batches_used = sinstrument->hash_batches_used;
+			memPeakKb = (sinstrument->hash_mem_peak + 1023) / 1024;
+
+			if (es->workers_state)
+				ExplainOpenWorker(n, es);
+
+			if (es->format == EXPLAIN_FORMAT_TEXT)
+			{
+				ExplainIndentText(es);
+
+				appendStringInfo(es->str, "Peak Memory Usage: " INT64_FORMAT " kB",
+								 memPeakKb);
+
+				if (hash_batches_used > 0)
+					appendStringInfo(es->str, "  Disk Usage: " UINT64_FORMAT " kB  HashAgg Batches: %d",
+									 hash_disk_used, hash_batches_used);
+				appendStringInfoChar(es->str, '\n');
+			}
+			else
+			{
+				ExplainPropertyInteger("Peak Memory Usage", "kB", memPeakKb,
+									   es);
+				if (hash_batches_used > 0)
+				{
+					ExplainPropertyInteger("Disk Usage", "kB", hash_disk_used,
+										   es);
+					ExplainPropertyInteger("HashAgg Batches", NULL,
+										   hash_batches_used, es);
+				}
+			}
+
+			if (es->workers_state)
+				ExplainCloseWorker(n, es);
+		}
+	}
+}
 
 /*
  * If it's EXPLAIN ANALYZE, show exact/lossy pages for a BitmapHeapScan node

src/backend/executor/execParallel.c

@@ -25,6 +25,7 @@
 
 #include "executor/execParallel.h"
 #include "executor/executor.h"
+#include "executor/nodeAgg.h"
 #include "executor/nodeAppend.h"
 #include "executor/nodeBitmapHeapscan.h"
 #include "executor/nodeCustom.h"
@@ -288,7 +289,10 @@ ExecParallelEstimate(PlanState *planstate, ExecParallelEstimateContext *e)
 			/* even when not parallel-aware, for EXPLAIN ANALYZE */
 			ExecIncrementalSortEstimate((IncrementalSortState *) planstate, e->pcxt);
 			break;
-
+		case T_AggState:
+			/* even when not parallel-aware, for EXPLAIN ANALYZE */
+			ExecAggEstimate((AggState *) planstate, e->pcxt);
+			break;
 		default:
 			break;
 	}
@@ -505,7 +509,10 @@ ExecParallelInitializeDSM(PlanState *planstate,
 			/* even when not parallel-aware, for EXPLAIN ANALYZE */
 			ExecIncrementalSortInitializeDSM((IncrementalSortState *) planstate, d->pcxt);
 			break;
-
+		case T_AggState:
+			/* even when not parallel-aware, for EXPLAIN ANALYZE */
+			ExecAggInitializeDSM((AggState *) planstate, d->pcxt);
+			break;
 		default:
 			break;
 	}
@@ -1048,6 +1055,9 @@ ExecParallelRetrieveInstrumentation(PlanState *planstate,
 		case T_HashState:
 			ExecHashRetrieveInstrumentation((HashState *) planstate);
 			break;
+		case T_AggState:
+			ExecAggRetrieveInstrumentation((AggState *) planstate);
+			break;
 		default:
 			break;
 	}
@@ -1336,7 +1346,10 @@ ExecParallelInitializeWorker(PlanState *planstate, ParallelWorkerContext *pwcxt)
 			ExecIncrementalSortInitializeWorker((IncrementalSortState *) planstate,
 												pwcxt);
 			break;
-
+		case T_AggState:
+			/* even when not parallel-aware, for EXPLAIN ANALYZE */
+			ExecAggInitializeWorker((AggState *) planstate, pwcxt);
+			break;
 		default:
 			break;
 	}

src/backend/executor/nodeAgg.c

@@ -240,6 +240,7 @@
 #include "postgres.h"
 
 #include "access/htup_details.h"
+#include "access/parallel.h"
 #include "catalog/objectaccess.h"
 #include "catalog/pg_aggregate.h"
 #include "catalog/pg_proc.h"
@@ -4483,6 +4484,22 @@ ExecEndAgg(AggState *node)
 	int			numGroupingSets = Max(node->maxsets, 1);
 	int			setno;
 
+	/*
+	 * When ending a parallel worker, copy the statistics gathered by the
+	 * worker back into shared memory so that it can be picked up by the main
+	 * process to report in EXPLAIN ANALYZE.
+	 */
+	if (node->shared_info && IsParallelWorker())
+	{
+		AggregateInstrumentation *si;
+
+		Assert(ParallelWorkerNumber <= node->shared_info->num_workers);
+		si = &node->shared_info->sinstrument[ParallelWorkerNumber];
+		si->hash_batches_used = node->hash_batches_used;
+		si->hash_disk_used = node->hash_disk_used;
+		si->hash_mem_peak = node->hash_mem_peak;
+	}
+
 	/* Make sure we have closed any open tuplesorts */
 
 	if (node->sort_in)
@@ -4854,3 +4871,89 @@ aggregate_dummy(PG_FUNCTION_ARGS)
 		 fcinfo->flinfo->fn_oid);
 	return (Datum) 0;			/* keep compiler quiet */
 }
+
+/* ----------------------------------------------------------------
+ *						Parallel Query Support
+ * ----------------------------------------------------------------
+ */
+
+ /* ----------------------------------------------------------------
+  *		ExecAggEstimate
+  *
+  *		Estimate space required to propagate aggregate statistics.
+  * ----------------------------------------------------------------
+  */
+void
+ExecAggEstimate(AggState *node, ParallelContext *pcxt)
+{
+	Size		size;
+
+	/* don't need this if not instrumenting or no workers */
+	if (!node->ss.ps.instrument || pcxt->nworkers == 0)
+		return;
+
+	size = mul_size(pcxt->nworkers, sizeof(AggregateInstrumentation));
+	size = add_size(size, offsetof(SharedAggInfo, sinstrument));
+	shm_toc_estimate_chunk(&pcxt->estimator, size);
+	shm_toc_estimate_keys(&pcxt->estimator, 1);
+}
+
+/* ----------------------------------------------------------------
+ *		ExecAggInitializeDSM
+ *
+ *		Initialize DSM space for aggregate statistics.
+ * ----------------------------------------------------------------
+ */
+void
+ExecAggInitializeDSM(AggState *node, ParallelContext *pcxt)
+{
+	Size		size;
+
+	/* don't need this if not instrumenting or no workers */
+	if (!node->ss.ps.instrument || pcxt->nworkers == 0)
+		return;
+
+	size = offsetof(SharedAggInfo, sinstrument)
+		+ pcxt->nworkers * sizeof(AggregateInstrumentation);
+	node->shared_info = shm_toc_allocate(pcxt->toc, size);
+	/* ensure any unfilled slots will contain zeroes */
+	memset(node->shared_info, 0, size);
+	node->shared_info->num_workers = pcxt->nworkers;
+	shm_toc_insert(pcxt->toc, node->ss.ps.plan->plan_node_id,
+				   node->shared_info);
+}
+
+/* ----------------------------------------------------------------
+ *		ExecAggInitializeWorker
+ *
+ *		Attach worker to DSM space for aggregate statistics.
+ * ----------------------------------------------------------------
+ */
+void
+ExecAggInitializeWorker(AggState *node, ParallelWorkerContext *pwcxt)
+{
+	node->shared_info =
+		shm_toc_lookup(pwcxt->toc, node->ss.ps.plan->plan_node_id, true);
+}
+
+/* ----------------------------------------------------------------
+ *		ExecAggRetrieveInstrumentation
+ *
+ *		Transfer aggregate statistics from DSM to private memory.
+ * ----------------------------------------------------------------
+ */
+void
+ExecAggRetrieveInstrumentation(AggState *node)
+{
+	Size		size;
+	SharedAggInfo *si;
+
+	if (node->shared_info == NULL)
+		return;
+
+	size = offsetof(SharedAggInfo, sinstrument)
+		+ node->shared_info->num_workers * sizeof(AggregateInstrumentation);
+	si = palloc(size);
+	memcpy(si, node->shared_info, size);
+	node->shared_info = si;
+}

src/include/executor/nodeAgg.h

@@ -14,6 +14,7 @@
 #ifndef NODEAGG_H
 #define NODEAGG_H
 
+#include "access/parallel.h"
 #include "nodes/execnodes.h"
 
 
@@ -323,4 +324,10 @@ extern void hash_agg_set_limits(double hashentrysize, uint64 input_groups,
 								int used_bits, Size *mem_limit,
 								uint64 *ngroups_limit, int *num_partitions);
 
+/* parallel instrumentation support */
+extern void ExecAggEstimate(AggState *node, ParallelContext *pcxt);
+extern void ExecAggInitializeDSM(AggState *node, ParallelContext *pcxt);
+extern void ExecAggInitializeWorker(AggState *node, ParallelWorkerContext *pwcxt);
+extern void ExecAggRetrieveInstrumentation(AggState *node);
+
 #endif							/* NODEAGG_H */

src/include/nodes/execnodes.h

@@ -2101,6 +2101,27 @@ typedef struct GroupState
 	bool		grp_done;		/* indicates completion of Group scan */
 } GroupState;
 
+/* ---------------------
+ *	per-worker aggregate information
+ * ---------------------
+ */
+typedef struct AggregateInstrumentation
+{
+	Size		hash_mem_peak;	/* peak hash table memory usage */
+	uint64		hash_disk_used; /* kB of disk space used */
+	int			hash_batches_used;	/* batches used during entire execution */
+} AggregateInstrumentation;
+
+/* ----------------
+ *	 Shared memory container for per-worker aggregate information
+ * ----------------
+ */
+typedef struct SharedAggInfo
+{
+	int			num_workers;
+	AggregateInstrumentation sinstrument[FLEXIBLE_ARRAY_MEMBER];
+} SharedAggInfo;
+
 /* ---------------------
  *	AggState information
  *
@@ -2190,6 +2211,7 @@ typedef struct AggState
 	AggStatePerGroup *all_pergroups;	/* array of first ->pergroups, than
 										 * ->hash_pergroup */
 	ProjectionInfo *combinedproj;	/* projection machinery */
+	SharedAggInfo *shared_info; /* one entry per worker */
 } AggState;
 
 /* ----------------