Disk-based Hash Aggregation.

While performing hash aggregation, track memory usage when adding new
groups to a hash table. If the memory usage exceeds work_mem, enter
"spill mode".

In spill mode, new groups are not created in the hash table(s), but
existing groups continue to be advanced if input tuples match. Tuples
that would cause a new group to be created are instead spilled to a
logical tape to be processed later.

The tuples are spilled in a partitioned fashion. When all tuples from
the outer plan are processed (either by advancing the group or
spilling the tuple), finalize and emit the groups from the hash
table. Then, create new batches of work from the spilled partitions,
and select one of the saved batches and process it (possibly spilling
recursively).

Author: Jeff Davis
Reviewed-by: Tomas Vondra, Adam Lee, Justin Pryzby, Taylor Vesely, Melanie Plageman
Discussion: https://postgr.es/m/507ac540ec7c20136364b5272acbcd4574aa76ef.camel@j-davis.com

1 files changed, 37 insertions, 0 deletions

diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c
index d901dc4a50e..58141d8393c 100644
--- a/src/backend/commands/explain.c
+++ b/src/backend/commands/explain.c
@@ -104,6 +104,7 @@ static void show_tablesample(TableSampleClause *tsc, PlanState *planstate,
 							 List *ancestors, ExplainState *es);
 static void show_sort_info(SortState *sortstate, ExplainState *es);
 static void show_hash_info(HashState *hashstate, ExplainState *es);
+static void show_hashagg_info(AggState *hashstate, ExplainState *es);
 static void show_tidbitmap_info(BitmapHeapScanState *planstate,
 								ExplainState *es);
 static void show_instrumentation_count(const char *qlabel, int which,
@@ -1882,6 +1883,7 @@ ExplainNode(PlanState *planstate, List *ancestors,
 		case T_Agg:
 			show_agg_keys(castNode(AggState, planstate), ancestors, es);
 			show_upper_qual(plan->qual, "Filter", planstate, ancestors, es);
+			show_hashagg_info((AggState *) planstate, es);
 			if (plan->qual)
 				show_instrumentation_count("Rows Removed by Filter", 1,
 										   planstate, es);
@@ -2770,6 +2772,41 @@ show_hash_info(HashState *hashstate, ExplainState *es)
 }
 
 /*
+ * Show information on hash aggregate memory usage and batches.
+ */
+static void
+show_hashagg_info(AggState *aggstate, ExplainState *es)
+{
+	Agg		*agg	   = (Agg *)aggstate->ss.ps.plan;
+	long	 memPeakKb = (aggstate->hash_mem_peak + 1023) / 1024;
+
+	Assert(IsA(aggstate, AggState));
+
+	if (agg->aggstrategy != AGG_HASHED &&
+		agg->aggstrategy != AGG_MIXED)
+		return;
+
+	if (es->costs && aggstate->hash_planned_partitions > 0)
+	{
+		ExplainPropertyInteger("Planned Partitions", NULL,
+							   aggstate->hash_planned_partitions, es);
+	}
+
+	if (!es->analyze)
+		return;
+
+	/* EXPLAIN ANALYZE */
+	ExplainPropertyInteger("Peak Memory Usage", "kB", memPeakKb, es);
+	if (aggstate->hash_batches_used > 0)
+	{
+		ExplainPropertyInteger("Disk Usage", "kB",
+							   aggstate->hash_disk_used, es);
+		ExplainPropertyInteger("HashAgg Batches", NULL,
+							   aggstate->hash_batches_used, es);
+	}
+}
+
+/*
  * If it's EXPLAIN ANALYZE, show exact/lossy pages for a BitmapHeapScan node
  */
 static void