From a57c837e5cdf601d6ec05e5e10a40d01f1d2b84e Mon Sep 17 00:00:00 2001
From: David Rowley <drowley@postgresql.org>
Date: Wed, 29 Jul 2020 11:43:11 +1200
Subject: [PATCH] Make EXPLAIN ANALYZE of HashAgg more similar to Hash Join

There were various unnecessary differences between Hash Agg's EXPLAIN
ANALYZE output and Hash Join's.  Here we modify the Hash Agg output so
that it's better aligned to Hash Join's.

The following changes have been made:
1. Start batches counter at 1 instead of 0.
2. Always display the "Batches" property, even when we didn't spill to
   disk.
3. Use the text "Batches" instead of "HashAgg Batches" for text format.
4. Use the text "Memory Usage" instead of "Peak Memory Usage" for text
   format.
5. Include "Batches" before "Memory Usage" in both text and non-text
   formats.

In passing also modify the "Planned Partitions" property so that we show
it regardless of if the value is 0 or not for non-text EXPLAIN formats.
This was pointed out by Justin Pryzby and probably should have been part
of 40efbf870.

Reviewed-by: Justin Pryzby, Jeff Davis
Discussion: https://postgr.es/m/CAApHDvrshRnA6C0VFnu7Fb9TVvgGo80PUMm5+2DiaS1gEkPvtw@mail.gmail.com
Backpatch-through: 13, where HashAgg batching was introduced
---
 src/backend/commands/explain.c | 35 +++++++++++++++++-----------------
 src/backend/executor/nodeAgg.c |  3 +++
 2 files changed, 20 insertions(+), 18 deletions(-)

diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c
index a283e4d45c8..54e3797a15b 100644
--- a/src/backend/commands/explain.c
+++ b/src/backend/commands/explain.c
@@ -3059,21 +3059,19 @@ show_hashagg_info(AggState *aggstate, ExplainState *es)
 	if (es->format != EXPLAIN_FORMAT_TEXT)
 	{
 
-		if (es->costs && aggstate->hash_planned_partitions > 0)
-		{
+		if (es->costs)
 			ExplainPropertyInteger("Planned Partitions", NULL,
 								   aggstate->hash_planned_partitions, es);
-		}
 
 		if (!es->analyze)
 			return;
 
 		/* EXPLAIN ANALYZE */
+		ExplainPropertyInteger("HashAgg Batches", NULL,
+							   aggstate->hash_batches_used, es);
 		ExplainPropertyInteger("Peak Memory Usage", "kB", memPeakKb, es);
 		ExplainPropertyInteger("Disk Usage", "kB",
 							   aggstate->hash_disk_used, es);
-		ExplainPropertyInteger("HashAgg Batches", NULL,
-							   aggstate->hash_batches_used, es);
 	}
 	else
 	{
@@ -3099,13 +3097,13 @@ show_hashagg_info(AggState *aggstate, ExplainState *es)
 		else
 			appendStringInfoString(es->str, "  ");
 
-		appendStringInfo(es->str, "Peak Memory Usage: " INT64_FORMAT "kB",
-						 memPeakKb);
+		appendStringInfo(es->str, "Batches: %d  Memory Usage: " INT64_FORMAT "kB",
+						 aggstate->hash_batches_used, 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);
+		/* Only display disk usage if we spilled to disk */
+		if (aggstate->hash_batches_used > 1)
+			appendStringInfo(es->str, "  Disk Usage: " UINT64_FORMAT "kB",
+							 aggstate->hash_disk_used);
 		appendStringInfoChar(es->str, '\n');
 	}
 
@@ -3130,21 +3128,22 @@ show_hashagg_info(AggState *aggstate, ExplainState *es)
 			{
 				ExplainIndentText(es);
 
-				appendStringInfo(es->str, "Peak Memory Usage: " INT64_FORMAT "kB",
-								 memPeakKb);
+				appendStringInfo(es->str, "Batches: %d  Memory Usage: " INT64_FORMAT "kB",
+								 hash_batches_used, memPeakKb);
 
-				if (hash_batches_used > 0)
-					appendStringInfo(es->str, "  Disk Usage: " UINT64_FORMAT "kB  HashAgg Batches: %d",
-									 hash_disk_used, hash_batches_used);
+				/* Only display disk usage if we spilled to disk */
+				if (hash_batches_used > 1)
+					appendStringInfo(es->str, "  Disk Usage: " UINT64_FORMAT "kB",
+									 hash_disk_used);
 				appendStringInfoChar(es->str, '\n');
 			}
 			else
 			{
+				ExplainPropertyInteger("HashAgg Batches", NULL,
+									   hash_batches_used, es);
 				ExplainPropertyInteger("Peak Memory Usage", "kB", memPeakKb,
 									   es);
 				ExplainPropertyInteger("Disk Usage", "kB", hash_disk_used, es);
-				ExplainPropertyInteger("HashAgg Batches", NULL,
-									   hash_batches_used, es);
 			}
 
 			if (es->workers_state)
diff --git a/src/backend/executor/nodeAgg.c b/src/backend/executor/nodeAgg.c
index ea928d5cb7c..5ca1751f956 100644
--- a/src/backend/executor/nodeAgg.c
+++ b/src/backend/executor/nodeAgg.c
@@ -3641,6 +3641,9 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
 		find_hash_columns(aggstate);
 		build_hash_tables(aggstate);
 		aggstate->table_filled = false;
+
+		/* Initialize this to 1, meaning nothing spilled, yet */
+		aggstate->hash_batches_used = 1;
 	}
 
 	/*