From 0e3e1c4e1cea68073132fe817fb3a98cb5c1b805 Mon Sep 17 00:00:00 2001
From: David Rowley <drowley@postgresql.org>
Date: Wed, 29 Jul 2020 11:42:21 +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 a283e4d45c..54e3797a15 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 bbfc4af1ec..7d7bfa9456 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;
 	}
 
 	/*