From 0588ee63aa2d8c5765d086991555cd9acdd4d86f Mon Sep 17 00:00:00 2001
From: Jeff Davis <jdavis@postgresql.org>
Date: Fri, 3 Apr 2020 19:52:16 -0700
Subject: [PATCH] Include chunk overhead in hash table entry size estimate.

Don't try to be precise about it, just use a constant 16 bytes of
chunk overhead. Being smarter would require knowing the memory context
where the chunk will be allocated, which is not known by all callers.

Discussion: https://postgr.es/m/20200325220936.il3ni2fj2j2b45y5@alap3.anarazel.de
---
 src/backend/executor/nodeAgg.c | 36 ++++++++++++++++++++++++++++------
 src/include/executor/nodeAgg.h |  2 +-
 2 files changed, 31 insertions(+), 7 deletions(-)

diff --git a/src/backend/executor/nodeAgg.c b/src/backend/executor/nodeAgg.c
index 4c8c5cfc07..4e100e5755 100644
--- a/src/backend/executor/nodeAgg.c
+++ b/src/backend/executor/nodeAgg.c
@@ -297,6 +297,12 @@
 /* minimum number of initial hash table buckets */
 #define HASHAGG_MIN_BUCKETS 256
 
+/*
+ * Estimate chunk overhead as a constant 16 bytes. XXX: should this be
+ * improved?
+ */
+#define CHUNKHDRSZ 16
+
 /*
  * Track all tapes needed for a HashAgg that spills. We don't know the maximum
  * number of tapes needed at the start of the algorithm (because it can
@@ -1639,14 +1645,32 @@ find_hash_columns(AggState *aggstate)
  * Estimate per-hash-table-entry overhead.
  */
 Size
-hash_agg_entry_size(int numAggs, Size tupleWidth, Size transitionSpace)
+hash_agg_entry_size(int numTrans, Size tupleWidth, Size transitionSpace)
 {
+	Size    tupleChunkSize;
+	Size    pergroupChunkSize;
+	Size    transitionChunkSize;
+	Size    tupleSize	 = (MAXALIGN(SizeofMinimalTupleHeader) +
+							tupleWidth);
+	Size    pergroupSize = numTrans * sizeof(AggStatePerGroupData);
+
+	tupleChunkSize = CHUNKHDRSZ + tupleSize;
+
+	if (pergroupSize > 0)
+		pergroupChunkSize = CHUNKHDRSZ + pergroupSize;
+	else
+		pergroupChunkSize = 0;
+
+	if (transitionSpace > 0)
+		transitionChunkSize = CHUNKHDRSZ + transitionSpace;
+	else
+		transitionChunkSize = 0;
+
 	return
-		MAXALIGN(SizeofMinimalTupleHeader) +
-		MAXALIGN(tupleWidth) +
-		MAXALIGN(sizeof(TupleHashEntryData) +
-				 numAggs * sizeof(AggStatePerGroupData)) +
-		transitionSpace;
+		sizeof(TupleHashEntryData) +
+		tupleChunkSize +
+		pergroupChunkSize +
+		transitionChunkSize;
 }
 
 /*
diff --git a/src/include/executor/nodeAgg.h b/src/include/executor/nodeAgg.h
index a5b8a004d1..c2b55728bf 100644
--- a/src/include/executor/nodeAgg.h
+++ b/src/include/executor/nodeAgg.h
@@ -314,7 +314,7 @@ extern AggState *ExecInitAgg(Agg *node, EState *estate, int eflags);
 extern void ExecEndAgg(AggState *node);
 extern void ExecReScanAgg(AggState *node);
 
-extern Size hash_agg_entry_size(int numAggs, Size tupleWidth,
+extern Size hash_agg_entry_size(int numTrans, Size tupleWidth,
 								Size transitionSpace);
 extern void hash_agg_set_limits(double hashentrysize, uint64 input_groups,
 								int used_bits, Size *mem_limit,