v5-0002-adjustments.patch

From 27544e1504a0f014165e1120e5230e22a424b8b4 Mon Sep 17 00:00:00 2001
From: Tomas Vondra <tomas@vondra.me>
Date: Fri, 10 Oct 2025 00:38:05 +0200
Subject: [PATCH v5 2/3] adjustments

---
 src/backend/executor/nodeHash.c | 43 ++++++++++++++++++++++-----------
 1 file changed, 29 insertions(+), 14 deletions(-)

diff --git a/src/backend/executor/nodeHash.c b/src/backend/executor/nodeHash.c
index 0f815674c9f..264c5b7ed40 100644
--- a/src/backend/executor/nodeHash.c
+++ b/src/backend/executor/nodeHash.c
@@ -883,37 +883,51 @@ ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,
 	 *
 	 * Note that we can only change nbuckets during initial hashtable sizing.
 	 * Once we start building the hash, nbuckets is fixed.
+	 *
+	 * We will double a number of parameters (space_allowed, nbuckets and
+	 * num_skew_mcvs), which brings the overflow risk. We simply stop the loop
+	 * if that happens. We could do something smarter (e.g. cap nbuckets and
+	 * continue), but the complexity is not worth it. It's extremely rare and
+	 * this is best-effort attempt to reduce memory usage.
 	 */
 	while (nbatch > 1)
 	{
 		/*
-		 * Ensure that nbuckets can be allocated. nbuckets may have been
-		 * increased above hash_table_size / sizeof(HashJoinTuple) if work_mem
-		 * was very small, so checking if MaxAllocSize is enough for the
-		 * buckets is the safest check.
+		 * Check that nbuckets wont't overflow MaxAllocSize.
+		 *
+		 * With extremely low work_mem values, nbuckets may have been set
+		 * higher than hash_table_size / sizeof(HashJoinTuple). We don't try
+		 * to correct that here, we accept nbuckets to be oversized.
 		 */
 		if (nbuckets > MaxAllocSize / sizeof(HashJoinTuple) / 2)
 			break;
 
 		/*
-		 * We check that space_allowed wouldn't overflow because it will be
-		 * larger than hash_table_bytes if there is a skew hashtable.
+		 * Check that space_allowed won't overflow.
+		 *
+		 * We don't check hash_table_bytes, because we subtracted space for
+		 * skew hashtable from it (so it may not be equal to space_allowed).
 		 */
 		if ((*space_allowed) > SIZE_MAX / 2)
 			break;
 
 		/*
-		 * This is the same as:
+		 * Check that num_skew_mcvs won't overflow.
+		 */
+		if ((*num_skew_mcvs) > INT_MAX / 2)
+			break;
+
+		/*
+		 * Will halving the number of batches and doubling the size of the
+		 * hashtable reduce overall memory usage?
 		 *
-		 * hash_table_bytes + 2 * nbatch * BLCKSZ < hash_table_bytes * 2 +
-		 * nbatch * BLCKSZ
+		 * This is the same as (S = space_allowed):
 		 *
-		 * avoiding intermediate overflow.
+		 * (S + 2 * nbatch * BLCKSZ) < (S * 2 + nbatch * BLCKSZ)
 		 *
-		 * which is to say: will halving the number of batches and doubling
-		 * the size of the hashtable reduce overall memory usage?
+		 * but avoiding intermediate overflow.
 		 */
-		if (nbatch < hash_table_bytes / BLCKSZ)
+		if (nbatch < *space_allowed / BLCKSZ)
 			break;
 
 		/*
@@ -921,7 +935,8 @@ ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,
 		 * overflowing nbuckets.
 		 */
 		nbuckets *= 2;
-		hash_table_bytes *= 2;
+
+		*num_skew_mcvs = (*num_skew_mcvs) * 2;
 		*space_allowed = (*space_allowed) * 2;
 
 		nbatch /= 2;
-- 
2.43.0