From 770ddfad484ffcc555c0797465299677350a5c25 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Wed, 13 Aug 2025 17:40:13 +0900
Subject: [PATCH v1 02/11] Minimize footprint of TOAST_MAX_CHUNK_SIZE in heap
 and amcheck

This eases a follow-up change to support 8-byte TOAST value IDs, as the
maximum chunk size allowed for a single chunk of TOASTed data depends on
the size of the value ID.
---
 src/backend/access/heap/heaptoast.c | 20 ++++++++++++--------
 contrib/amcheck/verify_heapam.c     | 13 +++++++++----
 2 files changed, 21 insertions(+), 12 deletions(-)

diff --git a/src/backend/access/heap/heaptoast.c b/src/backend/access/heap/heaptoast.c
index 747b8c56fc95..d1f9c25e93cd 100644
--- a/src/backend/access/heap/heaptoast.c
+++ b/src/backend/access/heap/heaptoast.c
@@ -634,11 +634,12 @@ heap_fetch_toast_slice(Relation toastrel, Oid8 valueid, int32 attrsize,
 	SysScanDesc toastscan;
 	HeapTuple	ttup;
 	int32		expectedchunk;
-	int32		totalchunks = ((attrsize - 1) / TOAST_MAX_CHUNK_SIZE) + 1;
+	int32		totalchunks;
 	int			startchunk;
 	int			endchunk;
 	int			num_indexes;
 	int			validIndex;
+	int32		max_chunk_size;
 
 	/* Look for the valid index of toast relation */
 	validIndex = toast_open_indexes(toastrel,
@@ -646,8 +647,11 @@ heap_fetch_toast_slice(Relation toastrel, Oid8 valueid, int32 attrsize,
 									&toastidxs,
 									&num_indexes);
 
-	startchunk = sliceoffset / TOAST_MAX_CHUNK_SIZE;
-	endchunk = (sliceoffset + slicelength - 1) / TOAST_MAX_CHUNK_SIZE;
+	max_chunk_size = TOAST_MAX_CHUNK_SIZE;
+
+	totalchunks = ((attrsize - 1) / max_chunk_size) + 1;
+	startchunk = sliceoffset / max_chunk_size;
+	endchunk = (sliceoffset + slicelength - 1) / max_chunk_size;
 	Assert(endchunk <= totalchunks);
 
 	/* Set up a scan key to fetch from the index. */
@@ -747,8 +751,8 @@ heap_fetch_toast_slice(Relation toastrel, Oid8 valueid, int32 attrsize,
 									 curchunk,
 									 startchunk, endchunk, valueid,
 									 RelationGetRelationName(toastrel))));
-		expected_size = curchunk < totalchunks - 1 ? TOAST_MAX_CHUNK_SIZE
-			: attrsize - ((totalchunks - 1) * TOAST_MAX_CHUNK_SIZE);
+		expected_size = curchunk < totalchunks - 1 ? max_chunk_size
+			: attrsize - ((totalchunks - 1) * max_chunk_size);
 		if (chunksize != expected_size)
 			ereport(ERROR,
 					(errcode(ERRCODE_DATA_CORRUPTED),
@@ -763,12 +767,12 @@ heap_fetch_toast_slice(Relation toastrel, Oid8 valueid, int32 attrsize,
 		chcpystrt = 0;
 		chcpyend = chunksize - 1;
 		if (curchunk == startchunk)
-			chcpystrt = sliceoffset % TOAST_MAX_CHUNK_SIZE;
+			chcpystrt = sliceoffset % max_chunk_size;
 		if (curchunk == endchunk)
-			chcpyend = (sliceoffset + slicelength - 1) % TOAST_MAX_CHUNK_SIZE;
+			chcpyend = (sliceoffset + slicelength - 1) % max_chunk_size;
 
 		memcpy(VARDATA(result) +
-			   curchunk * TOAST_MAX_CHUNK_SIZE - sliceoffset + chcpystrt,
+			   (curchunk * max_chunk_size - sliceoffset) + chcpystrt,
 			   chunkdata + chcpystrt,
 			   (chcpyend - chcpystrt) + 1);
 
diff --git a/contrib/amcheck/verify_heapam.c b/contrib/amcheck/verify_heapam.c
index b67fe0cc3f37..22bba6161de4 100644
--- a/contrib/amcheck/verify_heapam.c
+++ b/contrib/amcheck/verify_heapam.c
@@ -1558,15 +1558,19 @@ check_toast_tuple(HeapTuple toasttup, HeapCheckContext *ctx,
 				  uint32 extsize)
 {
 	int32		chunk_seq;
-	int32		last_chunk_seq = (extsize - 1) / TOAST_MAX_CHUNK_SIZE;
+	int32		last_chunk_seq;
 	Pointer		chunk;
 	bool		isnull;
 	int32		chunksize;
 	int32		expected_size;
 	Oid8		toast_valueid;
+	int32		max_chunk_size;
 
 	toast_valueid = ta->toast_pointer.va_valueid;
 
+	max_chunk_size = TOAST_MAX_CHUNK_SIZE;
+	last_chunk_seq = (extsize - 1) / max_chunk_size;
+
 	/* Sanity-check the sequence number. */
 	chunk_seq = DatumGetInt32(fastgetattr(toasttup, 2,
 										  ctx->toast_rel->rd_att, &isnull));
@@ -1631,8 +1635,8 @@ check_toast_tuple(HeapTuple toasttup, HeapCheckContext *ctx,
 		return;
 	}
 
-	expected_size = chunk_seq < last_chunk_seq ? TOAST_MAX_CHUNK_SIZE
-		: extsize - (last_chunk_seq * TOAST_MAX_CHUNK_SIZE);
+	expected_size = chunk_seq < last_chunk_seq ? max_chunk_size
+		: extsize - (last_chunk_seq * max_chunk_size);
 
 	if (chunksize != expected_size)
 		report_toast_corruption(ctx, ta,
@@ -1874,9 +1878,10 @@ check_toasted_attribute(HeapCheckContext *ctx, ToastedAttribute *ta)
 	int32		expected_chunk_seq = 0;
 	int32		last_chunk_seq;
 	Oid8		toast_valueid;
+	int32		max_chunk_size = TOAST_MAX_CHUNK_SIZE;
 
 	extsize = VARATT_EXTERNAL_GET_EXTSIZE(ta->toast_pointer);
-	last_chunk_seq = (extsize - 1) / TOAST_MAX_CHUNK_SIZE;
+	last_chunk_seq = (extsize - 1) / max_chunk_size;
 
 	/*
 	 * Setup a scan key to find chunks in toast table with matching va_valueid
-- 
2.54.0