From: Peter Geoghegan <pg@bowt.ie>
Date: Sat, 4 Apr 2026 15:45:33 +0000 (-0400)
Subject: heapam: Track heap block in IndexFetchHeapData.
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=c7d09595e46ff513ae0a23ae80e58871a321174f;p=thirdparty%2Fpostgresql.git

heapam: Track heap block in IndexFetchHeapData.

Add an explicit BlockNumber field (xs_blk) to IndexFetchHeapData that
tracks which heap block is currently pinned in xs_cbuf.

heapam_index_fetch_tuple now uses xs_blk to determine when buffer
switching is needed, replacing the previous approach that compared
buffer identities via ReleaseAndReadBuffer on every non-HOT-chain call.

This is preparatory work for an upcoming commit that will add index
prefetching using a read stream.  Delegating the release of a currently
pinned buffer to ReleaseAndReadBuffer won't work anymore -- at least not
when the next buffer that the scan needs to pin is one returned by
read_stream_next_buffer (not a buffer returned by ReadBuffer).

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAH2-Wz=g=JTSyDB4UtB5su2ZcvsS7VbP+ZMvvaG6ABoCb+s8Lw@mail.gmail.com
---

diff --git a/src/backend/access/heap/heapam_indexscan.c b/src/backend/access/heap/heapam_indexscan.c
index c36b804d1e3..bbd8a165ddc 100644
--- a/src/backend/access/heap/heapam_indexscan.c
+++ b/src/backend/access/heap/heapam_indexscan.c
@@ -32,6 +32,7 @@ heapam_index_fetch_begin(Relation rel, uint32 flags)
 	hscan->xs_base.rel = rel;
 	hscan->xs_base.flags = flags;
 	hscan->xs_cbuf = InvalidBuffer;
+	hscan->xs_blk = InvalidBlockNumber;
 	hscan->xs_vmbuffer = InvalidBuffer;
 
 	return &hscan->xs_base;
@@ -46,6 +47,7 @@ heapam_index_fetch_reset(IndexFetchTableData *scan)
 	{
 		ReleaseBuffer(hscan->xs_cbuf);
 		hscan->xs_cbuf = InvalidBuffer;
+		hscan->xs_blk = InvalidBlockNumber;
 	}
 
 	if (BufferIsValid(hscan->xs_vmbuffer))
@@ -240,25 +242,30 @@ heapam_index_fetch_tuple(struct IndexFetchTableData *scan,
 
 	Assert(TTS_IS_BUFFERTUPLE(slot));
 
-	/* We can skip the buffer-switching logic if we're in mid-HOT chain. */
-	if (!*heap_continue)
+	/* We can skip the buffer-switching logic if we're on the same page. */
+	if (hscan->xs_blk != ItemPointerGetBlockNumber(tid))
 	{
-		/* Switch to correct buffer if we don't have it already */
-		Buffer		prev_buf = hscan->xs_cbuf;
+		Assert(!*heap_continue);
 
-		hscan->xs_cbuf = ReleaseAndReadBuffer(hscan->xs_cbuf,
-											  hscan->xs_base.rel,
-											  ItemPointerGetBlockNumber(tid));
+		/* Remember this buffer's block number for next time */
+		hscan->xs_blk = ItemPointerGetBlockNumber(tid);
+
+		if (BufferIsValid(hscan->xs_cbuf))
+			ReleaseBuffer(hscan->xs_cbuf);
+
+		hscan->xs_cbuf = ReadBuffer(hscan->xs_base.rel, hscan->xs_blk);
 
 		/*
-		 * Prune page, but only if we weren't already on this page
+		 * Prune page when it is pinned for the first time
 		 */
-		if (prev_buf != hscan->xs_cbuf)
-			heap_page_prune_opt(hscan->xs_base.rel, hscan->xs_cbuf,
-								&hscan->xs_vmbuffer,
-								hscan->xs_base.flags & SO_HINT_REL_READ_ONLY);
+		heap_page_prune_opt(hscan->xs_base.rel, hscan->xs_cbuf,
+							&hscan->xs_vmbuffer,
+							hscan->xs_base.flags & SO_HINT_REL_READ_ONLY);
 	}
 
+	Assert(BufferGetBlockNumber(hscan->xs_cbuf) == hscan->xs_blk);
+	Assert(hscan->xs_blk == ItemPointerGetBlockNumber(tid));
+
 	/* Obtain share-lock on the buffer so we can examine visibility */
 	LockBuffer(hscan->xs_cbuf, BUFFER_LOCK_SHARE);
 	got_heap_tuple = heap_hot_search_buffer(tid,
diff --git a/src/include/access/heapam.h b/src/include/access/heapam.h
index cc90c821be5..5176478c295 100644
--- a/src/include/access/heapam.h
+++ b/src/include/access/heapam.h
@@ -122,10 +122,11 @@ typedef struct IndexFetchHeapData
 	IndexFetchTableData xs_base;	/* AM independent part of the descriptor */
 
 	/*
-	 * Current heap buffer in scan, if any. NB: if xs_cbuf is not
-	 * InvalidBuffer, we hold a pin on that buffer.
+	 * Current heap buffer in scan (and its block number), if any.  NB: if
+	 * xs_blk is not InvalidBlockNumber, we hold a pin in xs_cbuf.
 	 */
 	Buffer		xs_cbuf;
+	BlockNumber xs_blk;
 
 	/* Current heap block's corresponding page in the visibility map */
 	Buffer		xs_vmbuffer;