From e1e533243b98e723f4e6991e4165b1c9859722dc Mon Sep 17 00:00:00 2001
From: Konstantin Knizhnik <knizhnik@garret.ru>
Date: Thu, 15 Dec 2022 10:37:44 +0200
Subject: [PATCH] Do not set last written LSN for target page prefetch request

---
 pgxn/neon/pagestore_smgr.c | 54 +++++++++++++++++++-------------------
 1 file changed, 27 insertions(+), 27 deletions(-)

diff --git a/pgxn/neon/pagestore_smgr.c b/pgxn/neon/pagestore_smgr.c
index a7745713cf..bf0af964a4 100644
--- a/pgxn/neon/pagestore_smgr.c
+++ b/pgxn/neon/pagestore_smgr.c
@@ -675,39 +675,39 @@ prefetch_do_request(PrefetchRequest *slot, bool *force_latest, XLogRecPtr *force
 		request.req.lsn = lsn;
 		prefetch_lsn = Max(prefetch_lsn, lsn);
 		slot->effective_request_lsn = prefetch_lsn;
+
+		/*
+		 * Remember request LSN in the last-written LSN cache to avoid false
+		 * prefetch invalidations.
+		 *
+		 * Imagine what would happen without this, when you perform a large
+		 * sequential scan with UPDATE. The sequential scan issues a prefetch
+		 * request for each page in order, and every page is also dirtied. On
+		 * each page, the oldest page in the last-written LSN cache is evicted,
+		 * which advances the global last-written LSN. The pages being scanned are
+		 * not in the last-written cache, so each prefetch request will use the
+		 * global last-written LSN in the request and memorize that in the
+		 * slot. However, when we receive the response to the prefetch request,
+		 * the global last-written LSN has already moved forwards, and the
+		 * cross-check we make that the last-written LSN matches will fail, and we
+		 * discard the prefetched response unnecessary.
+		 *
+		 * Inserting the LSN we use in the prefetch request to the last-written LSN
+		 * cache avoids that problem. With that, we will use the cached value in
+		 * the cross-check, instead of the more recent global last-written LSN value.
+		 */
+		SetLastWrittenLSNForBlock(
+			request.req.lsn,
+			slot->buftag.rnode,
+			slot->buftag.forkNum,
+			slot->buftag.blockNum
+			);
 	}
 
 	Assert(slot->response == NULL);
 	Assert(slot->my_ring_index == MyPState->ring_unused);
 	page_server->send((NeonRequest *) &request);
 
-	/*
-	 * Remember request LSN in the last-written LSN cache to avoid false
-	 * prefetch invalidations.
-	 *
-	 * Imagine what would happen without this, when you perform a large
-	 * sequential scan with UPDATE. The sequential scan issues a prefetch
-	 * request for each page in order, and every page is also dirtied. On
-	 * each page, the oldest page in the last-written LSN cache is evicted,
-	 * which advances the global last-written LSN. The pages being scanned are
-	 * not in the last-written cache, so each prefetch request will use the
-	 * global last-written LSN in the request and memorize that in the
-	 * slot. However, when we receive the response to the prefetch request,
-	 * the global last-written LSN has already moved forwards, and the
-	 * cross-check we make that the last-written LSN matches will fail, and we
-	 * discard the prefetched response unnecessary.
-	 *
-	 * Inserting the LSN we use in the prefetch request to the last-written LSN 
-	 * cache avoids that problem. With that, we will use the cached value in
-	 * the cross-check, instead of the more recent global last-written LSN value.
-	 */
-	SetLastWrittenLSNForBlock(
-		request.req.lsn,
-		slot->buftag.rnode,
-		slot->buftag.forkNum,
-		slot->buftag.blockNum
-	);
-
 	/* update prefetch state */
 	MyPState->n_requests_inflight += 1;
 	MyPState->n_unused -= 1;