From 24c8f1da03f5bb887a23581fced19f74a47fb2d1 Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplageman@gmail.com>
Date: Wed, 15 Jan 2025 20:16:16 -0500
Subject: [PATCH v13 2/2] Use streaming I/O in VACUUM's third phase

Now vacuum's third phase (its second pass over the heap), which removes
dead items referring to dead tuples collected in the first phase, uses a
read stream that looks ahead in the TidStore.
---
 src/backend/access/heap/vacuumlazy.c | 39 +++++++++++++++++++++++-----
 1 file changed, 33 insertions(+), 6 deletions(-)

diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
index dce5dae9d02..c07ce578931 100644
--- a/src/backend/access/heap/vacuumlazy.c
+++ b/src/backend/access/heap/vacuumlazy.c
@@ -2247,6 +2247,24 @@ lazy_vacuum_all_indexes(LVRelState *vacrel)
 	return allindexes;
 }
 
+static BlockNumber
+vacuum_reap_lp_read_stream_next(ReadStream *stream,
+								void *callback_private_data,
+								void *per_buffer_data)
+{
+	TidStoreIter *iter = callback_private_data;
+	TidStoreIterResult *iter_result;
+
+	iter_result = TidStoreIterateNext(iter);
+	if (iter_result == NULL)
+		return InvalidBlockNumber;
+
+	/* Save the TidStoreIterResult for later, so we can extract the offsets. */
+	memcpy(per_buffer_data, iter_result, sizeof(*iter_result));
+
+	return iter_result->blkno;
+}
+
 /*
  *	lazy_vacuum_heap_rel() -- second pass over the heap for two pass strategy
  *
@@ -2267,6 +2285,8 @@ lazy_vacuum_all_indexes(LVRelState *vacrel)
 static void
 lazy_vacuum_heap_rel(LVRelState *vacrel)
 {
+	Buffer		buf;
+	ReadStream *stream;
 	BlockNumber vacuumed_pages = 0;
 	Buffer		vmbuffer = InvalidBuffer;
 	LVSavedErrInfo saved_err_info;
@@ -2287,10 +2307,18 @@ lazy_vacuum_heap_rel(LVRelState *vacrel)
 							 InvalidBlockNumber, InvalidOffsetNumber);
 
 	iter = TidStoreBeginIterate(vacrel->dead_items);
-	while ((iter_result = TidStoreIterateNext(iter)) != NULL)
+	stream = read_stream_begin_relation(READ_STREAM_MAINTENANCE,
+										vacrel->bstrategy,
+										vacrel->rel,
+										MAIN_FORKNUM,
+										vacuum_reap_lp_read_stream_next,
+										iter,
+										sizeof(TidStoreIterResult));
+
+	while (BufferIsValid(buf = read_stream_next_buffer(stream,
+													   (void **) &iter_result)))
 	{
 		BlockNumber blkno;
-		Buffer		buf;
 		Page		page;
 		Size		freespace;
 		OffsetNumber offsets[MaxOffsetNumber];
@@ -2298,8 +2326,7 @@ lazy_vacuum_heap_rel(LVRelState *vacrel)
 
 		vacuum_delay_point();
 
-		blkno = iter_result->blkno;
-		vacrel->blkno = blkno;
+		vacrel->blkno = blkno = BufferGetBlockNumber(buf);
 
 		num_offsets = TidStoreGetBlockOffsets(iter_result, offsets, lengthof(offsets));
 		Assert(num_offsets <= lengthof(offsets));
@@ -2312,8 +2339,6 @@ lazy_vacuum_heap_rel(LVRelState *vacrel)
 		visibilitymap_pin(vacrel->rel, blkno, &vmbuffer);
 
 		/* We need a non-cleanup exclusive lock to mark dead_items unused */
-		buf = ReadBufferExtended(vacrel->rel, MAIN_FORKNUM, blkno, RBM_NORMAL,
-								 vacrel->bstrategy);
 		LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
 		lazy_vacuum_heap_page(vacrel, blkno, buf, offsets,
 							  num_offsets, vmbuffer);
@@ -2326,6 +2351,8 @@ lazy_vacuum_heap_rel(LVRelState *vacrel)
 		RecordPageWithFreeSpace(vacrel->rel, blkno, freespace);
 		vacuumed_pages++;
 	}
+
+	read_stream_end(stream);
 	TidStoreEndIterate(iter);
 
 	vacrel->blkno = InvalidBlockNumber;
-- 
2.45.2