pgstattuple: fix free space calculation

[Frédéric Yhuel]

Hello,

I think that pgstattuple should use PageGetExactFreeSpace() instead of 
PageGetHeapFreeSpace() or PageGetFreeSpace(). The latter two compute the 
free space minus the space of a line pointer. They are used like this in 
the rest of the code (heapam.c):

pagefree = PageGetHeapFreeSpace(page);

if (newtupsize > pagefree) { we need a another page for the tuple }

... so it makes sense to take the line pointer into account in this context.

But it in the pgstattuple context, I think we want the exact free space.

I have attached a patch.

Best regards,
FrédéricFrom 350abec004fe472922800135d5d94ca3d8212da4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Fr=C3=A9d=C3=A9ric=20Yhuel?= <frederic.yh...@dalibo.com>
Date: Wed, 21 Aug 2024 15:05:11 +0200
Subject: [PATCH] pgstattuple: use PageGetExactFreeSpace()

instead of PageGetHeapFreeSpace() and PageGetFreeSpace()

We want the exact free space, we don't care if there's enough room for
another line pointer.
---
 contrib/pgstattuple/pgstatapprox.c | 2 +-
 contrib/pgstattuple/pgstatindex.c  | 2 +-
 contrib/pgstattuple/pgstattuple.c  | 6 +++---
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/contrib/pgstattuple/pgstatapprox.c b/contrib/pgstattuple/pgstatapprox.c
index c84c642355..a27dea621e 100644
--- a/contrib/pgstattuple/pgstatapprox.c
+++ b/contrib/pgstattuple/pgstatapprox.c
@@ -111,7 +111,7 @@ statapprox_heap(Relation rel, output_type *stat)
 		 * treat them as being free space for our purposes.
 		 */
 		if (!PageIsNew(page))
-			stat->free_space += PageGetHeapFreeSpace(page);
+			stat->free_space += PageGetExactFreeSpace(page);
 		else
 			stat->free_space += BLCKSZ - SizeOfPageHeaderData;
 
diff --git a/contrib/pgstattuple/pgstatindex.c b/contrib/pgstattuple/pgstatindex.c
index 5c06ba6db4..1b6b768cf8 100644
--- a/contrib/pgstattuple/pgstatindex.c
+++ b/contrib/pgstattuple/pgstatindex.c
@@ -311,7 +311,7 @@ pgstatindex_impl(Relation rel, FunctionCallInfo fcinfo)
 
 			max_avail = BLCKSZ - (BLCKSZ - ((PageHeader) page)->pd_special + SizeOfPageHeaderData);
 			indexStat.max_avail += max_avail;
-			indexStat.free_space += PageGetFreeSpace(page);
+			indexStat.free_space += PageGetExactFreeSpace(page);
 
 			indexStat.leaf_pages++;
 
diff --git a/contrib/pgstattuple/pgstattuple.c b/contrib/pgstattuple/pgstattuple.c
index 3bd8b96197..7e2a7262a3 100644
--- a/contrib/pgstattuple/pgstattuple.c
+++ b/contrib/pgstattuple/pgstattuple.c
@@ -372,7 +372,7 @@ pgstat_heap(Relation rel, FunctionCallInfo fcinfo)
 			buffer = ReadBufferExtended(rel, MAIN_FORKNUM, block,
 										RBM_NORMAL, hscan->rs_strategy);
 			LockBuffer(buffer, BUFFER_LOCK_SHARE);
-			stat.free_space += PageGetHeapFreeSpace((Page) BufferGetPage(buffer));
+			stat.free_space += PageGetExactFreeSpace((Page) BufferGetPage(buffer));
 			UnlockReleaseBuffer(buffer);
 			block++;
 		}
@@ -385,7 +385,7 @@ pgstat_heap(Relation rel, FunctionCallInfo fcinfo)
 		buffer = ReadBufferExtended(rel, MAIN_FORKNUM, block,
 									RBM_NORMAL, hscan->rs_strategy);
 		LockBuffer(buffer, BUFFER_LOCK_SHARE);
-		stat.free_space += PageGetHeapFreeSpace((Page) BufferGetPage(buffer));
+		stat.free_space += PageGetExactFreeSpace((Page) BufferGetPage(buffer));
 		UnlockReleaseBuffer(buffer);
 		block++;
 	}
@@ -565,7 +565,7 @@ pgstat_index_page(pgstattuple_type *stat, Page page,
 {
 	OffsetNumber i;
 
-	stat->free_space += PageGetFreeSpace(page);
+	stat->free_space += PageGetExactFreeSpace(page);
 
 	for (i = minoff; i <= maxoff; i = OffsetNumberNext(i))
 	{
-- 
2.39.2