Hi,
This patch fixes a couple of small inaccuracies in the doc and the comment for
FSM about index handling.
1. In the doc for pg_freespacemap, it currently says:
For indexes, what is tracked is entirely-unused pages, rather than free space
within pages. Therefore, the values are not meaningful, just whether a page is
full or empty.
However, as what is tracked is entirely-unused pages, the values mean whether a page is
"in-use or empty", rather than "full or empty".
2. In indexfsm.c the header comment says:
* This is similar to the FSM used for heap, in freespace.c, but instead
* of tracking the amount of free space on pages, we only track whether
* pages are completely free or in-use. We use the same FSM implementation
* as for heaps, using BLCKSZ - 1 to denote used pages, and 0 for unused.
However, in the code we see that used pages are marked with 0:
/*
* RecordUsedIndexPage - mark a page as used in the FSM
*/
void
RecordUsedIndexPage(Relation rel, BlockNumber usedBlock)
{
RecordPageWithFreeSpace(rel, usedBlock, 0);
}
And free pages are marked with BLCKSZ - 1:
/*
* RecordFreeIndexPage - mark a page as free in the FSM
*/
void
RecordFreeIndexPage(Relation rel, BlockNumber freeBlock)
{
RecordPageWithFreeSpace(rel, freeBlock, BLCKSZ - 1);
}
And so, this patch also fixes the comment's "using BLCKSZ - 1 to denote used pages, and 0 for
unused" to be "using 0 to denote used pages, and BLCKSZ - 1 for unused".
While these changes are minor, I've seen how this can cause a bit of confusion,
and it would be good to clarify it.
Best regards,
Alex Friedman
From a1b78438343fca053aa0014687eaba34d5e160e0 Mon Sep 17 00:00:00 2001
From: Alex Friedman <alex...@gmail.com>
Date: Tue, 25 Feb 2025 19:12:53 +0200
Subject: [PATCH v1] A small correction to doc and comment of FSM for indexes.
---
doc/src/sgml/pgfreespacemap.sgml | 2 +-
src/backend/storage/freespace/indexfsm.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/doc/src/sgml/pgfreespacemap.sgml b/doc/src/sgml/pgfreespacemap.sgml
index 829ad60f32f..3774a9f8c6b 100644
--- a/doc/src/sgml/pgfreespacemap.sgml
+++ b/doc/src/sgml/pgfreespacemap.sgml
@@ -67,7 +67,7 @@
<para>
For indexes, what is tracked is entirely-unused pages, rather than free
space within pages. Therefore, the values are not meaningful, just
- whether a page is full or empty.
+ whether a page is in-use or empty.
</para>
</sect2>
diff --git a/src/backend/storage/freespace/indexfsm.c
b/src/backend/storage/freespace/indexfsm.c
index 1fc263892a7..3cd2437599d 100644
--- a/src/backend/storage/freespace/indexfsm.c
+++ b/src/backend/storage/freespace/indexfsm.c
@@ -16,7 +16,7 @@
* This is similar to the FSM used for heap, in freespace.c, but instead
* of tracking the amount of free space on pages, we only track whether
* pages are completely free or in-use. We use the same FSM implementation
- * as for heaps, using BLCKSZ - 1 to denote used pages, and 0 for unused.
+ * as for heaps, using 0 to denote used pages, and BLCKSZ - 1 for unused.
*
*-------------------------------------------------------------------------
*/
--
2.41.0
