From: Melanie Plageman <melanieplageman@gmail.com>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: Eliminate use of cached VM value in lazy_scan_prune()
Date: Mon, 26 Jan 2026 22:04:37 +0000
Message-ID: <E1vkUhJ-002qH8-0I@gemulon.postgresql.org> (raw)
Eliminate use of cached VM value in lazy_scan_prune()
lazy_scan_prune() takes a parameter from lazy_scan_heap() indicating
whether the page was marked all-visible in the VM at the time it was
last checked in find_next_unskippable_block(). This behavior is
historical, dating back to commit 608195a3a365, when we did not pin the
VM page until deciding we must read it. Now that the VM page is already
pinned, there is no meaningful benefit to relying on a cached VM status.
Removing this cached value simplifies the logic in both lazy_scan_heap()
and lazy_scan_prune(). It also clarifies future work that will set the
visibility map on-access: such paths will not have a cached value
available, which would make the logic harder to reason about. And
eliminating it enables us to detect and repair VM corruption on-access.
Along with removing the cached value and unconditionally checking the
visibility status of the heap page, this commit also moves the VM
corruption handling to occur first. This reordering should have no
performance impact, since the checks are inexpensive and performed only
once per page. It does, however, make the control flow easier to
understand. The new restructuring also makes it possible to set the VM
after fixing corruption (if pruning found the page all-visible).
Now that no callers of visibilitymap_set() use its return value, change
its (and visibilitymap_set_vmbits()) return type to void.
Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/5CEAA162-67B1-44DA-B60D-8B65717E8B05%40gmail.com
Branch
------
master
Details
-------
https://git.postgresql.org/pg/commitdiff/648a7e28d7c28754ca46caa09864435bc45e543d
Modified Files
--------------
src/backend/access/heap/vacuumlazy.c | 180 ++++++++++++++------------------
src/backend/access/heap/visibilitymap.c | 9 +-
src/include/access/visibilitymap.h | 18 ++--
3 files changed, 92 insertions(+), 115 deletions(-)
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-committers@postgresql.org
Cc: melanieplageman@gmail.com, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Eliminate use of cached VM value in lazy_scan_prune()
In-Reply-To: <E1vkUhJ-002qH8-0I@gemulon.postgresql.org>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox