Horton

Re: eliminate xl_heap_visible to reduce WAL (and eventually set VM on-access)

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Kirill Reshke <reshkekirill@gmail.com>, Andrey Borodin <x4mmm@yandex-team.ru>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Heikki Linnakangas <hlinnaka@iki.fi>
Date: 2025-09-08T20:14:47Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Remove table_scan_analyze_next_tuple unneeded parameter OldestXmin

  2. Simplify visibility check in heap_page_would_be_all_visible()

  3. Eliminate use of cached VM value in lazy_scan_prune()

  4. Combine visibilitymap_set() cases in lazy_scan_prune()

  5. Fix const qualification in prune_freeze_setup()

  6. Simplify vacuum visibility assertion

  7. Split heap_page_prune_and_freeze() into helpers

  8. Assert that cutoffs are provided if freezing will be attempted

  9. Split PruneFreezeParams initializers to one field per line

  10. Refactor heap_page_prune_and_freeze() parameters into a struct

  11. Make heap_page_is_all_visible independent of LVRelState

  12. Inline TransactionIdFollows/Precedes[OrEquals]()

  13. Add helper for freeze determination to heap_page_prune_and_freeze

  14. Bump XLOG_PAGE_MAGIC after xl_heap_prune change

  15. Correct prune WAL record opcode name in comment

  16. Add error codes when vacuum discovers VM corruption

  17. Remove unused xl_heap_prune member, reason

  18. Remove unneeded VM pin from VM replay

  19. Add assert and log message to visibilitymap_set

  20. Add error codes to some corruption log messages

Reviewing 0003:

+               /*
+                * If we're only adding already frozen rows to a
previously empty
+                * page, mark it as all-frozen and update the
visibility map. We're
+                * already holding a pin on the vmbuffer.
+                */
                else if (all_frozen_set)
+               {
                        PageSetAllVisible(page);
+                       LockBuffer(vmbuffer, BUFFER_LOCK_EXCLUSIVE);
+                       visibilitymap_set_vmbits(relation,
+
  BufferGetBlockNumber(buffer),
+
  vmbuffer,
+
  VISIBILITYMAP_ALL_VISIBLE |
+
  VISIBILITYMAP_ALL_FROZEN);

Locking a buffer in a critical section violates the order of
operations proposed in the 'Write-Ahead Log Coding' section of
src/backend/access/transam/README.

+        * Now read and update the VM block. Even if we skipped
updating the heap
+        * page due to the file being dropped or truncated later in
recovery, it's
+        * still safe to update the visibility map.  Any WAL record that clears
+        * the visibility map bit does so before checking the page LSN, so any
+        * bits that need to be cleared will still be cleared.
+        *
+        * It is only okay to set the VM bits without holding the heap page lock
+        * because we can expect no other writers of this page.

The first paragraph of this paraphrases a similar content in
xlog_heap_visible(), but I don't see the variation in phrasing as an
improvement.

The second paragraph does not convince me at all. I see no reason to
believe that this is safe, or that it is a good idea. The code in
xlog_heap_visible() thinks its OK to unlock and relock the page to
make visibilitymap_set() happy, which is cringy but probably safe for
lack of concurrent writers, but skipping locking altogether seems
deeply unwise.

- *             visibilitymap_set        - set a bit in a previously pinned page
+ *             visibilitymap_set        - set bit(s) in a previously
pinned page and log
+ *      visibilitymap_set_vmbits - set bit(s) in a pinned page

I suspect the indentation was done with a different mix of spaces and
tabs here, because this doesn't align for me.

In general, this idea makes some sense to me -- there doesn't seem to
be any particularly good reason why the visibility-map update should
be handled by a different WAL record than the all-visible flag on the
page itself. It's a little hard for me to make that statement too
conclusively without studying more of the patches than I've had time
to do today, but off the top of my head it seems to make sense.
However, I'm not sure you've taken enough care with the details here.

--
Robert Haas
EDB: http://www.enterprisedb.com