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

Alexander Lakhin <exclusion@gmail.com>

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From: Alexander Lakhin <exclusion@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Melanie Plageman <melanieplageman@gmail.com>, Tomas Vondra <tomas@vondra.me>, David Rowley <dgrowleyml@gmail.com>, Kirill Reshke <reshkekirill@gmail.com>, Chao Li <li.evan.chao@gmail.com>, Andrey Borodin <x4mmm@yandex-team.ru>, Xuneng Zhou <xunengzhou@gmail.com>, Robert Haas <robertmhaas@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Heikki Linnakangas <hlinnaka@iki.fi>
Date: 2026-04-18T18:00:00Z
Lists: pgsql-hackers 
18.04.2026 19:25, Andres Freund wrote:
> The interesting column to show here would presumably be relallvisible.
>
> What I assume is happening is that occasionally analyze now sees enough all
> visible pages (due to on-access pruning marking the pages all visible) to
> consider the index only scan worthwhile, whereas before that wasn't (or only
> very rarely) happened.

Indeed, with c.relallvisible added, I can see:
--- .../contrib/btree_gist/expected/enum.out        2026-04-18 19:37:51.041565543 +0300
+++ .../contrib/btree_gist/results/enum.out 2026-04-18 19:40:59.077264981 +0300
@@ -88,18 +88,16 @@
  where c.relname in ('enumtmp', 'enumidx');
   relname | relpages | reltuples | autovacuum_count | autoanalyze_count | relallvisible
  ---------+----------+-----------+------------------+-------------------+---------------
- enumtmp |        3 |       595 |                0 |                 0 |             0
+ enumtmp |        3 |       595 |                0 |                 0 |             2
   enumidx |        4 |       595 | |                   |             0
  (2 rows)

  EXPLAIN (COSTS OFF)
  SELECT count(*) FROM enumtmp WHERE a >= 'g'::rainbow;
-                  QUERY PLAN
------------------------------------------------
+                   QUERY PLAN
+------------------------------------------------
   Aggregate
-   ->  Bitmap Heap Scan on enumtmp
-         Recheck Cond: (a >= 'g'::rainbow)
-         ->  Bitmap Index Scan on enumidx
-               Index Cond: (a >= 'g'::rainbow)
-(5 rows)
+   ->  Index Only Scan using enumidx on enumtmp
+         Index Cond: (a >= 'g'::rainbow)
+(3 rows)

At 378a21618~1, it stays zero.

> Maybe I'm daft, but what would prevent this from happening before? The path
> for it would be a bit more complicated, you'd have to have an autovacuum
> instead of just an analyze - but that seems possible. It might require running
> against a pre-existing install to be likely enough.

Yes, with VACUUM enumtmp; instead of ANALYZE enumtmp; the plan change is
reproduced at 378a21618~1:
@@ -88,18 +88,16 @@
  where c.relname in ('enumtmp', 'enumidx');
   relname | relpages | reltuples | autovacuum_count | autoanalyze_count | relallvisible
  ---------+----------+-----------+------------------+-------------------+---------------
- enumtmp |        3 |       595 |                0 |                 0 |             0
+ enumtmp |        3 |       595 |                0 |                 0 |             3
   enumidx |        4 |       595 | |                   |             0
  (2 rows)

  EXPLAIN (COSTS OFF)
  SELECT count(*) FROM enumtmp WHERE a >= 'g'::rainbow;
-                  QUERY PLAN
------------------------------------------------
+                   QUERY PLAN
+------------------------------------------------
   Aggregate
-   ->  Bitmap Heap Scan on enumtmp
-         Recheck Cond: (a >= 'g'::rainbow)
-         ->  Bitmap Index Scan on enumidx
-               Index Cond: (a >= 'g'::rainbow)
-(5 rows)
+   ->  Index Only Scan using enumidx on enumtmp
+         Index Cond: (a >= 'g'::rainbow)
+(3 rows)

And this diff is produced even at f7946a92 (from 2017-03-21), which added
the test case.

So, given that this is the only failure of btree_gist in two last years
at least, it looks like the probability of vacuuming the table there is
much lower than of analyzing.

>> Could you please look if this can be fixed?
> When you say fix, I assume you mean address the test instability, rather than
> actual code changes?

Sure, I didn't mean the new behavior is wrong. Probably changing that
table to temporary would work, but I wonder if there are other queries,
which plans can change due to the same reason.

Best regards,
Alexander

Commits

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

  1. Stabilize btree_gist test against on-access VM setting

  2. Stabilize plancache test against on-access VM setting

  3. Make local buffers pin limit more conservative

  4. Update FSM during prune/freeze replay even if freespace is zero

  5. Pass down information on table modification to scan nodes

  6. Thread flags through begin-scan APIs

  7. Remove PlannedStmt->resultRelations in favor of resultRelationRelids

  8. Make it cheap to check if a relation is modified by a query

  9. Keep newest live XID up-to-date even if page not all-visible

  10. Add pruning fast path for all-visible and all-frozen pages

  11. Detect and fix visibility map corruption in more cases

  12. Save vmbuffer in heap-specific scan descriptors for on-access pruning

  13. Avoid BufferGetPage() calls in heap_update()

  14. Initialize missing fields in CreateExecutorState()

  15. Prefix PruneState->all_{visible,frozen} with set_

  16. Add PageGetPruneXid() helper

  17. Move commonly used context into PruneState and simplify helpers

  18. Remove table_scan_analyze_next_tuple unneeded parameter OldestXmin

  19. Simplify visibility check in heap_page_would_be_all_visible()

  20. Rename LVRelState VM-related logging counters

  21. Fix flakiness in the pg_visibility VM-only vacuum test by using a temporary table.

  22. Refactor lazy_scan_prune() VM clear logic into helper

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

  24. Combine visibilitymap_set() cases in lazy_scan_prune()

  25. Fix const qualification in prune_freeze_setup()

  26. Simplify vacuum visibility assertion

  27. Split heap_page_prune_and_freeze() into helpers

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

  29. Split PruneFreezeParams initializers to one field per line

  30. Refactor heap_page_prune_and_freeze() parameters into a struct

  31. Make heap_page_is_all_visible independent of LVRelState

  32. Inline TransactionIdFollows/Precedes[OrEquals]()

  33. Add helper for freeze determination to heap_page_prune_and_freeze

  34. Bump XLOG_PAGE_MAGIC after xl_heap_prune change

  35. Correct prune WAL record opcode name in comment

  36. Add error codes when vacuum discovers VM corruption

  37. Remove unused xl_heap_prune member, reason

  38. Remove unneeded VM pin from VM replay

  39. Add assert and log message to visibilitymap_set

  40. Stamp HEAD as 19devel.

  41. Add error codes to some corruption log messages