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

Hello Melanie and Andres,

03.04.2026 08:00, Alexander Lakhin wrote:
>
> 31.03.2026 19:19, Melanie Plageman wrote:
>> Thanks for the reply! I have committed the patches in this thread and
>> marked the CF entry accordingly.
>
> I've come across an interesting failure produced starting from 378a21618:
> ...

I've discovered one more behaviour change introduced in 378a21618. I
investigated a yesterday's skink failure [1]:
# --- /home/bf/bf-build/skink-master/HEAD/pgsql/contrib/btree_gist/expected/enum.out 2025-06-23 20:17:56.295775456 +0200
# +++ /home/bf/bf-build/skink-master/HEAD/pgsql.build/testrun/btree_gist/regress/results/enum.out 2026-04-17
22:35:37.212061309 +0200
# @@ -83,12 +83,10 @@
#
#  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)
#
# 1 of 32 tests failed.

pgsql.build/testrun/btree_gist/regress/log/postmaster.log contains
2026-04-17 22:35:36.909 CEST autovacuum worker[4020330] LOG: automatic analyze of table
"regression_btree_gist.public.enumtmp"
    avg read rate: 0.000 MB/s, avg write rate: 0.000 MB/s
    buffer usage: 128 hits, 0 reads, 0 dirtied
    WAL usage: 2 records, 0 full page images, 322 bytes, 0 full page image bytes, 0 buffers full
    system usage: CPU: user: 0.05 s, system: 0.00 s, elapsed: 0.11 s

and managed to reproduce it locally under Valgrind on a slowed down VM so
that the enum test takes ~10 sec: With
+select c.relname,c.relpages,c.reltuples,s.autovacuum_count,s.autoanalyze_count
+from pg_class c
+left join pg_stat_all_tables s on c.oid = s.relid
+where c.relname in ('enumtmp', 'enumidx');
added to the test for diagnostics and the test repeated 100 times, I got:
...
ok 46        - enum                                    10635 ms
ok 47        - enum                                    10559 ms
# diff -U3 /home/vagrant/postgres/contrib/btree_gist/expected/enum.out
/home/vagrant/postgres/contrib/btree_gist/results/enum.out
# --- /home/vagrant/postgres/contrib/btree_gist/expected/enum.out 2026-04-18 11:41:17.224063241 +0000
# +++ /home/vagrant/postgres/contrib/btree_gist/results/enum.out 2026-04-18 11:52:43.870049782 +0000
# @@ -91,18 +91,16 @@
#  where c.relname in ('enumtmp', 'enumidx');
#   relname | relpages | reltuples | autovacuum_count | autoanalyze_count
# ---------+----------+-----------+------------------+-------------------
# - enumtmp |        3 |       595 |                0 |                 0
# + enumtmp |        3 |       595 |                0 |                 1
#   enumidx |        4 |       595 | |
#  (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)
#
not ok 48    - enum                                    10596 ms
ok 49        - enum                                    11693 ms
ok 50        - enum                                    11098 ms
...
# 6 of 131 tests failed.

I could also reproduce the same diff with just:
--- a/contrib/btree_gist/sql/enum.sql
+++ b/contrib/btree_gist/sql/enum.sql
@@ -40,2 +40,3 @@ SELECT count(*) FROM enumtmp WHERE a > 'g'::rainbow;

+ANALYZE enumtmp;
 EXPLAIN (COSTS OFF)

It's not reproduced at 378a21618~1, though.

Could you please look if this can be fixed?

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=skink&dt=2026-04-17%2019%3A10%3A50

Best regards,
Alexander