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