Re: BUG #17245: Index corruption involving deduplicated entries

Hi,

On 2021-10-29 19:37:40 -0700, Andres Freund wrote:
> Amit, this looks to be a parallel vacuum related bug, see below.
> 
> 
> Whew, we found the bug, I think. Or at least one that can create exactly this
> situation.
> [explanation]
> I haven't yet checked whether this is a bug introduced in 14, or whether it
> was possible to hit before as well.

It looks like a v14 issue. I can't reproduce it in 13, and I think a
change in

commit b4af70cb210393c9c8f41643acf6b213e21178e7
Author: Peter Geoghegan <pg@bowt.ie>
Date:   2021-04-05 13:21:44 -0700

    Simplify state managed by VACUUM.

which changed the logic for which relations are done in the leader:

-        /* Process the indexes skipped by parallel workers */
-        if (shared_indstats == NULL ||
-            skip_parallel_vacuum_index(Irel[i], lps->lvshared))
-            vacuum_one_index(Irel[i], &(vacrelstats->indstats[i]), lps->lvshared,
-                             shared_indstats, vacrelstats->dead_tuples,
-                             vacrelstats);
+        /* Skip already-complete indexes */
+        if (shared_istat != NULL)
+            continue;
+
+        indrel = vacrel->indrels[idx];
+
+        /*
+         * We're only here for the unsafe indexes
+         */
+        if (parallel_processing_is_safe(indrel, lvshared))
+            continue;
+

without realizing that the "shared_indstats == NULL || " piece is
important to handle parallel-safe but too-small indexes correctly.


Going a bit further than this, ISTM that once we decide to use
parallelism for any index, there's no point not using parallelism for
all the parallel-safe indexes...

Greetings,

Andres Freund