Обсуждение: Infinite Autovacuum loop caused by failing virtual generated column expression

On Fri, 10 Apr 2026 at 21:19, SATYANARAYANA NARLAPURAM
<satyanarlapuram@gmail.com> wrote:
>
> PG19 added support for stats on virtual generated columns [1]. Creating extended statistics on a virtual generated
columnwhose expression can raise an error leads to ANALYZE failing repeatedly, and autovacuum retrying indefinitely.
Thisfloods the server logs and also wastes resources. Vacuum analyze on that column (without extended stats) succeeds. 
>

True, though this is nothing new. The same thing can happen with
expression statistics on an expression that raises an error, which has
been possible since PG14.

> In order to avoid retry storms, I think we have two options. (1) skipping the offending row from the sample, (2)
skippingthe extended stats computation for that table with a warning message. At least this avoid autovacuum infinite
retry.Attached a draft patch for the option (2). Thoughts? 
>

I'm not sure. The default retry interval is 1 minute, so it won't
exactly be a flood of messages. Also, if the error only occurs for a
small subset of rows, it's possible that retrying might succeed.

Regards,
Dean

On Sat, 11 Apr 2026 17:33:13 +0100
Dean Rasheed <dean.a.rasheed@gmail.com> wrote:

> On Fri, 10 Apr 2026 at 21:19, SATYANARAYANA NARLAPURAM
> <satyanarlapuram@gmail.com> wrote:
> >
> > PG19 added support for stats on virtual generated columns [1]. Creating extended statistics on a virtual generated
columnwhose expression can raise an error leads to ANALYZE failing repeatedly, and autovacuum retrying indefinitely.
Thisfloods the server logs and also wastes resources. Vacuum analyze on that column (without extended stats) succeeds.
 
> >
> 
> True, though this is nothing new. The same thing can happen with
> expression statistics on an expression that raises an error, which has
> been possible since PG14.

Yes, this issue is not new, and I’m not aware of a way to prevent it a priori.

> 
> > In order to avoid retry storms, I think we have two options. (1) skipping the offending row from the sample, (2)
skippingthe extended stats computation for that table with a warning message. At least this avoid autovacuum infinite
retry.Attached a draft patch for the option (2). Thoughts?
 
> >
> 
> I'm not sure. The default retry interval is 1 minute, so it won't
> exactly be a flood of messages. Also, if the error only occurs for a
> small subset of rows, it's possible that retrying might succeed.

I think it would be good to skip ANALYZE for the extended statistics that cause
errors and just emit a warning, rather than aborting ANALYZE for the entire table.
It seems reasonable to treat this as the user’s responsibility to notice the warning
and address the underlying issue.

Regards,
Yugo Nagata

-- 
Yugo Nagata <nagata@sraoss.co.jp>

On Tue, 14 Apr 2026 00:16:42 -0700
SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com> wrote:

> Hi
> 
> On Mon, Apr 13, 2026 at 11:24 PM Yugo Nagata <nagata@sraoss.co.jp> wrote:
> 
> > On Sat, 11 Apr 2026 17:33:13 +0100
> > Dean Rasheed <dean.a.rasheed@gmail.com> wrote:
> >
> > > On Fri, 10 Apr 2026 at 21:19, SATYANARAYANA NARLAPURAM
> > > <satyanarlapuram@gmail.com> wrote:
> > > >
> > > > PG19 added support for stats on virtual generated columns [1].
> > Creating extended statistics on a virtual generated column whose expression
> > can raise an error leads to ANALYZE failing repeatedly, and autovacuum
> > retrying indefinitely. This floods the server logs and also wastes
> > resources. Vacuum analyze on that column (without extended stats) succeeds.
> > > >
> > >
> > > True, though this is nothing new. The same thing can happen with
> > > expression statistics on an expression that raises an error, which has
> > > been possible since PG14.
> >
> > Yes, this issue is not new, and I’m not aware of a way to prevent it a
> > priori.
> >
> > >
> > > > In order to avoid retry storms, I think we have two options. (1)
> > skipping the offending row from the sample, (2) skipping the extended stats
> > computation for that table with a warning message. At least this avoid
> > autovacuum infinite retry. Attached a draft patch for the option (2).
> > Thoughts?
> > > >
> > >
> > > I'm not sure. The default retry interval is 1 minute, so it won't
> > > exactly be a flood of messages. Also, if the error only occurs for a
> > > small subset of rows, it's possible that retrying might succeed.
> >
> > I think it would be good to skip ANALYZE for the extended statistics that
> > cause
> > errors and just emit a warning, rather than aborting ANALYZE for the
> > entire table.
> > It seems reasonable to treat this as the user’s responsibility to notice
> > the warning
> > and address the underlying issue.
> >
> 
> Yugo, thanks for the comments. Could you please review the v1 patch when you
> get a chance. It is in the direction you suggested.

I've looked into the patch and have some comments.

The child ResourceOwner is created and released in BuildRelationExtStatistics(),
but I don't think it is necessary if we add other PG_TRY block in make_build_data()
and compute_expr_stats(). For example in make_build_data():

+                       PG_TRY();
+                       {
+                               datum = ExecEvalExpr(exprstate,
+                                                                        GetPerTupleExprContext(estate),
+                                                                        &isnull);
+                               }
+                       PG_CATCH();
+                       {
+                               ExecDropSingleTupleTableSlot(slot);
+                               FreeExecutorState(estate);
+                               PG_RE_THROW();
+                       }
+                       PG_END_TRY();

Also, we could add tests for extended statistics that do not involve virtual generated
columns, since those are not the cause root of the issue. In addition, it might be useful
to verify that non-skipped extended statistics are still computed successfully.
For example:

+CREATE TABLE expr_err (a int);
+INSERT INTO expr_err VALUES (1), (2), (3);
+CREATE STATISTICS expr_err_s1 ON ((a/0)) FROM expr_err;
+CREATE STATISTICS expr_err_s2 ON (a/0),(a+1) FROM expr_err;
+CREATE STATISTICS expr_err_s3 ON ((a+1)) FROM expr_err;
+ANALYZE expr_err;  -- should warn, not fail
+SELECT statistics_name from pg_stats_ext x
+    WHERE tablename = 'expr_err' ORDER BY ROW(x.*);

Regards,
Yugo Nagata

-- 
Yugo Nagata <nagata@sraoss.co.jp>