Can concurrent create index concurrently block each other?

I noticed this on PG 10 recently, while I agree it is an obsolete version.
pg_blocking_pids() showed that one of the CIC on a Table is blocked
by a CIC on another Table.

I saw them both created over a period of time after which I doubted
what was reported by pg_blocking_pids().

As it was PG 10, I could not see the phase of CIC and interestingly no wait
events.
Anyways, PG 10 is unsupported but I would try it on a newer version.

Curious to understand why this would have happened.

Regards,
Avi Vallarapu.

On Sun, Oct 15, 2023 at 2:35 PM Konstantin Knizhnik <knizhnik@garret.ru>
wrote:

One our customer complains that he spawned two `create index concurrently`
for two different tables and both stuck in "waiting for old snapshots".
I wonder if two CIC can really block each other in `
WaitForOlderSnapshots`?
I found the similar question in hacker archive:

/messages/by-id/MWHPR20MB1421AEC7CEC67B159AC188F6A19A0@MWHPR20MB1421.namprd20.prod.outlook.com

but it is quite old (2016). Was the problem fixed since that time? And if
not, why there it is not mentioned in CIC documentation that performing
several CIC in parallel can cause "deadlock"?

Thanks in advance,
Konstantin

--
Regards,
Avinash Vallarapu
+1-902-221-5976

Konstantin Knizhnik <knizhnik@garret.ru> writes:

One our customer complains that he spawned two `create index
concurrently` for two different tables and both stuck in"waiting for old
snapshots".
I wonder if two CIC can really block each other in `WaitForOlderSnapshots`?

Since v14, we won't wait for another CIC unless it is processing a
partial or expressional index. (According to the comments for
WaitForOlderSnapshots, anyway.) What PG version is this, and what
kind of indexes are being rebuilt?

In any case, if they were blocking each other that would be reported
as a deadlock, since they'd use VirtualXactLock() which relies on
the heavyweight lock manager. What seems more likely is that your
customer had some other old transaction sitting idle and blocking both
of them. Looking into pg_locks would provide more definitive evidence
about what they are waiting for.

regards, tom lane

On 15/10/2023 10:59 pm, Tom Lane wrote:

Konstantin Knizhnik <knizhnik@garret.ru> writes:

One our customer complains that he spawned two `create index
concurrently` for two different tables and both stuck in"waiting for old
snapshots".
I wonder if two CIC can really block each other in `WaitForOlderSnapshots`?

Since v14, we won't wait for another CIC unless it is processing a
partial or expressional index. (According to the comments for
WaitForOlderSnapshots, anyway.) What PG version is this, and what
kind of indexes are being rebuilt?

In any case, if they were blocking each other that would be reported
as a deadlock, since they'd use VirtualXactLock() which relies on
the heavyweight lock manager. What seems more likely is that your
customer had some other old transaction sitting idle and blocking both
of them. Looking into pg_locks would provide more definitive evidence
about what they are waiting for.

Sorry, for false alarm. We have found long running truncation which
actually blocks CIC in this case.
I have asked this question because customer has wrote that there was no
other long living active transactions, but it was not true.