On 2/9/23 07:45, Kiriakos Georgiou wrote:
> Hello,
>
> I have an interesting concurrency issue with DROP INDEX CONCURRENTLY that I can summarize with this test scenario:
>
> /**************************************************/
>
> — suppose we have this table and index
> create table test(x int);
> create index idx1 on test(x);
>
> — now suppose with the database “live” and the above table super busy (lots of queries on the table using index
idx1),I decide to make the index unique
> create unique index concurrently idx2 on test(x); — runs fine
> drop index concurrently idx1; — took 3 hours to finish, since the table is super busy
>
> /**************************************************/
>
> Taking 3 hours to drop the index is not surprising (lots of queries on the table using idx1). What surprises me is
thedrop index causes havoc with concurrency on the table, causing queries to pile up.
> Once the drop index finishes, everything goes back to normal.
>
> I thought by using the CONCURRENTLY option, the drop index is “safe” from concurrency issues for the underlying
table,but in the above scenario it doesn’t appear to be “safe”.
>
> I am trying to formulate a theory to explain this. Any ideas?
1)From here:
https://www.postgresql.org/docs/current/sql-dropindex.html
"With this option, the command instead waits until conflicting
transactions have completed."
2) Probably too late for this case, but info from
https://www.postgresql.org/docs/current/view-pg-locks.html
and
https://www.postgresql.org/docs/15/monitoring-stats.html#MONITORING-PG-STAT-ACTIVITY-VIEW
would be useful.
>
> Regards,
> Kiriakos
>
--
Adrian Klaver
adrian.klaver@aklaver.com