Re: reindex concurrently and two toast indexes

On Sun, Feb 16, 2020 at 01:08:35PM -0600, Justin Pryzby wrote:
> Forking old, long thread:
> https://www.postgresql.org/message-id/36712441546604286%40sas1-890ba5c2334a.qloud-c.yandex.net
> On Fri, Jan 04, 2019 at 03:18:06PM +0300, Sergei Kornilov wrote:
>> About reindex invalid indexes - i found one good question in archives [1]: how about toast indexes?
>> I check it now, i am able drop invalid toast index, but i can not drop reduntant valid index.
>> Reproduce:
>> session 1: begin; select from test_toast ... for update;
>> session 2: reindex table CONCURRENTLY test_toast ;
>> session 2: interrupt by ctrl+C
>> session 1: commit
>> session 2: reindex table test_toast ;
>> and now we have two toast indexes. DROP INDEX is able to remove
>> only invalid ones. Valid index gives "ERROR: permission denied:
>> "pg_toast_16426_index_ccnew" is a system catalog"
>> [1]: https://www.postgresql.org/message-id/CAB7nPqT%2B6igqbUb59y04NEgHoBeUGYteuUr89AKnLTFNdB8Hyw%40mail.gmail.com
>
> It looks like this was never addressed.

On HEAD, this exact scenario leads to the presence of an old toast
index pg_toast.pg_toast_*_index_ccold, causing the index to be skipped
on a follow-up concurrent reindex:
=# reindex table CONCURRENTLY test_toast ;
WARNING: XX002: cannot reindex invalid index
"pg_toast.pg_toast_16385_index_ccold" concurrently, skipping
LOCATION: ReindexRelationConcurrently, indexcmds.c:2863
REINDEX

And this toast index can be dropped while it remains invalid:
=# drop index pg_toast.pg_toast_16385_index_ccold;
DROP INDEX

I recall testing that stuff for all the interrupts which could be
triggered and in this case, this waits at step 5 within
WaitForLockersMultiple(). Now, in your case you take an extra step
with a plain REINDEX, which forces a rebuild of the invalid toast
index, making it per se valid, and not droppable.

Hmm. There could be an argument here for skipping invalid toast
indexes within reindex_index(), because we are sure about having at
least one valid toast index at anytime, and these are not concerned
with CIC.

Any thoughts?
--
Michael