Re: Getting rid of "tuple concurrently updated" elog()s with concurrent DDLs (at least ALTER TABLE)

On December 26, 2017 5:55:03 AM GMT+01:00, Michael Paquier <michael.paquier@gmail.com> wrote:
>Hi all,
>
>Triggering "tuple concurrently updated" from heapam.c, which is an
>elog(),
>is not that difficult for some DDL commands. For example with ALTER
>ROLE,
>just create a role:
>psql --no-psqlrc -c 'create role popo'
>
>And then run that in two sessions and the error will show up, triggered
>from CatalogTupleUpdate():
>while true; do psql --no-psqlrc -c "alter role popo password 'foo'";
>done
>
>In this case, upgrading the lock taken on pg_authid from
>RowExclusiveLock
>to ShareRowExclusiveLock prevents concurrent sessions to update each
>other,
>which is what the patch attached does.
>
>I think that we should get rid of all those errors, for many
>applications
>doing concurrent DDLs getting this error is surprising, and could be
>thought
>as a corrupted instance. I am just touching the tip of the iceberg
>here, as
>I have the gut feeling that there are other objects where it is
>possible to
>trigger the failure, and an analysis effort is going to be costly. So
>I'd
>like to get first the temperature about such analysis.
>
>So, opinions?

You're proposing to lock the entire relation against many forms of concurrent DDL, just to get rid of that error? That
seemsunacceptable. 
Isn't the canonical way to solve this to take object locks?

Andres

Andres
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