Re: BUG #13660: serializable snapshotting hangs

On Wednesday, October 7, 2015 8:37 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Kevin Grittner <kgrittn@ymail.com> writes:

>> Incorrect.  Like vacuum and other visibility-related operations,
>> long-running transactions in other databases in the same "cluster"
>> (in the sense of a group of backends running under a single
>> postmaster) will be an issue.  I seem to recall that it was
>> non-trivial to limit things in general to a single database, but
>> perhaps the DEFERRABLE feature could be enhanced to do that without
>> too much pain.
>
> How will that work in the case of serialized access to a shared catalog?

The Serializable Snapshot Isolation techniques only work on top of
MVCC snapshot isolation.  At the time of the SSI patch, catalogs
were not accessed according to those rules, so we excluded them
from SSI handling -- it just could not have worked in a meaningful
way.  (So catalog access is currently identical under REPEATABLE
READ and SERIALIZABLE transaction isolation levels.)  I am not
clear on whether MVCC snapshot access changes to the catalogs since
then have made it possible to extend SSI into the catalogs, or what
the pros and cons of that would be.  So, until such time as as SSI
is changed to cover catalogs, it would be safe to ignore that issue
in implementing what the OP wants -- nothing would break.

The question is whether providing this feature would paint us into
a corner in terms of the possibility of later extending SSI to
cover shared catalogs.  It's worth taking a close look at that; but
for right now this is basically a few spots below the top of my
TODO list, and I don't have time available right now to do the
analysis that requires.  Thanks for pointing it out, though.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
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