Re: ALTER TABLE SET STATISTICS requires AccessExclusiveLock

On Fri, Jul 9, 2010 at 1:18 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On Fri, 2010-07-09 at 13:04 -0400, Robert Haas wrote:
>> Tom asked what happens when two transactions attempt to do concurrent
>> actions on the same table.  Your response was that we should handle it
>> like CREATE INDEX, and handle the update of the pg_class row
>> non-transactionally.  But of course, if you use a self-conflicting
>> lock at the relation level, then the relation locks conflict and you
>> never have to worry about how to update the pg_class entry in the face
>> of concurrent updates.
>
> From memory, Tom was also worried about the prospect of people updating
> pg_class directly using SQL. That seems a rare, yet valid concern.

Yes, and it's another another reason why we shouldn't use
non-transactional updates.

http://archives.postgresql.org/pgsql-hackers/2008-11/msg00744.php

> I've already agreed with your point that we should use SHARE UPDATE
> EXCLUSIVE.

The point you seem to be missing is that once we make that decision,
we can throw all the heap_inplace_update() stuff out the window, and
the whole problem becomes much simpler.

--
Robert Haas
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