Re: Pending 9.4 patches

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On 04/09/2014 01:54 AM, Stephen Frost wrote:
> * Robert Haas (robertmhaas(at)gmail(dot)com) wrote:
>> On Tue, Apr 8, 2014 at 11:59 AM, Gregory Smith
>> <gregsmithpgsql(at)gmail(dot)com> wrote:
>>> I have no other projects ahead of this for the remainder of
>>> this month. I just can't figure out what to do next until
>>> there's a committer (or committers, if someone else is going to
>>> take on the locking bug) identified. I looked at the locking
>>> problem enough to know that one is over my head.
>>
>> What post or thread should I read for details about this locking
>> bug?
>
> I think it was discussed a couple times, but it's here:
>
> http://www.postgresql.org/message-id/531D3355.6010403@2ndquadrant.com

Yep.
>
The follow-ups are important though - showing that it's actually
a pre-existing bug, and while it might also occur in updatable
security barrier views, there's already a locking problem in the
existing security barrier views that needs to be fixed.

The short version is:

When you have a RowMark (SELECT ... FOR UPDATE/SHARE, UPDATE, or
DELETE) on the results of a query, security barrier views are
incorrectly pushing this row mark down into the security_barrier
subquery they generate.

That means that if you:

SELECT * FROM some_view FOR UPDATE WHERE id % 2 = 0;

you should be locking only *even numbered* rows that match the
predicate of "some_view", but in fact, what'll get run looks like the
pseudo-SQL:

SELECT * FROM (
SELECT * FROM underlying_table WHERE view_predicate FOR UPDATE
) some_view
FOR UPDATE;

i.e. we're locking *all rows that match the view predicate*, failing
to apply the *user* predicate before locking.

- --
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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