Re: Concurrently updating an updatable view

Richard Huxton wrote:
> Heikki Linnakangas wrote:
>> The problem is that the new tuple version is checked only against the 
>> condition in the update rule, id=OLD.id, but not the condition in the 
>> original update-claus, dt='a'.
>>
>> Yeah, that's confusing :(.
> 
> Bit more than just normal rule confusion I'd say. Try the following two 
> statements in parallel (assuming you've just run the previous):
> 
> UPDATE test SET dt='c';
> UPDATE test SET dt='x' FROM test t2 WHERE test.id=t2.id AND t2.dt='b';
> 
> This isn't a problem with the view mechanism - it's a problem with 
> re-checking clauses involving subqueries or joins I'd guess.
> 
> I'm trying to decide if it's unexpected or just plain wrong, and I think 
> I'd have to argue wrong.

Or perhaps I'd not argue that :-/

This is really about MVCC in read committed mode, and the "just right 
for simpler cases":
http://www.postgresql.org/docs/8.2/interactive/transaction-iso.html#XACT-READ-COMMITTED

Clearly there needs to be a change to the sentence: "Because of the 
above rule, it is possible for an updating command to see an 
inconsistent snapshot: it can see the effects of concurrent updating 
commands that affected the same rows it is trying to update"

Not true if there's a subquery/join involved.

--   Richard Huxton  Archonet Ltd