Re: Re: new patch of MERGE (merge_204) & a question about duplicated ctid

On 03.01.2011 18:02, Robert Haas wrote:
> On Mon, Jan 3, 2011 at 10:58 AM, Heikki Linnakangas
> <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
>> On 03.01.2011 17:56, Stephen Frost wrote:
>>>
>>> * Robert Haas (robertmhaas(at)gmail(dot)com) wrote:
>>>>
>>>> Like Heikki, I'd rather have the feature without a workaround for the
>>>> concurrency issues than no feature.
>>>
>>> I'm still trying to figure out the problem with having the table-level
>>> lock, unless we really think people will be doing concurrent MERGE's
>>> where they won't overlap..? I'm also a bit nervous about if the result
>>> of concurrent MERGE's would actually be correct if we're not taking a
>>> bigger lock than row-level (I assume we're taking row-level locks as it
>>> goes through..).
>>>
>>> In general, I also thought/expected to have some kind of UPSERT type
>>> capability with our initial MERGE support, even if it requires a big
>>> lock and won't operate concurrently, etc.
>>
>> You can of course LOCK TABLE as a work-around, if that's what you want.
>
> That work-around completely fails to solve the concurrency problem.
> Just because you have a lock on the table doesn't mean that there
> aren't already tuples in the table which are invisible to your
> snapshot (for example because the inserting transactions haven't
> committed yet).

It works in read committed mode, because you acquire a new snapshot
after the LOCK TABLE, and anyone else who modified the table must commit
before the lock is granted. In serializable mode you get a serialization
error.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com