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

On Mon, Jan 3, 2011 at 10:58 AM, Heikki Linnakangas
<heikki.linnakangas@enterprisedb.com> wrote:
> On 03.01.2011 17:56, Stephen Frost wrote:
>>
>> * Robert Haas (robertmhaas@gmail.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).

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company