On 09/30/2014 02:51 PM, Kevin Grittner wrote:
> Josh Berkus <josh@agliodbs.com> wrote:
>> On 09/30/2014 02:39 PM, Kevin Grittner wrote:
>>> Josh Berkus <josh@agliodbs.com> wrote:
>>>> On 09/30/2014 07:15 AM, Kevin Grittner wrote:
>>>>
>>>>> At the risk of pushing people away from this POV, I'll point out
>>>>> that this is somewhat similar to what we do for unlogged bulk loads
>>>>> -- if all the conditions for doing it the fast way are present, we
>>>>> do it the fast way; otherwise it still works, but slower.
>>>>
>>>> Except that switching between fast/slow bulk loads affects *only* the
>>>> speed of loading, not the locking rules. Having a statement silently
>>>> take a full table lock when we were expecting it to be concurrent
>>>> (because, for example, the index got rebuilt and someone forgot the
>>>> UNIQUE) violates POLA from my perspective.
>>>
>>> I would not think that an approach which took a full table lock to
>>> implement the more general case would be accepted.
>>
>> Why not? There are certainly cases ... like bulk loading ... where
>> users would find it completely acceptable. Imagine that you're merging
>> 3 files into a single unlogged table before processing them into
>> finished data.
>
> So the expectation is that when we implement MERGE it will, by
> default, take out an EXCLUSIVE lock for the entire target table for
> the entire duration of the command? I would have expected a bit
> more finesse.
I don't know that that is the *expectation*. However, I personally
would find it *acceptable* if it meant that we could get efficient merge
semantics on other aspects of the syntax, since my primary use for MERGE
is bulk loading.
Regardless, I don't think there's any theoretical way to support UPSERT
without a unique constraint. Therefore eventual support of this would
require a full table lock. Therefore having it use the same command as
UPSERT with a unique constraint is a bit of a booby trap for users.
This is a lot like the "ADD COLUMN with a default rewrites the whole
table" booby trap which hundreds of our users complain about every
month. We don't want to add more such unexpected consequences for users.
--
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com