Re: ask for review of MERGE - Mailing list pgsql-hackers

Robert Haas wrote:
> I am also wondering exactly what the semantics are supposed to be
> under concurrency.  We can't assume that it's OK to treat WHEN NOT
> MATCHED as WHEN MATCHED if a unique-key violation is encountered.  The
> join condition could be something else entirely.  I guess we could
> scan the target table with a dirty snapshot and block on any in-doubt
> tuples, similar to what EXCLUDE constraints do internally, but
> throwing MVCC out the window doesn't seem right either.
>   

You've answered Tom's question of why not to just INSERT and trap the 
error here.  Presuming a unique key violation is what will kick back in 
this situation and to just follow the other side doesn't cover all the 
ways this can get used.

I'm thinking of this problem now as being like the one that happens when 
a concurrent UPDATE happens.  To quote from the docs here:

"a target row might have already been updated (or deleted or locked) by 
another concurrent transaction by the time it is found. In this case, 
the would-be updater will wait for the first updating transaction to 
commit or roll back (if it is still in progress). If the first updater 
rolls back, then its effects are negated and the second updater can 
proceed with updating the originally found row. If the first updater 
commits, the second updater will ignore the row if the first updater 
deleted it, otherwise it will attempt to apply its operation to the 
updated version of the row. The search condition of the command (the 
WHERE clause) is re-evaluated to see if the updated version of the row 
still matches the search condition. If so, the second updater proceeds 
with its operation using the updated version of the row."

What I think we can do with adding a key reservation is apply the same 
sort of logic to INSERTs too, given a way to lock an index entry before 
the row itself is complete.  If MERGE hits a WHERE condition that finds 
a key lock entry in the index, it has to sit and wait for that other 
command to finish.  There isn't any other sensible behavior I can see in 
that situation, just like there isn't one for UPDATE right now.

If the transaction that was locking the index entry commits, it has to 
start over with re-evaluating the WHEN MATCHED part (the equivilent of 
the WHERE in the UPDATE case).  But it shouldn't need to do a new JOIN 
again, because that condition was proven to be met already by the lock 
squatting on that index key, without even having the rest of the row 
there yet to check its data.  If the other entry commits, it would then 
follow the WHEN MATCHED path and in the UPSERT case do an UPDATE.  If 
the transaction who had the key reservervation rolls back, the 
re-evaluation of the MERGE WHEN MATCHED will fail, at which point it 
follows the WHERE FOUND INSERT path.  As it will now be the locker of 
the key reservation it had been waiting on earlier at that point, it 
will be free to do the INSERT with no concerns about race conditions or 
retries.  In the SERIALIZABLE case, again just try to follow the same 
slightly different rules that exist for concurrent UPDATE commands.

There may be a tricky point here that we will need to warn people that 
UPSERT implementations need to make sure they only reference the columns 
of the primary key in the MERGE conditions for this to work as expected, 
because otherwise they might get back a unique key violation error 
anyway.  I don't know how other databases deal with that particular 
problem.  I have considered following the somewhat distasteful path of 
installing SQL Server or Oracle here just to see how they handle some of 
the pathological test cases I'm now thinking about for this command.

This particular weak spot in MVCC is already there for UPDATE, and this 
approach seems to me to be the most logical way to extend the same basic 
implementation to also eliminate some concurrent MERGE race conditions, 
including the most common one the UPSERT case is stuck with.  But I have 
no intention of halting work on the basic MERGE to wait for this problem 
to be solved even if I'm completely wrong about what I've outlined 
above.  That style of thinking--don't even start until every a perfect 
solution for every possible problem is known--is something I consider a 
problem in this community's development model, one that has contributed 
to why no work has been done on this feature until now.  This one splits 
nicely into "get the implemenation working" and "improve the 
concurrency" parts, and I haven't heard a good reason yet why those need 
to coupled together.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services and Support        www.2ndQuadrant.us