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

On 12/30/2010 02:02 AM, Greg Smith wrote:
> Marko Tiikkaja wrote:
>> I have no idea why it worked in the past, but the patch was never 
>> designed to work for UPSERT.  This has been discussed in the past and 
>> some people thought that that's not a huge deal.
>
> It takes an excessively large lock when doing UPSERT, which means its 
> performance under a heavy concurrent load can't be good.  The idea is 
> that if the syntax and general implementation issues can get sorted 
> out, fixing the locking can be a separate performance improvement to 
> be implemented later.  Using MERGE for UPSERT is the #1 use case for 
> this feature by a gigantic margin.  If that doesn't do what's 
> expected, the whole implementation doesn't provide the community 
> anything really worth talking about.  That's why I keep hammering on 
> this particular area in all my testing.
>
> One of the reflexive "I can't switch to PostgreSQL easily" stopping 
> points for MySQL users is "I can't convert my ON DUPLICATE KEY UPDATE 
> code".  Every other use for MERGE is a helpful side-effect of adding 
> the implementation in my mind, but not the primary driver of why this 
> is important.  My hints in this direction before didn't get adopted, 
> so I'm saying it outright now:  this patch must have an UPSERT 
> implementation in its regression tests.  And the first thing I'm going 
> to do every time a new rev comes in is try and break it with the 
> pgbench test I attached.  If Boxuan can start doing that as part of 
> his own testing, I think development here might start moving forward 
> faster.  I don't care so much about the rate at which concurrent 
> UPSERT-style MERGE happens, so long as it doesn't crash.  But that's 
> where this has been stuck at for a while now.

I strongly agree. It *is* a huge deal.

cheers

andrew