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

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "Heikki Linnakangas" <heikki(dot)linnakangas(at)enterprisedb(dot)com>, "Robert Haas" <robertmhaas(at)gmail(dot)com>
Cc: "Greg Smith" <greg(at)2ndquadrant(dot)com>, "Simon Riggs" <simon(at)2ndquadrant(dot)com>, "Marko Tiikkaja" <marko(dot)tiikkaja(at)cs(dot)helsinki(dot)fi>, "Boxuan Zhai" <bxzhai2010(at)gmail(dot)com>, <pgsql-hackers(at)postgresql(dot)org>, "Stephen Frost" <sfrost(at)snowman(dot)net>
Subject: Re: Re: new patch of MERGE (merge_204) & a question about duplicated ctid
Date: 2011-01-03 18:18:57
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Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> Users hate having to do explicit locking (especially users whose
> names rhyme with Bevin Bittner)
Before you decide to taunt me again, I guess I should point out a
few things here.
Should SSI and MERGE both make it into 9.1, there's every reason to
believe that running just about any DML, including MERGE, at
REPEATABLE READ would generate the same behavior which running at
REPEATABLE READ or SERIALIZABLE does now.  If MERGE is susceptible
to such anomalies as testing for the presence of a row and then
trying to UPDATE it if found, only to update zero rows because it
was concurrently deleted, SERIALIZABLE would prevent that with a
serialization failure.  I'd kind of expect that already with a
write-write conflict, but if that isn't happening, the SSI patch
should help.  Well, help prevent anomalies -- if you want it to work
out how to continue without rolling back it won't help at all.
The fact that SSI introduces predicate locking may ultimately allow
MERGE to do something more clever in terms of UPSERT logic, but I
*REALLY* think it's too late in the release cycle to start looking
at that.  Predicate locking for SSI was done exactly as was most
useful for SSI, on the basis (generally popular on this list) that
trying to generalize something with only one use case is doomed to
failure.  Trying to bend it in an additional direction this late in
the release would pretty much ensure that neither MERGE nor SSI
could make it in.
On the other hand, if we put SSI in with predicate locking more or
less as it is, and put MERGE in with more primitive concurrency
control, I fully expect that someone could figure out how to tease
apart SSI and its predicate locking during the next release cycle,
so that the predicate locking was more generally useful.

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