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

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>
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>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, 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 19:11:23
Message-ID: AANLkTimy0YDCYiKbxA1CfRSrAQxujcRpt1UYF3J_yvt0@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Mon, Jan 3, 2011 at 2:01 PM, Kevin Grittner
<Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
>
>>> Before you decide to taunt me again, I guess I should point out a
>>> few things here.
>>
>> Sorry, that was intended as good-natured humor, not taunting.
>
> Oh, I took it that way.  I guess my attempt at humor through an
> oblique reference to a line from "Monty Python and the Holy Grail"
> fell flat.  :-/  I guess I should have said "before you taunt me a
> second time" to make it more readily recognizable...

Ah!  I missed that.  I have actually seen that movie, but it's been,
well... OK, I feel old now.

>> I think that the work you are doing on the serializability stuff
>> is *exactly* the right fix for the concurrency issues associated
>> with MERGE.
>
> It's got a nice consistency with current behavior, with reads never
> blocking or being blocked, but I can see why people would want a
> MERGE which could dance around the concurrency problems and always
> succeed with UPSERT behavior.

I think the right thing to do about wanting UPSERT is to implement
UPSERT, though personally I prefer the name REPLACE from my long-ago
days as a MySQL user.  It may be easier to solve a special case of the
concurrency problem than to solve it in its full generality (and
fixing MERGE is pretty close to "solving it in its full generality").
And even if it isn't, the MERGE syntax is insane if what you really
want to do is insert or update ONE record.  If all we have is MERGE,
people will keep doing it with a PL/pgsql stored procedure or some
crummy application logic just so that they don't have to spend several
days trying to understand the syntax.  Heck, I understand the syntax
(or I think I do) and I still think it's more trouble than its worth.
There is certainly a use case for an F-15 fighter jet but sometimes
what you really want is a model rocket and a small bottle of silver
paint.

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

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