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

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, Greg Smith <greg(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
Subject: Re: Re: new patch of MERGE (merge_204) & a question about duplicated ctid
Date: 2011-01-03 16:49:23
Message-ID: 1294073363.19612.1555.camel@ebony (view raw or flat)
Thread:
Lists: pgsql-hackers
On Mon, 2011-01-03 at 18:35 +0200, Heikki Linnakangas wrote:
> On 03.01.2011 18:29, Simon Riggs wrote:
> > On Mon, 2011-01-03 at 18:08 +0200, Heikki Linnakangas wrote:
> >
> >> It works in read committed mode, because you acquire a new snapshot
> >> after the LOCK TABLE, and anyone else who modified the table must commit
> >> before the lock is granted. In serializable mode you get a serialization
> >> error.
> >
> > If its not safe without this
> >
> > LOCK TABLE ... IN SHARE ROW EXCLUSIVE MODE
> >
> > then we should do that automatically, and document that.
> 
> No we should not. The SQL standard doesn't require that, and it would 
> unnecessarily restrict concurrent updates on unrelated rows in the table.

If we do that, then we definitely need a catch-all WHEN statement, so
that we can say

WHEN NOT MATCHED
  INSERT
WHEN MATCHED
  UPDATE
ELSE
  { INSERT into another table so we can try again in a minute
 or RAISE error }

Otherwise we will silently drop rows. Throwing an error every time isn't
useful behaviour.

Of course, that then breaks the standard, just as all existing
implementations do.

-- 
 Simon Riggs           http://www.2ndQuadrant.com/books/
 PostgreSQL Development, 24x7 Support, Training and Services
 


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Next:From: Heikki LinnakangasDate: 2011-01-03 17:01:00
Subject: Re: Re: new patch of MERGE (merge_204) & a question about duplicated ctid
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