Horton

Re: ask for review of MERGE

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Simon Riggs <simon@2ndquadrant.com>
Cc: Greg Stark <gsstark@mit.edu>, Kevin Grittner <Kevin.Grittner@wicourts.gov>, Greg Smith <greg@2ndquadrant.com>, Marko Tiikkaja <marko.tiikkaja@cs.helsinki.fi>, Boxuan Zhai <bxzhai2010@gmail.com>, pgsql-hackers@postgresql.org, Tom Lane <tgl@sss.pgh.pa.us>, Martijn van Oosterhout <kleptog@svana.org>
Date: 2010-10-26T20:08:32Z
Lists: pgsql-hackers 
On Tue, Oct 26, 2010 at 8:10 AM, Simon Riggs <simon@2ndquadrant.com> wrote:
> I agree with your analysis. Let me review...
> [review]

Sounds like we're on the same page.

> Two options for resolution are
>
> 1) Throw SERIALIZABLE error
>
> 2) Implement something similar to EvalPlanQual
> As you say, we already resolve this situation for concurrent updates by
> following the update chain from a row that is visible to the latest row.
> For MERGE the semantics need to be subtely different here: we need to
> follow the update chain to the latest row, but from a row that we
> *can't* see.
>
> MERGE is still very useful without the need for 2), though fails in some
> cases for concurrent use. The failure rate would increase as the number
> of concurrent MERGErs and/or number of rows in source table. Those
> errors are no more serious than are possible now.
>
> So IMHO we should implement 1) now and come back later to implement 2).
> Given right now there may be other issues with 2) it seems unsafe to
> rely on the assumption that we'll fix them by end of release.

Yeah.  In fact, I'm not sure we're ever going to want to implement #2
- I think that needs more study to determine whether there's even
something there that makes sense to implement at all.  But certainly I
wouldn't count on it happening for 9.1.

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