Re: ALTER TABLE lock strength reduction patch is unsafe

From: Noah Misch <noah(at)leadboat(dot)com>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: ALTER TABLE lock strength reduction patch is unsafe
Date: 2012-01-02 18:06:26
Message-ID: 20120102180626.GA23436@tornado.leadboat.com
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On Mon, Jan 02, 2012 at 05:09:16PM +0000, Simon Riggs wrote:
> Attached patch makes SnapshotNow into an MVCC snapshot, initialised at
> the start of each scan iff SnapshotNow is passed as the scan's
> snapshot. It's fairly brief but seems to do the trick.

That's a neat trick. However, if you start a new SnapshotNow scan while one is
ongoing, the primordial scan's snapshot will change mid-stream.

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