| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
| Cc: | Noah Misch <noah(at)leadboat(dot)com>, 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 19:13:00 |
| Message-ID: | 27994.1325531580@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> On Mon, Jan 2, 2012 at 6:06 PM, Noah Misch <noah(at)leadboat(dot)com> wrote:
>> On Mon, Jan 02, 2012 at 05:09:16PM +0000, Simon Riggs wrote:
>>> Attached patch makes SnapshotNow into an MVCC snapshot,
>> 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.
> Do we ever do that?
Almost certainly yes. For example, a catcache load may invoke catcache
or relcache reload operations on its way to opening the table or index
needed to fetch the desired row.
I think you can only safely do this if each caller has its own snapshot
variable, a la SnapshotDirty, and that's going to be hugely more
invasive.
regards, tom lane
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