Horton

Re: ALTER TABLE lock strength reduction patch is unsafe

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Simon Riggs <simon@2ndQuadrant.com>
Cc: Noah Misch <noah@leadboat.com>, Robert Haas <robertmhaas@gmail.com>, Bruce Momjian <bruce@momjian.us>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2012-01-02T19:13:00Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Add bytea_agg, parallel to string_agg.

  2. Fix ALTER TABLE ONLY .. DROP CONSTRAINT.

Simon Riggs <simon@2ndQuadrant.com> writes:
> On Mon, Jan 2, 2012 at 6:06 PM, Noah Misch <noah@leadboat.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