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: Robert Haas <robertmhaas@gmail.com>
Cc: Simon Riggs <simon@2ndquadrant.com>, pgsql-hackers@postgresql.org
Date: 2011-06-17T22:41:24Z
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.

Robert Haas <robertmhaas@gmail.com> writes:
> On Thu, Jun 16, 2011 at 6:54 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> 4. Backend #2 visits the new, about-to-be-committed version of
>> pgbench_accounts' pg_class row just before backend #3 commits.
>> It sees the row as not good and keeps scanning. By the time it
>> reaches the previous version of the row, however, backend #3
>> *has* committed. So that version isn't good according to SnapshotNow
>> either.

> <thinks some more>

> Why isn't this a danger for every pg_class update?  For example, it
> would seem that if VACUUM updates relpages/reltuples, it would be
> prone to this same hazard.

VACUUM does that with an in-place, nontransactional update.  But yes,
this is a risk for every transactional catalog update.

			regards, tom lane