Re: ALTER TABLE lock strength reduction patch is unsafe

On 4 March 2014 12:18, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Tue, Mar 4, 2014 at 6:57 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>> The main impact I see is that this would block VACUUM while pg_dump runs.
>>
>> But then, while pg_dump runs VACUUM is ineffective anyway so perhaps
>> that is no bad thing.
>
> Well, a vacuum that's already running when pg_dump starts up may be
> doing a lot of good, so it would be a shame to see pg_dump kill them
> all off.
>
> Also, this would put us in the surprising situation that you can't run
> two simultaneous dumps of overlapping sets of tables, which doesn't
> strike me as a great thing.

These changes in concurrency are the most serious objections and a
definite change in previous behaviour. So we cannot pick a single lock
level that suits all goals the user may have.

> I'd really like to see us find a way to apply some version of this
> patch. I was in favor of the concept 3 years ago when we did this the
> first time, and I've subsequently done quite a bit of work (viz., MVCC
> catalog snapshots) to eliminate the main objection that was raised at
> that time. But it's really hard to reason about what might happen
> with lowered lock levels, and convince yourself that there's
> absolutely nothing that can ever go wrong. I don't know what to do
> about that tension, but I think even modest improvements in this area
> stand to benefit an awful lot of users.

Agreed. The question is, which subset? The issue just raised would
affect whichever subset we choose, so reducing the scope of the patch
does nothing to the impact of the pg_dump issue.

I will add the option to change lock level for pg_dump. It's simple to
use, clear as to why it would be needed and effective at removing this
as an obstacle.

--
Simon Riggs http://www.2ndQuadrant.com/
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