Re: ALTER TABLE ... NOREWRITE option

On 2012-12-01 18:27:08 +0000, Simon Riggs wrote:
> On 1 December 2012 16:38, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> >> It's hard to know whether your tables will be locked for long periods
> >> when implementing DDL changes.
> >
> >> The NOREWRITE option would cause an ERROR if the table would be
> >> rewritten by the command.
> >
> >> This would allow testing to highlight long running statements before
> >> code hits production.
> >
> > I'm not thrilled about inventing YA keyword for this. If you have a
> > problem with that sort of scenario, why aren't you testing your DDL
> > on a test server before you do it on production?
>
> That's the point. You run it on a test server first, and you can
> conclusively see that it will/will not run for a long time on
> production server.
>
> Greg Sabine Mullane wrote an interesting blog about a way of solving
> the problem in userspace.
>
> > Or even more to the point, you can always cancel the statement once
> > you realize it's taking too long.
>
> Which means you have to watch it, which is not always possible.
>
> > Also, I don't really like the idea of exposing syntax knobs for
> > what ought to be purely an internal optimization. If someday the
> > optimization becomes unnecessary or radically different in behavior,
> > you're stuck with dead syntax. Sometimes the knob is sufficiently
> > important to take that risk, but it doesn't seem to be so here.
>
> I think it was an interesting idea, but I agree with comments about
> weird syntax.
>
> We need something better and more general for impact assessment.

My first thought is to add more detailed EXPLAIN support for
DDL... Although that unfortunately broadens the scope of this a tiny
bit.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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