Re: Freeze avoidance of very large table.

At Wed, 2 Mar 2016 17:57:27 -0600, Jim Nasby <Jim.Nasby@BlueTreble.com> wrote in <56D77DE7.7080309@BlueTreble.com>
> On 3/2/16 5:41 PM, Tom Lane wrote:
> > Jim Nasby <Jim.Nasby@BlueTreble.com> writes:
> >> On 3/2/16 4:21 PM, Peter Geoghegan wrote:
> >>> I think you should commit this. The chances of anyone other than you
> >>> and Masahiko recalling that you developed this tool in 3 years is
> >>> essentially nil. I think that the cost of committing a developer-level
> >>> debugging tool like this is very low. Modules like pg_freespacemap
> >>> currently already have no chance of being of use to ordinary users.
> >>> All you need to do is restrict the functions to throw an error when
> >>> called by non-superusers, out of caution.
> >>>
> >>> It's a problem that modules like pg_stat_statements and
> >>> pg_freespacemap are currently lumped together in the documentation,
> >>> but we all know that.
> >
> >> +1.
> >
> > Would it make any sense to stick it under src/test/modules/ instead of
> > contrib/ ?  That would help make it clear that it's a debugging tool
> > and not something we expect end users to use.
> 
> I haven't looked at it in detail; is there something inherently
> dangerous about it?

I don't see any danger but the interface doesn't seem to fit use
by DBAs.

> When I'm forced to wear a DBA hat, I'd really love to be able to find
> out what VM status for a large table is. If it's in contrib they'll
> know the tool is there; if it's under src then there's about 0 chance
> of that. I'd think SU-only and any appropriate warnings would be
> enough heads-up for DBAs to be careful with it.

It looks to expose nothing about table contents. At lesast
anybody who can see the name of a table are safely allowable to
use this on it.

A possible usage (for me) of this would be directly couting
(un)vacuumed or (un)freezed pages in a relation. It would be
convenient that the 'freezed' and 'vacuumed' bits are in separate
integers. It would be usable even stats values for these bits are
shown in statistics views.

regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center