Re: Prepared transactions vs novice DBAs, again

On Wednesday 22 April 2009 15:49:32 Tom Lane wrote:
> I wrote:
> > Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
> >> Configuration affects what can be tested in installcheck, that's quite
> >> natural. I would be happy with simply adding an alternative expected
> >> output file for min_prepared_xacts=0 case. Like we've done for xml test
> >> cases, for example, though that's a compile-time option.
> >
> > Hmm, that's true; the xml case is a relevant precedent. This would be
> > a pretty low-effort way of addressing the problem. Another nice thing
> > about it is that we'd stop having a default max_prepared_transactions
> > value that's completely useless (5 is guaranteed to be either too much
> > or not enough...)
>
> The more I think about this the more I like it. The current default of
> 5 never had any justification beyond allowing the regression tests to
> run --- it's almost certainly not enough for production usage of the
> feature, but it exposes you to all of the downsides of accidental use.
> If we change it to zero, we could alter the Notes for PREPARE
> TRANSACTION to urge more strongly that the feature not be enabled
> without having set up appropriate external infrastructure.
>
> Warning about very old prepared transactions is something that we
> could think about doing as well; it doesn't have to be either-or.
> I think the need for it would decrease quite a bit if they weren't
> enabled by default, though.
>
> Comments? Anyone seriously opposed to making the default be zero?
>

I see this has already been committed, and I am not seriously opposed to
changing it, but I wanted to chime in on a point no one seemed to raise. I
used to recommend people set this to 0 pretty regularly, since most web shops
don't even know what prepared transactions are, let alone use them. I got
less agressive about this after a few people reported to me that they had run
out of lock slots on thier systems. Now, you'd think that ~300 lock slots
wouldn't make that much difference, but it did make me a little nervous; so I
thought I'd mention it.

--
Robert Treat
Conjecture: http://www.xzilla.net
Consulting: http://www.omniti.com