Re: Are we losing momentum?

ow said...
> --- Neil Conway <neilc(at)samurai(dot)com> wrote:
> > Are you talking about queries between databases on the same
> > postmaster
> > (i.e. running under the same PostgreSQL installation),
>
> Yes

Based on your later comments, the answer seems to /actually/ be "No."

> > or queries
> > between postmasters running on different systems? If the former, I
> > don't
> > see how putting your data into multiple schemas in a single database
> > is
> > significantly less reliable than putting it into multiple databases.
>
> I disagree. For example, suppose you have
> app12 that uses db1 and db2,
> app23 that uses db2 and db3,
> app3 that uses db3.
>
> If db3 goes down then app12 is not affected, app23 could be partially
> affected (e.g. user may not be able to run historic queries) and app3
> is completely unavailable. This is definitely better than all three
> apps are down. Besides, having one huge db makes everything more
> difficult and requires (much) more time for backups, restores, etc.
>
> Every major RDBMS vendor (and mySql) finds this feature important and
> they support it. Hope Postgresql will too.

If it's all running as just one PostgreSQL instance, then if db1 goes down,
then, since it's the same postmaster as is supporting db2 and db3, they
necessarily go down as well.

The only way that you get to take down one DB without affecting the others is
for them NOT to be running as part of the same PG installation.

By the way, if you only have one PG instance, then you may very well find it
challenging to suitably parallelize all the loads/dumps of data. If you have
three disks, or three arrays, it may make a lot of sense to have separate PG
instances on each one, as that allows I/O to not need to interfere between
instances. (There are, admittedly, other ways of tuning this sort of thing,
such as moving WAL to a separate disk, or perhaps even specific table files,
identified by OID...)

But the most general ways of separating things out lead to having quite
separate DB instances. And when you've got that, it certainly is attractive
to have 2PC, as is available for the "expensive guys."
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