Re: "Hot standby"?

On Tue, Aug 11, 2009 at 01:14:56PM -0400, Robert Haas wrote:
> On Tue, Aug 11, 2009 at 1:08 PM, Josh Berkus<josh(at)agliodbs(dot)com> wrote:
> > I believe we're just copying Oracle's terminology.  While that
> > terminology is not consistent, it is understood by the industry.  Oracle
> > defined their Hot Standby to have both asynchronous and synchronous modes:
> > http://www.oracle.com/technology/products/rdb/htdocs/dbms/hotstandby.html
>
> I think that you are wrong. Hot Standby is described in that document
> as a way of replication the database and providing failover in the
> event that the master dies. This feature has to do with allowing
> queries to be executed on the slave, which is not discussed in that
> document.

That web page contains only a brief summary of Oracle's Hot Standby;
by looking at the attached PDF guide you can see that the capability
of allowing read-only queries on the standby node is actually part of
their architecture.

Since we are comparing the features with Oracle's Hot Standby, it is
perhaps useful to mention two of the major differences:

* Oracle's Hot Standby allows only one Standby node, while on
PostgreSQL we can have as many as we want, since the Primary is
totally unaware of them.

Allowing N clones instead of only one may be a crucial advantage in
these use cases where read-only queries represent the vast majority
of the load.

* Oracle's Hot Standby allows cloning hash indexes, which on
PostgreSQL are not WAL safe (anyway, they are currently discouraged
by the documentation).

Best regards,
Dr. Gianni Ciolli - 2ndQuadrant Italia
PostgreSQL Training, Services and Support
gianni(dot)ciolli(at)2ndquadrant(dot)it | www.2ndquadrant.it