Wednesday, September 24, 2008

Re: [HACKERS] Hot Standby Design

On Wed, 2008-09-24 at 12:35 -0400, Robert Treat wrote:
> On Wednesday 24 September 2008 03:27:44 Simon Riggs wrote:
> > On Wed, 2008-09-24 at 00:30 -0400, Robert Treat wrote:
> > > * "However, some WAL redo actions will be for DDL actions. These DDL
> > > actions are repeating actions that have already committed on the primary
> > > node, so they must not fail on the standby node. These DDL locks take
> > > priority and will automatically cancel any read-only transactions that
> > > get in their way."
> > >
> > > Some people will want the option to stack-up ddl transactions behind
> > > long-running queries (one of the main use cases of a hot slave is
> > > reporting stye and other long running queries)
> >
> > Scheduling tools can help here. Run set of queries on Standby, then when
> > complete run DDL on Primary and have its changes filter through.
> >
> true... i am just reminded of Oracle 8's log replay, where you had to stop
> replay to run any queries... given that was a usefull feature, I suspect
> we'll hear complaints about it not going that way. I think one could argue
> that we might be able to provide such an option in the future, if not in the
> first release.

It probably sounds worse than it is. If you drop a table on the Primary,
then somebody running a query against it on the Standby is in for a
shock. But on the other hand, why are you dropping a table that people
still consider worth reading? DROP TABLE should be carefully researched
before use, so I don't think its a big problem.

We could make it wait for a while before cancelling, as an option, if
you think its important?

Simon Riggs
PostgreSQL Training, Services and Support

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