Re: Overriding the optimizer

Dnia 16-12-2005, pią o godzinie 16:16 +1300, Mark Kirkwood napisał(a):
> Craig A. James wrote:
>
> >
> > What would be cool would be some way the developer could alter the plan,
> > but they way of doing so would strongly encourage the developer to send
> > the information to this mailing list. Postgres would essentially say,
> > "Ok, you can do that, but we want to know why!"
> >
>
> Yeah it would - an implementation I have seen that I like is where the
> developer can supply the *entire* execution plan with a query. This is
> complex enough to make casual use unlikely :-), but provides the ability
> to try out other plans, and also fix that vital query that must run
> today.....

I think you could use SPI for that.
There is function SPI_prepare, which prepares plan,
and SPI_execute_plan, executing it.
These functions are defined in src/backend/executor/spi.c.

I think (someone please correct me if I'm wrong) you could
prepare plan yourself, instead of taking it from SPI_prepare,
and give it to SPI_execute_plan.

SPI_prepare calls _SPI_prepare_plan, which parses query and calls
pg_analyze_and_rewrite. In your version don't call this function,
but provide PostgreSQL with your own plan (not-optimised according to
PostrgeSQL, but meeting your criteria).

--
Tomasz Rybak <bogomips(at)post(dot)pl>