Re: Using a multi-valued function in a view

On Thu, Jun 4, 2009 at 11:15 AM, Radcon Entec <radconentec(at)yahoo(dot)com> wrote:
> Greetings!
>
> Having received the answer I needed to my question about using functions
> with OUT parameters from this list (thanks very much!), I find myself
> confused about how to use the function in a view.  The function chargeneeds
> takes one input parameter, a charge number, and has 3 output parameters:
> needsfs, needsdrygas and needsbigbase.  When I create a view, I always
> beginning by running the view's select statement in pgadmin query window.  I
> tried "select charge, (select * from chargeneeds(charge) from charge", and
> got a complaint that a subquery can only return one value.  If that is so,
> then all this effort has been wasted.  I can use my new function in a query
> like this:
>
> select charge,
>         (select needsfs from chargeneeds(charge)) as needsfs,
>         (select needsdrygas from chargeneeds(charge)) as needsdrygas,
>         (select needsbigbase from chargeneeds(charge)) as needsbigbase
> from charge
>
> But on the face of it, this appears to call chargeneeds(charge) three
> separate times, which not only defeats the purpose of combining the three
> calculations into one function, but is actually worse, because all three
> values will be calculated three times.
>
> So should I just go back to separate functions, or is PostgreSQL going to be
> smart enough to optimize the three calls to chargeneeds() into a single call
> internally?

make sure chargeneeds returns a registered composite type. do this by
1) returning an explicit composite type, or 2) out parameters in
function definition.

then you can have it return the composite type like so:
select charge, (cn).* from (select chargeneeds(charge) from charge);

don't be tempted to write your query like this:

select charge, (select chargeneeds(charge)).* from charge;

this will work, but because of the way .* works in postgresql, this
will still execute the function three times.

merlin