Обсуждение: More missing docs (aggs)

Folks,

So I don't forget this:

The docs do not have an example of creating a multi-column aggregate,
and the syntax is NOT obvious.  For example, what do you use for stype
in a multi-col aggregate?

--Josh

Josh Berkus <josh@agliodbs.com> writes:
> The docs do not have an example of creating a multi-column aggregate,

True...

> and the syntax is NOT obvious.  For example, what do you use for stype
> in a multi-col aggregate?

Uh, why do you think that's significant?  The stype is whatever you
need to use.

            regards, tom lane

Tom,

> Uh, why do you think that's significant?  The stype is whatever you
> need to use.

Right, but do you have two stypes for a multi-col, a single stype, or a
composite stype?  The docs don't say and it's not obvious.

--
Josh Berkus
PostgreSQL @ Sun
San Francisco

Josh Berkus <josh@agliodbs.com> writes:
>> Uh, why do you think that's significant?  The stype is whatever you
>> need to use.

> Right, but do you have two stypes for a multi-col, a single stype, or a
> composite stype?  The docs don't say and it's not obvious.

There is only one transition state variable, as I should think would be
reasonably obvious from the wording:

    An aggregate function is made from one or two ordinary functions: a
    state transition function sfunc, and an optional final calculation
    function ffunc. These are used as follows:

    sfunc( internal-state, next-data-values ) ---> next-internal-state
    ffunc( internal-state ) ---> aggregate-value

    PostgreSQL creates a temporary variable of data type stype to hold
    the current internal state of the aggregate. At each input row, the
    aggregate argument value(s) are calculated and the state transition
    function is invoked with the current state value and the new
    argument value(s) to calculate a new internal state value.

Whether the state value is a composite type or not is up to the
aggregate author.  There aren't any examples of using a composite type
in the docs, but there are examples of using an array as stype, so
I would hope that our users are bright enough to think of composites
when they need one.

            regards, tom lane

Tom,

> Whether the state value is a composite type or not is up to the
> aggregate author.  There aren't any examples of using a composite type
> in the docs, but there are examples of using an array as stype, so
> I would hope that our users are bright enough to think of composites
> when they need one.

Aha, got it.  OK, so this just needs a good example ... will try to think of
one.

--
Josh Berkus
PostgreSQL @ Sun
San Francisco