To: Josh Berkus <josh@agliodbs.com>
cc: pgsql-docs@postgresql.org
Subject: Re: More missing docs (aggs) 
In-reply-to: <200612110929.07744.josh@agliodbs.com> 
References: <45765819.20804@agliodbs.com> <18695.1165384231@sss.pgh.pa.us>
	<200612110929.07744.josh@agliodbs.com>
Comments: In-reply-to Josh Berkus <josh@agliodbs.com>
	message dated "Mon, 11 Dec 2006 09:29:07 -0800"
Date: Mon, 11 Dec 2006 12:38:20 -0500
Message-ID: <13946.1165858700@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

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