Обсуждение: example of aggregate function for product

I needed to write a product aggregate function, and just happened to
find this example in the nodes to the 8.0 manual:

Ansis <ansis_no_ AT inbox.lv>13 Jan 2006 16:41:05
An aggregate multiplication function, an analog of "sum" (the same
should be defined also for other numeric types):

CREATE OR REPLACE FUNCTION mul2(FLOAT,FLOAT)
RETURNS FLOAT AS '
DECLARE
a ALIAS FOR $1;
b ALIAS FOR $2;
BEGIN
RETURN a*b;
END;
' LANGUAGE plpgsql;

CREATE AGGREGATE mul (
    sfunc = mul2,
    basetype = FLOAT,
    stype = FLOAT,
    initcond = '1'
);

but that example looks pretty different than the ones found in the 8.3
manual (avg for instance):

CREATE AGGREGATE avg (float8)
(
    sfunc = float8_accum,
    stype = float8[],
    finalfunc = float8_avg,
    initcond = '{0,0}'
);

and float8_accum is defined like this (in the postgres function list).

CREATE OR REPLACE FUNCTION float8_accum(double precision[], double precision)
  RETURNS double precision[] AS
'float8_accum'
  LANGUAGE 'internal' IMMUTABLE STRICT
  COST 1;
ALTER FUNCTION float8_accum(double precision[], double precision)
OWNER TO postgres;
COMMENT ON FUNCTION float8_accum(double precision[], double precision)
IS 'aggregate transition function';


Are there any experts out there who have defined a product or
cumulative product function that abides best practices who would be
willing to share?

Thanks,
Whit

Whit Armstrong escribió:

> Ansis <ansis_no_ AT inbox.lv>13 Jan 2006 16:41:05
> An aggregate multiplication function, an analog of "sum" (the same
> should be defined also for other numeric types):
>
> CREATE OR REPLACE FUNCTION mul2(FLOAT,FLOAT)
> RETURNS FLOAT AS '
> DECLARE
> a ALIAS FOR $1;
> b ALIAS FOR $2;

[...]

> but that example looks pretty different than the ones found in the 8.3
> manual (avg for instance):

The main difference is that the 8.3 docs example piggybacks on C
language functions that you must compile and install separately, whereas
the comment function uses a plpgsql function and is self-contained.
Other than that (and the fact that the second one is for averages not
multiplication), both examples are technically identical ...

--
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.

Whit Armstrong wrote:
> I needed to write a product aggregate function, and just happened to
> find this example in the nodes to the 8.0 manual:
> 
[...]
> 
> but that example looks pretty different than the ones found in the 8.3
> manual (avg for instance):
> 
[...]
> 
> Are there any experts out there who have defined a product or
> cumulative product function that abides best practices who would be
> willing to share?

I don't know what "best practices" are, but the following works
fine on PostgreSQL 8.3 and 8.4:

CREATE AGGREGATE mul(double precision) (SFUNC=float8mul, STYPE=double precision, INITCOND=1);

Yours,
Laurenz Albe