GROUP BY bug or feature?

Hi,

we have come across a problem where we need an inverted index,
an array of IDs ordered by another condition. We came up
with this scheme:

-- final inverted index
CREATE TABLE product.t_product_inv (
word text primary key not null,
ids bigint[]
);

-- transition table, "word" contains a single lexeme
-- from an original table's "description" field
CREATE TABLE product.t_product_inv0 (
word text not null,
id bigint not null,
price numeric -- not null
);

CREATE INDEX t_product_inv0_idx ON product.t_product_inv0 (word, price
NULLS FIRST, id);

CREATE OR REPLACE FUNCTION array_append_1(bigint[], numeric, bigint)
RETURNS bigint[] AS $$select array_append($1, $3)$$ LANGUAGE SQL;

CREATE AGGREGATE array_accum_1 (numeric, bigint)
(
sfunc = array_append_1,
stype = bigint[],
initcond = '{}'
);

I would like the get the list of IDs ordered by the price field:

INSERT INTO product.t_product_inv
SELECT word, array_accum_1(price, id) FROM product.t_product_inv0
GROUP BY word ORDER BY word, price NULLS FIRST, id;

However, I get an error:

ERROR: column "t_product_inv0.price" must appear in the GROUP BY clause
or be used in an aggregate function
LINE 3: GROUP BY word ORDER BY word, price NULLS FIRST, id;
^

The condition in the error message is true, the field "price"
is indeed used in an aggregate function but ignored on purpose
in the sfunc. So I obviously didn't expect the error to happen.
Is the bug in our approach or in the check for the
GROUP BY/ORDER BY/aggregated variables?

This was tested on 8.4.1, 8.5CVS from 20090930 and from today.

Best regards,
Zoltán Böszörményi

--
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----------------------------------
Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
http://www.postgresql.at/