Expressing a result set as an array (and vice versa)?

Good day,

Is it possible to construct an array from an appropriate select
expression that generates a result set of unknown cardinality?
To focus on the simple case: Is it possible to construct a one-
dimensional array from a select of a single column in a table with an
unknown number of rows?

Conversely, is it possible to construct a (single column) result set
from a select expression on a one-dimensional array with an unknown
number of elements?

Thanks for any hints!

Regards,
Don Maier

CREATE OR REPLACE FUNCTION foreach( liste INTEGER[] ) RETURNS SETOF
INTEGER AS $$
DECLARE
i INTEGER;
BEGIN
FOR i IN 1..icount(liste) LOOP
RETURN NEXT liste[i];
END LOOP;
END;
$$ LANGUAGE plpgsql;

CREATE AGGREGATE array_accum (
sfunc = array_append,
basetype = anyelement,
stype = anyarray,
initcond = '{}'
);

SELECT array_accum( DISTINCT list_id ) FROM bookmarks;
array_accum
---------------
{1,2,3,4,5,7}

SELECT * FROM foreach( '{1,2,3,4,5,7}' );
foreach
---------
1
2
3
4
5
7

On Thu, 23 Mar 2006 20:44:32 +0100, Don Maier <dMaier(at)genome(dot)stanford(dot)edu>
wrote:

> Good day,
>
> Is it possible to construct an array from an appropriate select
> expression that generates a result set of unknown cardinality?
> To focus on the simple case: Is it possible to construct a one-
> dimensional array from a select of a single column in a table with an
> unknown number of rows?
>
> Conversely, is it possible to construct a (single column) result set
> from a select expression on a one-dimensional array with an unknown
> number of elements?
>
> Thanks for any hints!
>
> Regards,
> Don Maier
>
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 1: if posting/reading through Usenet, please send an appropriate
> subscribe-nomail command to majordomo(at)postgresql(dot)org so that your
> message can get through to the mailing list cleanly

On Thu, 23 Mar 2006 11:44:32 -0800
Don Maier <dMaier(at)genome(dot)stanford(dot)edu> threw this fish to the penguins:

> Is it possible to construct an array from an appropriate select
> expression that generates a result set of unknown cardinality?
> To focus on the simple case: Is it possible to construct a one-
> dimensional array from a select of a single column in a table with an
> unknown number of rows?

select array(select some_int_field from my_table where something);

produces an array of integers. No user defined function is required.

> Conversely, is it possible to construct a (single column) result set
> from a select expression on a one-dimensional array with an unknown
> number of elements?
Not so easy without a custom function.

> Thanks for any hints!
>
> Regards,
> Don Maier
>
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 1: if posting/reading through Usenet, please send an appropriate
> subscribe-nomail command to majordomo(at)postgresql(dot)org so that your
> message can get through to the mailing list cleanly
>

--
"Are the gods not just?" "Oh no, child.
What would become of us if they were?" (CSL)

On Mar 25 10:11, george young wrote:
> On Mar 23 11:44, Don Maier <dMaier(at)genome(dot)stanford(dot)edu> wrote:
> > Conversely, is it possible to construct a (single column) result set
> > from a select expression on a one-dimensional array with an unknown
> > number of elements?
>
> Not so easy without a custom function.

But not that hard:

test=> SELECT id, val FROM t_arr;
id | val
----+---------------
1 | {1,2,3}
2 | {4,5,6}
3 | {7,8,9}
4 | {10,11,12,13}
(4 rows)

--
-- First Way
--
test=> SELECT id, val[s.i]
test-> FROM t_arr
test-> LEFT OUTER JOIN
test-> (SELECT g.s
test(> FROM generate_series(1, (SELECT max(array_upper(val, 1)) FROM t_arr)) AS g(s)
test(> ) AS s(i)
test-> ON (s.i <= array_upper(val, 1));
id | val
----+-----
1 | 1
1 | 2
1 | 3
2 | 4
2 | 5
2 | 6
3 | 7
3 | 8
3 | 9
4 | 10
4 | 11
4 | 12
4 | 13
(13 rows)

--
-- Second Way (by using contrib/intagg)
--
SELECT id, int_array_enum(val) FROM t_arr;

Regards.

On Sat, Mar 25, 2006 at 12:17:08AM +0100, PFC wrote:
>
>
> CREATE OR REPLACE FUNCTION foreach( liste INTEGER[] ) RETURNS SETOF
> INTEGER AS $$
> DECLARE
> i INTEGER;
> BEGIN
> FOR i IN 1..icount(liste) LOOP
> RETURN NEXT liste[i];
> END LOOP;
> END;
> $$ LANGUAGE plpgsql;

Seems like this should really exist in the backend...

> CREATE AGGREGATE array_accum (
> sfunc = array_append,
> basetype = anyelement,
> stype = anyarray,
> initcond = '{}'
> );
>
> SELECT array_accum( DISTINCT list_id ) FROM bookmarks;
> array_accum
> ---------------
> {1,2,3,4,5,7}

Couldn't you just use array()?
--
Jim C. Nasby, Sr. Engineering Consultant jnasby(at)pervasive(dot)com
Pervasive Software http://pervasive.com work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461

>> SELECT array_accum( DISTINCT list_id ) FROM bookmarks;
>> array_accum
>> ---------------
>> {1,2,3,4,5,7}
>
> Couldn't you just use array()?

Yes, you can do this :

SELECT ARRAY( SELECT something with one column );

However, array_accum() as an aggregate is more interesting because you
can use GROUP BY. For instance :

SELECT parent, array_accum( child ) FROM table GROUP BY parent;

I have another question. Suppose I have these tables :

CREATE TABLE items (
id SERIAL PRIMARY KEY,
category INTEGER NOT NULL,
name TEXT NOT NULL,
);

CREATE TABLE comments (
item_id INTEGER NOT NULL REFERENCES items(id),
id SERIAL PRIMARY KEY,
comment TEXT NOT NULL,
added TIMESTAMP NOT NULL DEFAULT now()
)

Say I want to display some items and the associated comments :

SELECT * FROM items WHERE category = ...

Then, I gather the item ids which were returned by this query, and do :

SELECT * FROM comments WHERE item_id IN ( the ids ) ORDER BY item_id,
added;

Is there a more elegant and efficient way which would avoid making a big
IN() query ? I could join comments with items, but in my case the search
condition on items is quite complicated and slow ; hence I only want to do
the search once. And I have several different tables in the same style of
the "comments" table, and so I make several queries using the same IN
(...) term. It isn't very elegant... is there a better way ? Use a
temporary table ? How do you do it ?