How to apply a function to each element of an array column in Postgres?

Question

A Pg query returns an array. I would like to retrieve that with each element formatted to 3 decimal places. How can I apply a function to each element of an array? Something

Answer 1

You need to turn the array into a row set. For example, using generate_series:

SELECT ARRAY(SELECT ROUND(ARRAY[1.53224,0.23411234])[i], 2) FROM generate_series(1,2) AS s(i));    

I know that's pretty ugly. There should be a helper function to make such mappings easier.

Perhaps something like (yes it's horrible, slow, and brittle dynamic code):

CREATE OR REPLACE FUNCTION map_with_arg(TEXT, ANYARRAY, TEXT)
RETURNS ANYARRAY
IMMUTABLE STRICT
LANGUAGE 'plpgsql' AS
$$
DECLARE
    i INTEGER;
    t TEXT;
    cmd TEXT;
BEGIN
    FOR i IN array_lower($2, 1) .. array_upper($2, 1) LOOP
        cmd := 'SELECT ('||quote_ident($1)||'('||quote_nullable($2[i])||', '||quote_nullable($3)||'))::TEXT';
        EXECUTE cmd INTO t;
        $2[i] := t;
    END LOOP;
    RETURN $2;
END;
$$;

select map_with_arg('repeat', array['can','to']::TEXT[], '2');
 map_with_arg
---------------
 {cancan,toto}

Update It occurs to me that we could use a single dynamic statement for the whole loop. This could mitigate some of the performance concerns.

CREATE OR REPLACE FUNCTION map_with_arg(TEXT, ANYARRAY, TEXT)
RETURNS ANYARRAY
IMMUTABLE STRICT
LANGUAGE 'plpgsql' AS
$$
DECLARE
    cmd TEXT;
    rv TEXT;
BEGIN
    cmd := 'SELECT ARRAY(SELECT (' || quote_ident($1)||'($1[i], '||quote_nullable($3)||'))::TEXT FROM generate_subscripts($1, 1) AS gs(i))';
    EXECUTE cmd USING $2 INTO rv;
    RETURN rv;
END;
$$;