Make a new array with items derived from another array - arrays

Question

Given a PostgreSQL ARRAY of items of one type, how can I create a new array where each item is derived from the items in the initial array?
Example: I have an array of INTERVAL values. I want a new array where each item is a NUMERIC(10, 1) that is the total number of seconds in the corresponding INTERVAL value.
I know how to convert one INTERVAL value:
foo=> SELECT '00:01:20.000'::INTERVAL AS duration_interval;
duration_interval
-------------------
00:01:20
(1 row)
foo=> SELECT extract(EPOCH FROM date_trunc('second', '00:01:20.000'::INTERVAL))
::NUMERIC(10, 1) AS duration_seconds;
duration_seconds
------------------
80.0
(1 row)
The array does not exist in a table – this is a value returned from another function call – so the conversion code needs to operate on it as an array.
How can I convert an array of INTERVAL values to an array of corresponding NUMERIC values?

Answer 1

You need to unnest() the array, do the conversion and then aggregate back into an array.
Assuming you want to do this on a real table with a primary key:
SELECT pk, array_agg(extract(epoch from dur_int)::numeric(10,1)
ORDER BY ordinality) AS duration_seconds
FROM my_table, unnest(duration_interval) WITH ORDINALITY d(dur_int)
GROUP BY pk;
If you have a single array, such as the result from a function call:
SELECT array_agg(extract(epoch from dur_int)::numeric(10,1)
ORDER BY ordinality) AS duration_seconds
FROM unnest(function(...)) WITH ORDINALITY d(dur_int);
Note that you need the WITH ORDINALITY clause when unnesting the array. This will add a column ordinality to the result such that every row has two columns: (dur_int interval, ordinality bigint). When putting the array back again with seconds instead of an interval, you order the rows by the ordinality column. That way you ensure that the order in the resulting array of seconds is the same as in the original array of intervals. (In general, SQL row sources have no specific ordering, the server may present rows in any order it prefers.)
If you have access to the function and you are not breaking other uses of it, you might be better off by changing the function such that you can use its result directly.

Answer 2

If there is a primary key then #Patrick answer is enough. If not then use row_number to aggregate on:
with i(i) as (values
(array['00:01:20.000','00:00:30.000']::interval[]),
(array['00:02:10.000','00:01:30.000']::interval[])
)
select array_agg(extract(epoch from a)::numeric(10,1))
from (
select i, row_number() over() as r
from i
) s, unnest(i) a (a)
group by r
;
array_agg
--------------
{80.0,30.0}
{130.0,90.0}