If you try to use unnest with array_agg you will get the following error:
```sql
SELECT array_agg(unnest(ids)) FROM (
SELECT
month, array_agg(id) as ids
FROM x
GROUP BY month
);
ERROR: 0A000: aggregate function calls cannot contain set-returning function calls
LINE 14: array_agg(unnest(ids)) AS ids,
^
HINT: You might be able to move the set-returning function into a LATERAL FROM item.
```
You can use jsonb_agg and jsonb_array_elements_text to flatten or unnest an array of arrays in PostgreSQL:
```sql
SELECT
jsonb_array_elements_text(jsonb_agg(array_of_arrays))
FROM
x;
```
But multiple rows will be returned.
For other solutions, see:
https://stackoverflow.com/a/8142998
https://wiki.postgresql.org/wiki/Unnest_multidimensional_array