CREATE TYPE something AS (c1 INT, c2 TEXT, discriminator TEXT);

WITH my_table AS (SELECT * FROM (VALUES (1), (2), (3)) t(value))
 SELECT (unnesting.data).* FROM (
     SELECT unnest(q.v) AS data
       FROM (
              SELECT (CASE WHEN my_table.value = 1
                           THEN (SELECT array_agg(q0.* :: something) FROM (SELECT 111, NULL, 'Int') q0)
                           ELSE (SELECT array_agg(q1.* :: something) FROM (SELECT NULL, 'myString', 'String') q1)
                      END) AS v
              FROM my_table) q
  ) unnesting

... ROWS FROM (...) AS (INT, TEXT, TEXT)