Postgres 9.4

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CREATE TYPE activity_type_enum 
            AS ENUM ('a',
                     'b',
                     'c');
CREATE TABLE some_table
             (activities json);
INSERT INTO some_table
            (activities)
            VALUES ('{"a":"X1","b":"Y1","c":"Z1"}'),
                   ('{"a":"X2","b":"Y2","c":"Z2"}'),
                   ('{"a":"X3","b":"Y3","c":"Z3"}');
3 rows affected

DO
$$
BEGIN
  EXECUTE '
CREATE VIEW some_view
AS
  SELECT x.*
         FROM some_table t
              CROSS JOIN LATERAL json_to_record(t.activities) x(' || (SELECT string_agg(un.m || ' text', ', ')
                                                                             FROM unnest(enum_range(NULL::activity_type_enum)) un(m)) || ');
';
END;
$$
LANGUAGE plpgsql;
SELECT *
       FROM some_view;


a
b
c


X1
Y1
Z1

X2
Y2
Z2

X3
Y3
Z3