CREATE OR REPLACE FUNCTION count_distinct_elements_append(internal, anyarray)
    RETURNS internal
    AS 'count_distinct', 'count_distinct_elements_append'
    LANGUAGE C IMMUTABLE;

CREATE OR REPLACE FUNCTION array_agg_distinct(internal, anynonarray)
    RETURNS anyarray
    AS 'count_distinct', 'array_agg_distinct_type_by_element'
    LANGUAGE C IMMUTABLE;

CREATE OR REPLACE FUNCTION array_agg_distinct(internal, anyarray)
    RETURNS anyarray
    AS 'count_distinct', 'array_agg_distinct_type_by_array'
    LANGUAGE C IMMUTABLE;

DO $$
BEGIN
       IF (
              SELECT TRUE
              FROM pg_attribute
              WHERE attrelid = 'pg_catalog.pg_aggregate'::regclass
                AND attname = 'aggcombinefn'
       )
       THEN
              DROP AGGREGATE count_distinct(anyelement);
              /* Server supports parallel aggregation (9.6+) */

              /* serialize data */
              CREATE OR REPLACE FUNCTION count_distinct_serial(p_pointer internal)
                  RETURNS bytea
                  AS 'count_distinct', 'count_distinct_serial'
                  LANGUAGE C IMMUTABLE STRICT;

              /* deserialize data */
              CREATE OR REPLACE FUNCTION count_distinct_deserial(p_value bytea, p_dummy internal)
                  RETURNS internal
                  AS 'count_distinct', 'count_distinct_deserial'
                  LANGUAGE C IMMUTABLE STRICT;

              /* combine data */
              CREATE OR REPLACE FUNCTION count_distinct_combine(p_state_1 internal, p_state_2 internal)
                  RETURNS internal
                  AS 'count_distinct', 'count_distinct_combine'
                  LANGUAGE C IMMUTABLE;

              /* Create the aggregate functions */
              CREATE AGGREGATE count_distinct(anyelement) (
                     SFUNC = count_distinct_append,
                     STYPE = internal,
                     FINALFUNC = count_distinct,
                     COMBINEFUNC = count_distinct_combine,
                     SERIALFUNC = count_distinct_serial,
                     DESERIALFUNC = count_distinct_deserial,
                     PARALLEL = SAFE
              );

              CREATE AGGREGATE array_agg_distinct(anynonarray) (
                     SFUNC = count_distinct_append,
                     STYPE = internal,
                     FINALFUNC = array_agg_distinct,
                     FINALFUNC_EXTRA,
                     COMBINEFUNC = count_distinct_combine,
                     SERIALFUNC = count_distinct_serial,
                     DESERIALFUNC = count_distinct_deserial,
                     PARALLEL = SAFE
              );

              CREATE AGGREGATE count_distinct_elements(anyarray) (
                     SFUNC = count_distinct_elements_append,
                     STYPE = internal,
                     FINALFUNC = count_distinct,
                     COMBINEFUNC = count_distinct_combine,
                     SERIALFUNC = count_distinct_serial,
                     DESERIALFUNC = count_distinct_deserial,
                     PARALLEL = SAFE
              );

              CREATE AGGREGATE array_agg_distinct_elements(anyarray) (
                     SFUNC = count_distinct_elements_append,
                     STYPE = internal,
                     FINALFUNC = array_agg_distinct,
                     FINALFUNC_EXTRA,
                     COMBINEFUNC = count_distinct_combine,
                     SERIALFUNC = count_distinct_serial,
                     DESERIALFUNC = count_distinct_deserial,
                     PARALLEL = SAFE
              );
       ELSE
              /* Server does not support parallel aggregation (pre-9.6) */

              /* Create the aggregate function */
              CREATE AGGREGATE array_agg_distinct(anynonarray) (
                     SFUNC = count_distinct_append,
                     STYPE = internal,
                     FINALFUNC = array_agg_distinct,
                     FINALFUNC_EXTRA
              );

              CREATE AGGREGATE count_distinct_elements(anyarray) (
                     SFUNC = count_distinct_elements_append,
                     STYPE = internal,
                     FINALFUNC = count_distinct
              );

              CREATE AGGREGATE array_agg_distinct_elements(anyarray) (
                     SFUNC = count_distinct_elements_append,
                     STYPE = internal,
                     FINALFUNC = array_agg_distinct,
                     FINALFUNC_EXTRA
              );
       END IF;
END;
$$;