Parallel sequential scan not supported for stored procedure with RETURN QUERY EXECUTE ?

Hi,

We're using PostgreSQL 9.6.3 on Linux.

I have a pl/pgsql stored procedure that is not utilizing the new
parallel sequential scan feature although manually running the same
query does (assuming the same settings/optimizer hints are used ofc).
A rough outline of the stored procedure (omitting all the boring parts)
is given below ; basically I'm dynamically creating a SQL statement and
then using RETURN QUERY EXECUTE to run it.

EXPLAIN'ing the query that's printed by the "RAISE NOTICE" (with the
same options as the stored procedure) produces a plan that uses parallel
execution but invoking the stored procedure obviously does not as the
execution time is orders of magnitudes slower.

Any ideas ?

Thanks,
Tobias

---------------------------------
CREATE OR REPLACE FUNCTION do_stuff(....lots of parameters...)
RETURNS SETOF importer.statistic_type AS
$BODY$
DECLARE
  _sql text;
BEGIN
    _sql := 'SELECT ''' || _hostname || '''::text AS hostname,'
                  'interval_start, '
                  'total_filesize, '
                  'total_filecount, '
                  'EXTRACT(EPOCH FROM combined_import_time_seconds) AS
combined_import_time_seconds, '
                  'min_throughput, '
                  'max_throughput, '
                  ''''|| _filetype ||'''::text AS filetype, '
                  'busy_seconds '
            'FROM ( SELECT '
                          'vf_cut_func(starttime' ||
_cut_func_parameter || ') AS interval_start, '
                          'sum(filesize) AS total_filesize, '
                          'count(*) AS total_filecount, '
                          'sum( endtime-starttime ) AS
combined_import_time_seconds, '
                          'min(filesize/EXTRACT(EPOCH FROM
endtime-starttime)) AS min_throughput, '
                          'max(filesize/EXTRACT(EPOCH FROM
endtime-starttime)) AS max_throughput, '
                          'busy_time_seconds(
tstzrange(starttime,MIN(endtime,  vf_cut_func(starttime' ||
_cut_func_parameter || ') + ' || _interval || ') ) ) AS busy_seconds '
                    'FROM importer.log '
                    'WHERE filetype = '''|| _filetype ||''' '
                      'AND starttime >= ''' || _starttime || ''' '
                      'AND starttime < ''' || _endtime || ''' '
                      'AND hostname=''' || _hostname || ''' '
                    'GROUP BY vf_cut_func(starttime' ||
_cut_func_parameter || '), hostname) AS foo;';

  RAISE NOTICE '_sql:%', _sql;

  RETURN QUERY EXECUTE _sql;
END;
$BODY$
  LANGUAGE plpgsql VOLATILE
  SET "TimeZone" TO 'utc'
  set parallel_setup_cost TO 1
  set max_parallel_workers_per_gather TO 4
  set min_parallel_relation_size TO 1
  set enable_indexscan TO false
  set enable_bitmapscan TO false;