CREATE OR REPLACE FUNCTION sch.f_statistic(rowcount bigint, rowoffset bigint, sortString varchar)
 RETURNS TABLE(stat_id character varying, stat_datetime character varying, stat_sm_type character varying, page_size bigint, nrows bigint)
 LANGUAGE plpgsql
AS $function$  
declare
  queryStr text;
  	 
begin
             	
  queryStr = 'select * from ' ||
 	     '(select *, count(*) over() as page_size, row_number() over(order by ' || sortString || ') as nrows from sch.f_statistic_all()) as pre' || 
 	     ' where nrows >= ' || rowoffset ||
 	     ' and nrows < ' || rowoffset + rowcount || ' order by nrows;';	
  return query execute queryStr;
 
end;
$function$;

array_to_json(array_agg(row_to_json(c))) FROM <мой набор данных queryStr>