postgresql 自定义函数 循环打印日期

CREATE OR REPLACE FUNCTION db.test()
 RETURNS text
 LANGUAGE plpgsql
 NOT FENCED NOT SHIPPABLE
AS $$
DECLARE

    --declare 
    data_array VARCHAR2(19)[];
    field_tmp varchar2(19);
    month_numbers number(10);
    return_values text;
    i number(2);
    start_month timestamp;
    end_month varchar(10);
    
    
BEGIN
    i :=0;
    start_month := timestamp'2020-07-01';
    end_month := '2020-07-31';
   #存在日期bug
   -- select (select regexp_split_to_array(age(quote_literal(end_month),start_month),' '))[1] into month_numbers;
   #替换方案
   select extract(day from (quote_literal(end_month)-start_month)) into month_numbers;
    LOOP
           select date_trunc('day', date'2020-07-01')+i into data_array[i];
        --return_values:=return_values||i;
        EXIT WHEN i >=month_numbers;
        i := i + 1;
    END LOOP;
    
    foreach field_tmp in array data_array LOOP
        return_values := return_values || '__' || field_tmp;
    end loop;
    return return_values;
 
END$$
/