Callback function in Oracle PL/SQL

Question

I have been doing some data transformation/processing in PL/SQL and I want to eliminate duplicate code from my package. This are the relevant code parts.

Input type for the table function:

type t_legs_cur is ref cursor return legs%rowtype;  

Procedure that process one record:

procedure discontinuity_rule(p_leg in out nocopy legs%rowtype) as
begin
  null; --business logic here
end discontinuity_rule;

Table function that iterates over a cursor, process each row in cursors and pipes the output (if any):

function apply_discontinuity_rule(p_cur t_legs_cur)
  return t_legs pipelined
  order p_cur by (/* some fields */)    
  parallel_enable (partition p_cur by range (/* some fields */))
as
  v_leg legs%rowtype;
begin
  loop
    fetch p_cur into v_leg;
    exit when p_cur%notfound;

    discontinuity_rule(v_leg); --call back

    if v_leg.id is not null then
      pipe row (v_leg);
    end if;

  end loop;
end apply_discontinuity_rule;

There are several steps of transformation/processing, e.g. I would run the following select to do the some processing and apply some rules in a given order:

select * from table(trip_rules.generate_trips_from_legs(cursor(
  select * from table(trip_rules.apply_5_legs_rule(cursor(
    select * from table (trip_rules.apply_previous_city_rule(cursor(
      select * from table (trip_rules.apply_backhaul_rule(cursor(
        select * from table(trip_rules.apply_connection_time_rule(cursor(
          select * from table(trip_rules.apply_discontinuity_rule(cursor(
            select * from table(trip_rules.generate_legs_from_input(cursor(
              select * from INPUT_DATA
            )))
          )))
        )))
      )))
    )))
  )))
)));

This is all fine and dandy, only problem is, my trip_rule package contains many apply_*_rule functions. They all similar to the example apply_discontinuity_rule. The only difference is the actual procedure (discontinuity_rule) they call back.

SO, my question would be, how can I avoid to copy the code of the apply_* functions. Is there a more elegant way to do this then use a big if: if p_rule_name == 'discontinuity_rule' then

function apply_rule(p_cur t_legs_cur, p_rule_name in varchar2)
  return t_legs pipelined
  order p_cur by (/* some fields */)    
  parallel_enable (partition p_cur by range (/* some fields */))
as
  v_leg legs%rowtype;
begin
  loop
    fetch p_cur into v_leg;
    exit when p_cur%notfound;

    if p_rule_name == 'discontinuity_rule' then
      discontinuity_rule(v_leg);
    elsif p_rule_name == 'other_rule' then
      other_rule(v_leg);
    elsif p_rule_name == 'totally_other_rule' then
      totally_other_rule(v_leg);
    -- and so on...
    end if;

    if v_leg.id is not null then
      pipe row (v_leg);
    end if;

  end loop;
end apply_rule;

I also understand that it would be possible to create an anonymous PL/SQL block on the fly using the procedure name and execute it as dynamic SQL. I wonder if it can be done properly, without killing my performance. Any idea is appreciated.

Answer 1

Your giant IF statement is not duplicate code.

True, it has parts which resemble each other but this ...

elsif p_rule_name == 'other_rule' then
    other_rule(v_leg);

... is most definitely not the same as this ...

elsif p_rule_name == 'totally_other_rule' then
    totally_other_rule(v_leg); 

Dynamic PL/SQL is something we should avouid unless there really is no alternative. There is no need for it here.