Porting strategy pattern from SQL Server to Postgresql

If all called functions return a single value of the same data type and take a single parameter of the same data type, you can do that with dynamic SQL in Postgres as well:

create or replace function evaluate(p_input integer, p_method text)
  returns float
as
$$
declare
  l_result float;
begin
  execute 'select '||p_method||'($1)' 
     using p_input 
     into l_result;
  return l_result;
end;
$$
language plpgsql;

• select evaluate(42, 'sqrt'); returns 6.48074069840786
• select evaluate(1, 'exp'); returns 2.718281828459045

This works with multiple parameters as well:

create or replace function evaluate(p_arg_1 integer, p_arg_2 text, p_method text)
  returns float
as
$$
declare
  l_result float;
begin
  execute 'select '||p_method||'($1, $2)' 
     using p_arg_1, p_arg_2 
     into l_result;
  return l_result;
end;
$$
language plpgsql;