…here

is sure about the variable value - there is not some sophisticated inteligence).
This is used mainly for check of dynamic SQL, and should to reduce some false
alarms.

supported:

    var := 'select 1';
    execute var;

unsupported (the possible value is ambigonuous):

    var := 'select 1';
    if (expr) then
      var := 'select 2';
    end if;
    -- now, the var can be 'select 1' or 'select 2',
    -- and this case is unsupported
    execute var;

- add new option constants_tracing (true by default) for controling of possibility to trace constants.
- increase version to 2.6 - API change