Explain conditional compilation with $IF, $ELSE and $ERROR

Conditional compilation directives ($IF ... $THEN ... $ELSIF ... $ELSE ... $END) are evaluated at compile time. Conditions use inquiry directives ($$flag, set via PLSQL_CCFLAGS or ALTER), DBMS_DB_VERSION constants, or static package constants. $ERROR raises a compile-time error. Typical uses: leave instrumentation/logging in the source but compile it out of production, or maintain one code base across DB versions. Check the result with DBMS_PREPROCESSOR.PRINT_POST_PROCESSED_SOURCE.

ALTER SESSION SET PLSQL_CCFLAGS = 'debug_on:true';

CREATE OR REPLACE PROCEDURE do_work AS
BEGIN
  $IF $$debug_on $THEN
    DBMS_OUTPUT.PUT_LINE('DEBUG: entering do_work at ' || SYSTIMESTAMP);
  $END

  $IF DBMS_DB_VERSION.VERSION < 12 $THEN
    $ERROR 'This procedure requires Oracle 12c or later' $END
  $END
  NULL;
END;
/