Implement a built-in package DBMS_OUTPUT for sql_mode=ORACLE.
This feature is actively used by Oracle users, especially for debugging purposes.

The idea:

• A stored routine can send debug messages using DBMS_OUTPUT methods, such as DBMS_OUTPUT.PUT_LINE(), which are collected in the package internal buffers.
• The client program e.g. SQL*Plus retrieves the collected messages using methods such as DBMS_OUTPUT.GET_LINE()

SQL*Plus understands a special command SET SERVEROUTPUT ON; to enable displaying of the DBMS_OUTPUT messages after each statement executed.

Example:

DROP FUNCTION f1;

CREATE FUNCTION f1 (a INT) RETURN INT

AS

BEGIN

  DBMS_OUTPUT.PUT_LINE('a IS '||TO_CHAR(a));

  RETURN a;

END;

/

SET SERVEROUTPUT ON;

SELECT f1(10) FROM DUAL;

    F1(10)

----------

	10

a IS 10

Possible implementation using NOTEs

When we have MDEV-10591 implemented, we will be able just to create a package DBMS_OUTOUT with a procedure PUT_LINE('string') which will just send 'string' to the client as a NOTE. That would not be exactly the same how Oracle works, but very close, and useful for debugging purposes.