我正试图跟踪一个失败的用户登录到Oracle数据库进行监视的目的。计划是在数据库登录时使用触发器。表已创建，触发器已成功与表关联。表CREATE语句和TRIGGER成功。当检查表值时，oracle不返回任何记录。我怎么能做到这一点？

--create table

CREATE TABLE myschema.errors(
    user_name VARCHAR2(30) NOT NULL ENABLE,
        error_code VARCHAR2(50) NOT NULL ENABLE,
            error_message VARCHAR2(50) NOT NULL ENABLE,
               error_time timestamp)
                 TABLESPACE "USERS";

--create application context
CREATE CONTEXT myschema6_ctx USING myschema.myschema6_pkg; 

--create package body
CREATE OR REPLACE PACKAGE BODY myschema.myschema6_pkg 
    IS 
        c_context CONSTANT VARCHAR2(30) := 'myschema6_ctx';
    username  VARCHAR2(30);
    My_USER  VARCHAR2(30);
 
            PROCEDURE set_contexts IS 
            BEGIN 
            SELECT SYS_CONTEXT ('USERENV', 'SESSION_USER') into username FROM DUAL;
            SELECT SYS_CONTEXT ('USERENV', 'OS_USER') into My_USER FROM DUAL;
            DBMS_SESSION.SET_CONTEXT ( c_context, username, My_USER); 
    END; 
            END myschema6_pkg; 
                / 

--create trigger
CREATE OR REPLACE TRIGGER myschema6_ctx_trig AFTER LOGON ON DATABASE
    DECLARE 
    My_USER VARCHAR2(30);
        v_code VARCHAR2(30);
        v_errm VARCHAR2(30);
    BEGIN
        myschema.myschema6_pkg.set_contexts;
        SELECT SYS_CONTEXT ('USERENV', 'OS_USER') into My_USER FROM DUAL;
            EXCEPTION
                WHEN OTHERS THEN
                   v_code := SQLCODE;
                   v_errm := SUBSTR(SQLERRM, 1 , 64);

                   -- to insert information about errors.
                INSERT INTO myschema.errors(user_name,error_code,error_message,error_time)
                VALUES(My_USER,v_code,v_errm, SYSTIMESTAMP);
    END;
/