我得到了这个问题

SQL> CREATE USER DEMO IDENTIFIED BY DEMO DEFAULT TABLESPACE USERS;

User created.

SQL> GRANT CONNECT, RESOURCE TO DEMO;

Grant succeeded.

SQL> GRANT UNLIMITED TABLESPACE TO DEMO;

Grant succeeded.

字符串
第一届会议：

SQL> CONNECT DEMO/DEMO
Connected.
SQL> CREATE TABLE D_LOCK(DIGIT NUMBER, STRING VARCHAR(10));
Table created.

INSERT INTO D_LOCK VALUES (1, 'ONE');
SQL> 
1 row created.

INSERT INTO D_LOCK VALUES (2, 'TWO');
SQL> 
1 row created.

SQL> COMMIT;
Commit complete.

SQL> UPDATE D_LOCK SET STRING='001' WHERE DIGIT=1;
1 row updated.

型
第二届会议：

SQL> CONNECT DEMO/DEMO
Connected.

SQL> delete from D_LOCK WHERE DIGIT=1;

型
<-第二个会话在等待第一个会话提交时挂起
几秒钟后，我提出了下一个请求：

SELECT
    ash.snap_id,
    ash.sample_time,
    ash.blocking_session as ,
    ash.blocking_session_serial#,
     ' -> '                    as is_blocking,
    ash.session_id as blocked_sid,
    ash.session_serial# as blocked_serial,
    ash.sql_id as blocked_sql_id,
    sql.sql_text as blocked_sql_text,
    ash.sql_opname as blocked_sql_opname,
    ash.event as blocked_event,
    round(ash.time_waited / 1000000) AS blocked_sec
FROM
    dba_hist_active_sess_history   ash,
    v$sql                          sql
WHERE
    blocking_session IS NOT NULL
    AND ash.sql_id = sql.sql_id 
    and BLOCKING_SESSION_STATUS = 'VALID'
    --and time_waited>0 
ORDER BY
    sample_id DESC;

型
但是字段blocked_sec为0。为什么？应该还有一个等待时间（几秒）。