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Oracle数据库锁与阻塞分析与解决指南

作者:J.P.August

在Oracle数据库中,锁和阻塞是并发控制的关键概念,正确理解和管理它们对于确保数据一致性和提高系统性能至关重要,本文旨在提供关于锁和阻塞的全面分析,并给出相应的解决建议,感兴趣的小伙伴跟着小编一起来看看吧

1. 概述

在Oracle数据库中,锁和阻塞是并发控制的关键概念。正确理解和管理它们对于确保数据一致性和提高系统性能至关重要。本文旨在提供关于锁和阻塞的全面分析,并给出相应的解决建议。

2. 锁(Lock)的概念

定义

类型

目的

3. 阻塞(Blocking)的概念

定义

原因

影响

4. 锁和阻塞的区别

特征锁(Lock)阻塞(Blocking)
定义控制并发访问的机制由于锁冲突导致的等待状态
作用对象数据库资源(如表、行、模式对象)尝试获取锁的事务
目的确保数据一致性和完整性确保事务按顺序执行,避免冲突
结果可能引起阻塞事务暂停执行,等待锁释放
解决方法使用适当的隔离级别、优化查询终止长时间等待的事务、调整应用逻辑

5. 查询锁和阻塞情况

查询当前持有锁的会话及其相关信息

SELECT 
    o.owner AS "对象所属用户",
    o.object_name AS "对象名称",
    s.sid AS "会话SID",
    s.serial# AS "会话SERIAL#",
    s.username AS "用户名",
    s.osuser AS "操作系统用户",
    s.machine AS "客户端机器名",
    s.program AS "使用的程序",
    l.locked_mode AS "锁定模式"
FROM 
    v$locked_object l
JOIN 
    dba_objects o ON l.object_id = o.object_id
JOIN 
    v$session s ON l.session_id = s.sid;

查询正在等待特定对象上释放锁的会话

WITH locked_objects AS (
    SELECT 
        l.session_id,
        o.owner,
        o.object_name,
        o.object_type,
        l.locked_mode
    FROM 
        v$locked_object l
    JOIN 
        dba_objects o ON l.object_id = o.object_id
),
waiting_sessions AS (
    SELECT 
        s.sid,
        s.serial#,
        s.username,
        s.osuser,
        s.machine,
        s.program,
        s.status,
        s.logon_time,
        s.event,
        s.seconds_in_wait,
        s.wait_class,
        s.blocking_session,
        lo.owner,
        lo.object_name,
        lo.object_type,
        lo.locked_mode
    FROM 
        v$session s
    LEFT JOIN 
        locked_objects lo ON s.blocking_session = lo.session_id
    WHERE 
        s.wait_class != 'Idle' -- 排除空闲会话
        AND s.blocking_session IS NOT NULL -- 只选择有阻塞会话的记录
)
SELECT 
    ws.sid AS "等待会话SID",
    ws.serial# AS "等待会话SERIAL#",
    ws.username AS "等待会话用户名",
    ws.osuser AS "操作系统用户",
    ws.machine AS "客户端机器名",
    ws.program AS "使用的程序",
    ws.status AS "会话状态",
    ws.logon_time AS "登录时间",
    ws.event AS "等待事件",
    ws.seconds_in_wait AS "等待时间(秒)",
    ws.wait_class AS "等待类别",
    ws.blocking_session AS "阻塞会话SID",
    lo.session_id AS "持有锁的会话SID",
    lo.owner AS "对象所属用户",
    lo.object_name AS "对象名称",
    lo.object_type AS "对象类型",
    CASE lo.locked_mode
        WHEN 0 THEN 'None'
        WHEN 1 THEN 'Null (NULL)'
        WHEN 2 THEN 'Row Share (SS)'
        WHEN 3 THEN 'Row Exclusive (SX)'
        WHEN 4 THEN 'Share (S)'
        WHEN 5 THEN 'Share Row Exclusive (SSX)'
        WHEN 6 THEN 'Exclusive (X)'
        ELSE 'Unknown'
    END AS "锁定模式描述"
FROM 
    waiting_sessions ws
JOIN 
    locked_objects lo ON ws.blocking_session = lo.session_id
ORDER BY 
    ws.blocking_session, ws.sid;

查看阻塞情况并生成阻塞树

-- 查询阻塞树结构
SELECT 
    LPAD(' ', 5 * (LEVEL - 1)) || s.username AS username,
    LPAD(' ', 5 * (LEVEL - 1)) || s.inst_id || ',' || s.sid AS inst_sid,
    s.serial#,
    s.sql_id,
    s.row_wait_obj#,
    s.wait_class,
    s.event,
    s.p1,
    s.p2,
    s.p3,
    s.seconds_in_wait,
    s.blocking_instance || ',' || s.blocking_session AS blocking_inst_sid
FROM 
    gv$session s
WHERE 
    s.blocking_session IS NOT NULL
    OR (s.inst_id || ',' || s.sid) IN (
        SELECT DISTINCT blocking_instance || ',' || blocking_session 
        FROM gv$session
        WHERE blocking_instance IS NOT NULL AND blocking_session IS NOT NULL
    )
START WITH 
    s.blocking_session IS NULL -- 从不被其他会话阻塞的会话开始
CONNECT BY PRIOR 
    (s.inst_id || ',' || s.sid) = (s.blocking_instance || ',' || s.blocking_session)
ORDER SIBLINGS BY 
    s.inst_id, s.sid;

查看特定会话正在执行的SQL语句

SELECT 
    s.sid AS "会话SID",
    s.serial# AS "会话SERIAL#",
    s.username AS "用户名",
    s.status AS "会话状态",
    s.sql_id AS "SQL_ID",
    q.sql_text AS "SQL语句"
FROM 
    v$session s
LEFT JOIN 
    v$sql q ON s.sql_id = q.sql_id
WHERE 
    s.sid = &sid; -- 替换为实际的会话SID

6. 预防和解决问题的策略

预防措施

解决方案

7. 案例分析

案例1:频繁的行级锁冲突

问题描述:多个用户同时更新同一张表的不同行,但经常遇到行级锁冲突,导致性能下降。

解决方案

案例2:长时间的表级锁

问题描述:某张表被长时间锁定,影响了其他用户的正常操作。

解决方案

8. 总结

通过理解锁和阻塞的概念及其相互关系,可以更好地设计应用程序、配置数据库参数以及解决潜在的问题。有效管理和优化锁可以显著提升系统的并发处理能力和响应速度。

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