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<div><h1>MySQL 故障诊断：教你快速定位加锁的 SQL</h1>
|
||
<h3>为什么会加锁</h3>
|
||
<p>锁是可以协调并发连接访问 MySQL 数据库资源的一种技术，可以保证数据的一致性。</p>
|
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<p>有关 MySQL 锁的具体内容，可以详看我的另外一个 Chat，其中有一部分介绍的是“MySQL 锁机制（机智）”：</p>
|
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<blockquote>
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<p>[MySQL 地基基础：事务和锁的面纱](MySQL 地基基础：事务和锁的面纱.md)</p>
|
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</blockquote>
|
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<h3>数据库锁有什么威力</h3>
|
||
<p>在实际应用中你肯定遇到过锁问题，这个锁的威力很大，那出现了数据库锁，会造成什么影响呢？</p>
|
||
<p><strong>锁等待</strong></p>
|
||
<p>一个连接申请了锁资源，其他连接要申请资源，无法获取，等待资源释放。</p>
|
||
<p><strong>死锁</strong></p>
|
||
<p>你锁我，我也锁你，大家一起锁着吧。</p>
|
||
<h3>快速定位加锁的 SQL</h3>
|
||
<p>既然出现了锁，我们就要管理一下这个锁，找一找是什么 SQL 持有这个锁，早点发现锁，提前干预，下面我们看看如何快速定位加锁的 SQL。</p>
|
||
<p>首先我们创建一个测试表：</p>
|
||
<pre><code>mysql> create table t1(id decimal,v_name varchar(10));
|
||
mysql> insert into t1 values(1,'a'),(2,'b'),(3,'c');
|
||
mysql> select * from t1;
|
||
+------+--------+
|
||
| id | v_name |
|
||
+------+--------+
|
||
| 1 | a |
|
||
| 2 | b |
|
||
| 3 | c |
|
||
+------+--------+
|
||
3 rows in set (0.00 sec)
|
||
</code></pre>
|
||
<p>会话 1，开启事务，更新 id=1 的数据：</p>
|
||
<pre><code>mysql> begin;
|
||
mysql> update t1 set v_name='aa' where id=1;
|
||
</code></pre>
|
||
<p>会话 2，开启另一个事务，删除 id=1 的数据：</p>
|
||
<pre><code>mysql> begin;
|
||
mysql> delete from t1 where id=1;
|
||
</code></pre>
|
||
<p>此时会话 2 被锁定，出现锁等待。</p>
|
||
<p>我们再开一个会话 3，查查当前的 processlist，看看是否能发现什么？</p>
|
||
<pre><code>mysql> show processlist;
|
||
+----+------+-----------+------+---------+------+----------+---------------------------+
|
||
| Id | User | Host | db | Command | Time | State | Info |
|
||
+----+------+-----------+------+---------+------+----------+---------------------------+
|
||
| 38 | root | localhost | test | Sleep | 5 | | NULL |
|
||
| 41 | root | localhost | test | Query | 2 | updating | delete from t1 where id=1 |
|
||
| 42 | root | localhost | NULL | Query | 0 | starting | show processlist |
|
||
+----+------+-----------+------+---------+------+----------+---------------------------+
|
||
</code></pre>
|
||
<p>我们可以看到 delete 这个 SQL 的进程在执行中，并没有发现其他有价值的内容，那锁在哪里了。接下来的步骤带你一步步的定位出加锁的 SQL。</p>
|
||
<p><strong>定位锁等待</strong></p>
|
||
<pre><code>mysql> select * from information_schema.innodb_lock_waits;
|
||
+-------------------+-------------------+-----------------+------------------+
|
||
| requesting_trx_id | requested_lock_id | blocking_trx_id | blocking_lock_id |
|
||
+-------------------+-------------------+-----------------+------------------+
|
||
| 2207 | 2207:28:3:7 | 2206 | 2206:28:3:7 |
|
||
+-------------------+-------------------+-----------------+------------------+
|
||
1 row in set, 1 warning (0.00 sec)
|
||
</code></pre>
|
||
<p>结果显示有一个锁等待。</p>
|
||
<p><strong>定位锁</strong></p>
|
||
<pre><code>mysql> select * from information_schema.innodb_locks;
|
||
+-------------+-------------+-----------+-----------+-------------+-----------------+------------+-----------+----------+----------------+
|
||
| lock_id | lock_trx_id | lock_mode | lock_type | lock_table | lock_index | lock_space | lock_page | lock_rec | lock_data |
|
||
+-------------+-------------+-----------+-----------+-------------+-----------------+------------+-----------+----------+----------------+
|
||
| 2207:28:3:7 | 2207 | X | RECORD | `test`.`t1` | GEN_CLUST_INDEX | 28 | 3 | 7 | 0x000000000211 |
|
||
| 2206:28:3:7 | 2206 | X | RECORD | `test`.`t1` | GEN_CLUST_INDEX | 28 | 3 | 7 | 0x000000000211 |
|
||
+-------------+-------------+-----------+-----------+-------------+-----------------+------------+-----------+----------+----------------+
|
||
2 rows in set, 1 warning (0.00 sec)
|
||
</code></pre>
|
||
<p>结果显示有两个锁相关内容。</p>
|
||
<p><strong>定位事务</strong></p>
|
||
<pre><code>mysql> select trx_id,trx_started,trx_requested_lock_id,trx_query,trx_mysql_thread_id from information_schema.innodb_trx;
|
||
+--------+---------------------+-----------------------+---------------------------+---------------------+
|
||
| trx_id | trx_started | trx_requested_lock_id | trx_query | trx_mysql_thread_id |
|
||
+--------+---------------------+-----------------------+---------------------------+---------------------+
|
||
| 2207 | 2021-01-18 15:18:11 | 2207:28:3:7 | delete from t1 where id=1 | 41 |
|
||
| 2206 | 2021-01-18 15:18:08 | NULL | NULL | 38 |
|
||
+--------+---------------------+-----------------------+---------------------------+---------------------+
|
||
2 rows in set (0.01 sec)
|
||
</code></pre>
|
||
<p>结果有两个事务，MySQL 事务线程 id 为 38 和 41，很直观的看到 41 是我们的 delete 事务，被 38 锁定。</p>
|
||
<p><strong>定位线程</strong></p>
|
||
<pre><code>mysql> select * from performance_schema.threads where processlist_id=38;
|
||
+-----------+---------------------------+------------+----------------+------------------+------------------+----------------+---------------------+------------------+-------------------+------------------+------------------+------+--------------+---------+-----------------+--------------+
|
||
| THREAD_ID | NAME | TYPE | PROCESSLIST_ID | PROCESSLIST_USER | PROCESSLIST_HOST | PROCESSLIST_DB | PROCESSLIST_COMMAND | PROCESSLIST_TIME | PROCESSLIST_STATE | PROCESSLIST_INFO | PARENT_THREAD_ID | ROLE | INSTRUMENTED | HISTORY | CONNECTION_TYPE | THREAD_OS_ID |
|
||
+-----------+---------------------------+------------+----------------+------------------+------------------+----------------+---------------------+------------------+-------------------+------------------+------------------+------+--------------+---------+-----------------+--------------+
|
||
| 63 | thread/sql/one_connection | FOREGROUND | 38 | root | localhost | test | Sleep | 35 | NULL | NULL | NULL | NULL | YES | YES | Socket | 15070 |
|
||
+-----------+---------------------------+------------+----------------+------------------+------------------+----------------+---------------------+------------------+-------------------+------------------+------------------+------+--------------+---------+-----------------+--------------+
|
||
1 row in set (0.00 sec)
|
||
</code></pre>
|
||
<p>结果找到 MySQL 事务线程 38 对应的服务器线程 63。</p>
|
||
<p><strong>定位加锁 SQL</strong></p>
|
||
<pre><code>mysql> select * from performance_schema.events_statements_current where thread_id=63;
|
||
+-----------+----------+--------------+----------------------+--------------------------+---------------------+---------------------+------------+-----------+--------------------------------------+----------------------------------+----------------------------------------------+----------------+-------------+---------------+-------------+-----------------------+-------------+-------------------+------------------------------------------+--------+----------+---------------+-----------+---------------+-------------------------+--------------------+------------------+------------------------+--------------+--------------------+-------------+-------------------+------------+-----------+-----------+---------------+--------------------+------------------+--------------------+---------------------+
|
||
| THREAD_ID | EVENT_ID | END_EVENT_ID | EVENT_NAME | SOURCE | TIMER_START | TIMER_END | TIMER_WAIT | LOCK_TIME | SQL_TEXT | DIGEST | DIGEST_TEXT | CURRENT_SCHEMA | OBJECT_TYPE | OBJECT_SCHEMA | OBJECT_NAME | OBJECT_INSTANCE_BEGIN | MYSQL_ERRNO | RETURNED_SQLSTATE | MESSAGE_TEXT | ERRORS | WARNINGS | ROWS_AFFECTED | ROWS_SENT | ROWS_EXAMINED | CREATED_TMP_DISK_TABLES | CREATED_TMP_TABLES | SELECT_FULL_JOIN | SELECT_FULL_RANGE_JOIN | SELECT_RANGE | SELECT_RANGE_CHECK | SELECT_SCAN | SORT_MERGE_PASSES | SORT_RANGE | SORT_ROWS | SORT_SCAN | NO_INDEX_USED | NO_GOOD_INDEX_USED | NESTING_EVENT_ID | NESTING_EVENT_TYPE | NESTING_EVENT_LEVEL |
|
||
+-----------+----------+--------------+----------------------+--------------------------+---------------------+---------------------+------------+-----------+--------------------------------------+----------------------------------+----------------------------------------------+----------------+-------------+---------------+-------------+-----------------------+-------------+-------------------+------------------------------------------+--------+----------+---------------+-----------+---------------+-------------------------+--------------------+------------------+------------------------+--------------+--------------------+-------------+-------------------+------------+-----------+-----------+---------------+--------------------+------------------+--------------------+---------------------+
|
||
| 63 | 32 | 32 | statement/sql/update | socket_connection.cc:101 | 2757904906303653000 | 2757904906543381000 | 239728000 | 145000000 | update t1 set v_name='aa' where id=1 | 356a053ffb5eae2a35981b05090faa01 | UPDATE `t1` SET `v_name` = ? WHERE `id` = ? | test | NULL | NULL | NULL | NULL | 0 | 00000 | Rows matched: 1 Changed: 0 Warnings: 0 | 0 | 0 | 0 | 0 | 3 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | NULL | NULL | 0 |
|
||
+-----------+----------+--------------+----------------------+--------------------------+---------------------+---------------------+------------+-----------+--------------------------------------+----------------------------------+----------------------------------------------+----------------+-------------+---------------+-------------+-----------------------+-------------+-------------------+------------------------------------------+--------+----------+---------------+-----------+---------------+-------------------------+--------------------+------------------+------------------------+--------------+--------------------+-------------+-------------------+------------+-----------+-----------+---------------+--------------------+------------------+--------------------+---------------------+
|
||
1 row in set (0.00 sec)
|
||
</code></pre>
|
||
<p>结果中我们找到了加锁的 update 的 SQL 语句。</p>
|
||
<p><strong>总结</strong></p>
|
||
<p>在 MySQL 数据库中出现了锁，不要着急，我们通过这个方法可以快速定位加锁的 SQL，你学会了吗？</p>
|
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</div>
|
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</div>
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