2019独角兽企业重金招聘Python工程师标准>>>
一、关于锁的三张表(MEMORY引擎)
## 当前运行的所有事务 mysql> select * from information_schema.innodb_trx\G; *************************** 1. row *************************** trx_id: 613963 trx_state: LOCK WAIT #事务状态 trx_started: 2019-02-22 10:48:48 trx_requested_lock_id: 613963:460:3:4 trx_wait_started: 2019-02-22 11:08:41 trx_weight: 2 trx_mysql_thread_id: 140 trx_query: update tx1 set c1='heyfffff',c2='heyffffff' where id =3 trx_operation_state: starting index read trx_tables_in_use: 1 trx_tables_locked: 1 trx_lock_structs: 2 trx_lock_memory_bytes: 1136 trx_rows_locked: 7 #事务锁住的行数 trx_rows_modified: 0 trx_concurrency_tickets: 0 #事务并发票数 trx_isolation_level: REPEATABLE READ trx_unique_checks: 1 trx_foreign_key_checks: 1 trx_last_foreign_key_error: NULL trx_adaptive_hash_latched: 0 trx_adaptive_hash_timeout: 0 trx_is_read_only: 0 trx_autocommit_non_locking: 0 *************************** 2. row *************************** trx_id: 613962 trx_state: RUNNING trx_started: 2019-02-22 10:46:29 trx_requested_lock_id: NULL trx_wait_started: NULL trx_weight: 3 trx_mysql_thread_id: 138 trx_query: select * from information_schema.innodb_trx trx_operation_state: NULL trx_tables_in_use: 0 trx_tables_locked: 1 trx_lock_structs: 2 trx_lock_memory_bytes: 1136 trx_rows_locked: 1 trx_rows_modified: 1 trx_concurrency_tickets: 0 trx_isolation_level: REPEATABLE READ trx_unique_checks: 1 trx_foreign_key_checks: 1 trx_last_foreign_key_error: NULL trx_adaptive_hash_latched: 0 trx_adaptive_hash_timeout: 0 trx_is_read_only: 0 trx_autocommit_non_locking: 0 2 rows in set (0.00 sec) ## 当前出现的锁 mysql> select * from information_schema.innodb_locks\G; *************************** 1. row *************************** lock_id: 613963:460:3:4 lock_trx_id: 613963 lock_mode: X lock_type: RECORD lock_table: `test`.`tx1` lock_index: PRIMARY lock_space: 460 lock_page: 3 lock_rec: 4 lock_data: 3 *************************** 2. row *************************** lock_id: 613962:460:3:4 lock_trx_id: 613962 lock_mode: X lock_type: RECORD lock_table: `test`.`tx1` lock_index: PRIMARY lock_space: 460 lock_page: 3 lock_rec: 4 lock_data: 3 2 rows in set, 1 warning (0.00 sec) ## 锁等待的对应关系 mysql> select * from information_schema.innodb_lock_waits\G; *************************** 1. row *************************** requesting_trx_id: 613963 requested_lock_id: 613963:460:3:4 #请求锁的锁ID blocking_trx_id: 613962 #当前拥有锁的事务ID blocking_lock_id: 613962:460:3:4 1 row in set, 1 warning (0.00 sec)
二、查看锁的情况
mysql> show status like 'innodb_row_lock_%'; +-------------------------------+--------+ | Variable_name | Value | +-------------------------------+--------+ | Innodb_row_lock_current_waits | 1 | | Innodb_row_lock_time | 479764 | | Innodb_row_lock_time_avg | 39980 | | Innodb_row_lock_time_max | 51021 | | Innodb_row_lock_waits | 12 | +-------------------------------+--------+ 5 rows in set (0.00 sec) 解释如下: Innodb_row_lock_current_waits : 当前等待锁的数量 Innodb_row_lock_time : 系统启动到现在,锁定的总时间长度 Innodb_row_lock_time_avg : 每次平均锁定的时间 Innodb_row_lock_time_max : 最长一次锁定时间 Innodb_row_lock_waits : 系统启动到现在总共锁定的次数 # 查询是否锁表 mysql> show OPEN TABLES where In_use > 0; +----------+-------+--------+-------------+ | Database | Table | In_use | Name_locked | +----------+-------+--------+-------------+ | test | tx1 | 1 | 0 | +----------+-------+--------+-------------+ 1 row in set (0.00 sec)
三、杀死进程
# 1.查询进程 mysql> show processlist\G # \G 结构旋转90度变成纵向 *************************** 1. row *************************** Id: 138 User: root Host: localhost:55106 db: test Command: Query Time: 0 State: starting Info: show processlist *************************** 2. row *************************** Id: 140 User: root Host: localhost:56158 db: test Command: Sleep # 正在等待客户端向它发送执行语句 Time: 145 State: Info: NULL 2 rows in set (0.00 sec) # 2.杀死对应进程ID mysql> kill 140; Query OK, 0 rows affected (0.00 sec) mysql> show processlist\G *************************** 1. row *************************** Id: 138 User: root Host: localhost:55106 db: test Command: Query #该线程正在执行一个语句 Sleep:线程正在等待客户端向其发送新的语句。 Time: 0 State: starting Info: show processlist 1 row in set (0.00 sec)
四、SQL分析
explain使用方法
EXPLAIN命令,用于显示SQL语句的查询执行计划。EXPLAIN为用于SELECT语句中的每个表返回一行信息。
EXPLAIN EXTENDED命令:显示SQL语句的详细的查询执行计划;之后可以通过"SHOW WARNINGS"命令查看详细信息。 SHOW WARNINGS命令:可以查看MySQL优化器优化后的SQL语句。 EXPLAIN PARTITIONS命令:显示SQL语句的带有分区表信息的查询执行计划。 EXPLAIN命令的输出格式: TRADITIONAL:传统类型,按行隔离,每行标识一个自操作。 JSON:JSON格式。
使用方法为在SQL语句前加explain
得到结果如下:
mysql> explain select id,c1 from t1 where c1=4398825;
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 4992210 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)
各列功能如下:
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id: 按照sql语法解析后分层后的编号,可能重复
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select_type:
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SIMPLE,简单的select查询,不使用union及子查询
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PRIMARY,最外层的select查询
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UNION,UNION 中的第二个或随后的 select 查询,不依赖于外部查询的结果集
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DEPENDENT UNION,UNION 中的第二个或随后的 select 查询,依赖于外部查询的结果集
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SUBQUERY,子查询中的第一个 select 查询,不依赖于外部查询的结果集
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DEPENDENT SUBQUERY,子查询中的第一个 select 查询,依赖于外部查询的结果集
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DERIVED,用于 from子句里有子查询的情况。 MySQL会递归执行这些子查询, 把结果放在临时表里。
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UNCACHEABLE SUBQUERY,结果集不能被缓存的子查询,必须重新为外层查询的每一行进行评估。
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UNCACHEABLE UNION,UNION 中的第二个或随后的 select 查询,属于不可缓存的子查询
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table:涉及的表,如果SQL中表有赋别名,这里出现的是别名
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type:
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system,从系统表读一行。这是const联接类型的一个特例。
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const,表最多有一个匹配行,它将在查询开始时被读取。因为仅有一行,在这行的列值可被优化器剩余部分认为是常数。const表很快,因为它们只读取一次!
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eq_ref,查询条件为等于
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ref,条件查询不等于
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ref_or_null,同ref(条件查询),包含NULL值的行。
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index_merge,索引联合查询
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unique_subquery,利用唯一索引进行子查询
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index_subquery,用非唯一索引进行子查询
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range,索引范围扫描
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index,索引全扫描
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ALL,全表扫描。
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possible_keys:可能使用的索引
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key:sql中使用的索引
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key_len:索引长度
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ref:使用哪个列或常数与key一起从表中选择行。
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rows:显示MYSQL执行查询的行数,简单且重要,数值越大越不好,说明没有用好索引
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Extra:该列包含MySQL解决查询的详细信息。
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Distinct,去重,返回第一个满足条件的值
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Not exists 使用not exists查询
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Range checked for each record,有索引,但索引选择率很低
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Using filesort,有序查询
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Using index,索引全扫描
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Using index condition,索引查询
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Using temporary,临表表检索
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Using where,where条件查询
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Using sort_union,有序合并查询
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Using union,合并查询
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Using intersect,索引交叉合并
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Impossible WHERE noticed after reading const tables,读取const tables,查询结果为空
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No tables used,没有使用表
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Using join buffer (Block Nested Loop),使用join buffer(BNL算法)
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Using MRR(Multi-Range Read ) 使用辅助索引进行多范围读
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五、数据库的一些默认设置
# 查看数据库默认存储引擎 mysql> show engines; +--------------------+---------+----------------------------------------------------------------+--------------+------+------------+ | Engine | Support | Comment | Transactions | XA | Savepoints | +--------------------+---------+----------------------------------------------------------------+--------------+------+------------+ | InnoDB | DEFAULT | Supports transactions, row-level locking, and foreign keys | YES | YES | YES | | MRG_MYISAM | YES | Collection of identical MyISAM tables | NO | NO | NO | | MEMORY | YES | Hash based, stored in memory, useful for temporary tables | NO | NO | NO | | BLACKHOLE | YES | /dev/null storage engine (anything you write to it disappears) | NO | NO | NO | | MyISAM | YES | MyISAM storage engine | NO | NO | NO | | CSV | YES | CSV storage engine | NO | NO | NO | | ARCHIVE | YES | Archive storage engine | NO | NO | NO | | PERFORMANCE_SCHEMA | YES | Performance Schema | NO | NO | NO | | FEDERATED | NO | Federated MySQL storage engine | NULL | NULL | NULL | +--------------------+---------+----------------------------------------------------------------+--------------+------+------------+ 9 rows in set (0.00 sec) # 查看当前数据库默认隔离级别 mysql> select @@global.tx_isolation; +-----------------------+ | @@global.tx_isolation | +-----------------------+ | REPEATABLE-READ | +-----------------------+ 1 row in set, 1 warning (0.00 sec) # 默认自动提交事务 mysql> show global variables like 'autocommit'; +---------------+-------+ | Variable_name | Value | +---------------+-------+ | autocommit | ON | +---------------+-------+ 1 row in set (0.00 sec) mysql> select @@global.autocommit; +---------------------+ | @@global.autocommit | +---------------------+ | 1 | +---------------------+ 1 row in set (0.00 sec)