1、什么是performance_schema
MySQL的performance schema 用于监控MySQL server在一个较低级别的运行过程中的资源消耗、资源等待等情况，它具有以下特点：
- 提供了一种在数据库运行时实时检查server的内部执行情况的方法。performance_schema 数据库中的表使用performance_schema存储引擎。该数据库主要关注数据库运行过程中的性能相关的数据，与information_schema不同，information_schema主要关注server运行过程中的元数据信息
- performance_schema通过监视server的事件来实现监视server内部运行情况， “事件”就是server内部活动中所做的任何事情以及对应的时间消耗，利用这些信息来判断server中的相关资源消耗在了哪里？一般来说，事件可以是函数调用、操作系统的等待、SQL语句执行的阶段（如sql语句执行过程中的parsing 或 sorting阶段）或者整个SQL语句与SQL语句集合。事件的采集可以方便的提供server中的相关存储引擎对磁盘文件、表I/O、表锁等资源的同步调用信息。
- performance_schema中的事件与写入二进制日志中的事件（描述数据修改的events）、事件计划调度程序（这是一种存储程序）的事件不同。performance_schema中的事件记录的是server执行某些活动对某些资源的消耗、耗时、这些活动执行的次数等情况。
- performance_schema中的事件只记录在本地server的performance_schema中，其下的这些表中数据发生变化时不会被写入binlog中，也不会通过复制机制被复制到其他server中。
- 当前活跃事件、历史事件和事件摘要相关的表中记录的信息。能提供某个事件的执行次数、使用时长。进而可用于分析某个特定线程、特定对象（如mutex或file）相关联的活动。
- PERFORMANCE_SCHEMA存储引擎使用server源代码中的“检测点”来实现事件数据的收集。对于performance_schema实现机制本身的代码没有相关的单独线程来检测，这与其他功能（如复制或事件计划程序）不同
- 收集的事件数据存储在performance_schema数据库的表中。这些表可以使用SELECT语句查询，也可以使用SQL语句更新performance_schema数据库中的表记录（如动态修改performance_schema的setup_*开头的几个配置表，但要注意：配置表的更改会立即生效，这会影响数据收集）
- performance_schema的表中的数据不会持久化存储在磁盘中，而是保存在内存中，一旦服务器重启，这些数据会丢失（包括配置表在内的整个performance_schema下的所有数据）
- MySQL支持的所有平台中事件监控功能都可用，但不同平台中用于统计事件时间开销的计时器类型可能会有所差异。
2、performance_schema使用快速入门
2.1检查当前数据库版本是否支持
mysql> SHOW ENGINES; +--------------------+---------+----------------------------------------------------------------+--------------+------+------------+ | Engine | Support | Comment | Transactions | XA | Savepoints | +--------------------+---------+----------------------------------------------------------------+--------------+------+------------+ | FEDERATED | NO | Federated MySQL storage engine | NULL | NULL | NULL | | MEMORY | YES | Hash based, stored in memory, useful for temporary tables | NO | NO | NO | | InnoDB | DEFAULT | Supports transactions, row-level locking, and foreign keys | YES | YES | YES | | PERFORMANCE_SCHEMA | YES | Performance Schema | NO | NO | NO | | MyISAM | YES | MyISAM storage engine | NO | NO | NO | | MRG_MYISAM | YES | Collection of identical MyISAM tables | NO | NO | NO | | BLACKHOLE | YES | /dev/null storage engine (anything you write to it disappears) | NO | NO | NO | | CSV | YES | CSV storage engine | NO | NO | NO | | ARCHIVE | YES | Archive storage engine | NO | NO | NO | +--------------------+---------+----------------------------------------------------------------+--------------+------+------------+ 9 rows in set (0.01 sec) mysql> SELECT * FROM INFORMATION_SCHEMA.ENGINES WHERE ENGINE ='PERFORMANCE_SCHEMA'; +--------------------+---------+--------------------+--------------+------+------------+ | ENGINE | SUPPORT | COMMENT | TRANSACTIONS | XA | SAVEPOINTS | +--------------------+---------+--------------------+--------------+------+------------+ | PERFORMANCE_SCHEMA | YES | Performance Schema | NO | NO | NO | +--------------------+---------+--------------------+--------------+------+------------+ 1 row in set (0.00 sec)
mysql: [Warning] Using a password on the command line interface can be insecure. Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 10 Server version: 8.0.27 MySQL Community Server - GPL Copyright (c) 2000, 2021, Oracle and/or its affiliates. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners. Type 'help;' or '\h' for help. Type '\c' to clear the current input statement. mysql> show tables from performance_schema; +------------------------------------------------------+ | Tables_in_performance_schema | +------------------------------------------------------+ | accounts | | binary_log_transaction_compression_stats | | cond_instances | | data_lock_waits | | data_locks | | error_log | | events_errors_summary_by_account_by_error | | events_errors_summary_by_host_by_error | | events_errors_summary_by_thread_by_error | | events_errors_summary_by_user_by_error | | events_errors_summary_global_by_error | | events_stages_current | | events_stages_history | | events_stages_history_long | | events_stages_summary_by_account_by_event_name | | events_stages_summary_by_host_by_event_name | | events_stages_summary_by_thread_by_event_name | | events_stages_summary_by_user_by_event_name | | events_stages_summary_global_by_event_name | | events_statements_current | | events_statements_histogram_by_digest | | events_statements_histogram_global | | events_statements_history | | events_statements_history_long | | events_statements_summary_by_account_by_event_name | | events_statements_summary_by_digest | | events_statements_summary_by_host_by_event_name | | events_statements_summary_by_program | | events_statements_summary_by_thread_by_event_name | | events_statements_summary_by_user_by_event_name | | events_statements_summary_global_by_event_name | | events_transactions_current | | events_transactions_history | | events_transactions_history_long | | events_transactions_summary_by_account_by_event_name | | events_transactions_summary_by_host_by_event_name | | events_transactions_summary_by_thread_by_event_name | | events_transactions_summary_by_user_by_event_name | | events_transactions_summary_global_by_event_name | | events_waits_current | | events_waits_history | | events_waits_history_long | | events_waits_summary_by_account_by_event_name | | events_waits_summary_by_host_by_event_name | | events_waits_summary_by_instance | | events_waits_summary_by_thread_by_event_name | | events_waits_summary_by_user_by_event_name | | events_waits_summary_global_by_event_name | | file_instances | | file_summary_by_event_name | | file_summary_by_instance | | global_status | | global_variables | | host_cache | | hosts | | keyring_component_status | | keyring_keys | | log_status | | memory_summary_by_account_by_event_name | | memory_summary_by_host_by_event_name | | memory_summary_by_thread_by_event_name | | memory_summary_by_user_by_event_name | | memory_summary_global_by_event_name | | metadata_locks | | mutex_instances | | objects_summary_global_by_type | | performance_timers | | persisted_variables | | prepared_statements_instances | | processlist | | replication_applier_configuration | | replication_applier_filters | | replication_applier_global_filters | | replication_applier_status | | replication_applier_status_by_coordinator | | replication_applier_status_by_worker | | replication_asynchronous_connection_failover | | replication_asynchronous_connection_failover_managed | | replication_connection_configuration | | replication_connection_status | | replication_group_member_stats | | replication_group_members | | rwlock_instances | | session_account_connect_attrs | | session_connect_attrs | | session_status | | session_variables | | setup_actors | | setup_consumers | | setup_instruments | | setup_objects | | setup_threads | | socket_instances | | socket_summary_by_event_name | | socket_summary_by_instance | | status_by_account | | status_by_host | | status_by_thread | | status_by_user | | table_handles | | table_io_waits_summary_by_index_usage | | table_io_waits_summary_by_table | | table_lock_waits_summary_by_table | | threads | | tls_channel_status | | user_defined_functions | | user_variables_by_thread | | users | | variables_by_thread | | variables_info | +------------------------------------------------------+ 110 rows in set (0.00 sec)
现在，我们知道了在 MySQL 8.0.27 版本中，performance_schema 下一共有110张表！！！
2.2. 启用performance_schema
performance_schema在5.7.x及其以上版本中默认启用（5.6.x及其以下版本默认关闭），
如果要显式启用或关闭时，我们需要使用参数performance_schema=ON|OFF设置，并在my.cnf中进行配置：
[mysqld] performance_schema = ON # 注意：该参数为只读参数，需要在实例启动之前设置才生效
mysql> SHOW VARIABLES LIKE 'performance_schema'; +--------------------+-------+ | Variable_name | Value | +--------------------+-------+ | performance_schema | OFF | +--------------------+-------+ 1 row in set (0.00 sec)
开启的时候：
CONTAINER ID NAME CPU % MEM USAGE / LIMIT MEM % NET I/O BLOCK I/O PIDS
025fc92b4ca6 mysql8 0.32% 347.6MiB / 978.9MiB 35.51% 0B / 0B 35.5MB / 13.5MB 38
关闭的时候：
CONTAINER ID NAME CPU % MEM USAGE / LIMIT MEM % NET I/O BLOCK I/O PIDS
025fc92b4ca6 mysql8 0.25% 131.8MiB / 978.9MiB 13.46% 0B / 0B 14.5MB / 13.5MB 38
PS：因为在我的云机器只有1G内存，所以我要关闭它，因为开启它的时候，mysql至少要300M内存，关闭之后，只有100多M！！！，效果非常明显！！！
参考
1.https://zhuanlan.zhihu.com/p/88422176
2.https://www.codeleading.com/article/60583826238/
performance_schema是个啥