当客户端连接到 MySQL 服务器时，它是以特定的用户名和来自特定主机进行连接的。性能模式提供了关于这些连接的统计信息，按账户（用户名和主机组合）进行跟踪，也可以分别按用户名和主机名进行跟踪，使用以下表：When a client connects to the MySQL server, it does so under a particular user name and from a particular host. The Performance Schema provides statistics about these connections, tracking them per account (user and host combination) as well as separately per user name and host name, using these tables:

在连接表中，“account” 的含义与 mysql 系统数据库中的 MySQL 授权表含义类似，即该术语指的是用户和主机值的组合。不同之处在于，对于授权表，账户的主机部分可以是一个模式，而对于 Performance Schema 表，主机值总是一个特定的非模式主机名。The meaning of “account” in the connection tables is similar to its meaning in the MySQL grant tables in the mysql system database, in the sense that the term refers to a combination of user and host values. They differ in that, for grant tables, the host part of an account can be a pattern, whereas for Performance Schema tables, the host value is always a specific nonpattern host name.

每个连接表都有 CURRENT_CONNECTIONS 和 TOTAL_CONNECTIONS 列，用于跟踪每个 “跟踪值” 的当前连接数和总连接数。各表使用的跟踪值不同。accounts 表有 USER 和 HOST 列，用于跟踪每个用户和主机组合的连接。users 表和 hosts 表分别有 USER 和 HOST 列，用于跟踪每个用户名和主机名的连接数。Each connection table has CURRENT_CONNECTIONS and TOTAL_CONNECTIONS columns to track the current and total number of connections per “tracking value” on which its statistics are based. The tables differ in what they use for the tracking value. The accounts table has USER and HOST columns to track connections per user and host combination. The users and hosts tables have a USER and HOST column, respectively, to track connections per user name and host name.

性能模式还会统计内部线程以及未能通过认证的用户会话线程，使用USER和HOST列值为NULL的行。The Performance Schema also counts internal threads and threads for user sessions that failed to authenticate, using rows with USER and HOST column values of NULL.

假设名为user1和user2的客户端分别从hosta和hostb各连接一次。性能模式会如下跟踪这些连接：Suppose that clients named user1 and user2 each connect one time from hosta and hostb. The Performance Schema tracks the connections as follows:

当客户端连接时，性能模式会使用每个表相应的跟踪值确定每个连接表中的哪一行适用。如果没有这样的行，则会添加一行。然后性能模式会将该行中的 CURRENT_CONNECTIONS 和 TOTAL_CONNECTIONS 列的值都增加一。When a client connects, the Performance Schema determines which row in each connection table applies, using the tracking value appropriate to each table. If there is no such row, one is added. Then the Performance Schema increments by one the CURRENT_CONNECTIONS and TOTAL_CONNECTIONS columns in that row.

当客户端断开连接时，性能模式会将该行中的 CURRENT_CONNECTIONS 列的值减一，而 TOTAL_CONNECTIONS 列保持不变。When a client disconnects, the Performance Schema decrements by one the CURRENT_CONNECTIONS column in the row and leaves the TOTAL_CONNECTIONS column unchanged.

TRUNCATE TABLE 允许用于连接表。它具有以下效果：TRUNCATE TABLE is permitted for connection tables. It has these effects:

性能模式维护汇总表，这些表按账户、主机或用户汇总各种事件类型的连接统计信息。这些表的名称中包含_summary_by_account、_summary_by_host或_summary_by_user。要识别它们，请使用以下查询：The Performance Schema maintains summary tables that aggregate connection statistics for various event types by account, host, or user. These tables have _summary_by_account, _summary_by_host, or _summary_by_user in the name. To identify them, use this query:

mysql> SELECT TABLE_NAME FROM INFORMATION_SCHEMA.TABLES
       WHERE TABLE_SCHEMA = 'performance_schema'
       AND TABLE_NAME REGEXP '_summary_by_(account|host|user)'
       ORDER BY TABLE_NAME;
+------------------------------------------------------+
| TABLE_NAME                                           |
+------------------------------------------------------+
| events_errors_summary_by_account_by_error            |
| events_errors_summary_by_host_by_error               |
| events_errors_summary_by_user_by_error               |
| events_stages_summary_by_account_by_event_name       |
| events_stages_summary_by_host_by_event_name          |
| events_stages_summary_by_user_by_event_name          |
| events_statements_summary_by_account_by_event_name   |
| events_statements_summary_by_host_by_event_name      |
| events_statements_summary_by_user_by_event_name      |
| events_transactions_summary_by_account_by_event_name |
| events_transactions_summary_by_host_by_event_name    |
| events_transactions_summary_by_user_by_event_name    |
| events_waits_summary_by_account_by_event_name        |
| events_waits_summary_by_host_by_event_name           |
| events_waits_summary_by_user_by_event_name           |
| memory_summary_by_account_by_event_name              |
| memory_summary_by_host_by_event_name                 |
| memory_summary_by_user_by_event_name                 |
+------------------------------------------------------+

有关各个连接汇总表的详细信息，请查阅描述汇总事件类型的表的部分：For details about individual connection summary tables, consult the section that describes tables for the summarized event type:

TRUNCATE TABLE 允许用于连接汇总表。它会删除没有连接的帐户、主机或用户的行，并将剩余行的汇总列重置为零。此外，每个按帐户、主机、用户或线程汇总的汇总表，都会因为其依赖的连接表被截断而被隐式截断。下表描述了连接表截断与隐式截断表之间的关系。TRUNCATE TABLE is permitted for connection summary tables. It removes rows for accounts, hosts, or users with no connections, and resets the summary columns to zero for the remaining rows. In addition, each summary table that is aggregated by account, host, user, or thread is implicitly truncated by truncation of the connection table on which it depends. The following table describes the relationship between connection table truncation and implicitly truncated tables.

截断 _summary_global 汇总表也会隐式截断其对应的连接和线程汇总表。例如，截断 events_waits_summary_global_by_event_name 会隐式截断按账户、主机、用户或线程汇总的等待事件汇总表。Truncating a _summary_global summary table also implicitly truncates its corresponding connection and thread summary tables. For example, truncating events_waits_summary_global_by_event_name implicitly truncates the wait event summary tables that are aggregated by account, host, user, or thread.