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:

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.

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.

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.

Suppose that clients named user1 and user2 each connect one time from hosta and hostb. The Performance Schema tracks the connections as follows:

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.

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 is permitted for connection tables. It has these effects:

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_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                 |
+------------------------------------------------------+

For details about individual connection summary tables, consult the section that describes tables for the summarized event type:

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.

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.