Performance Schema Connection 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: 4989 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 | •Summary tables that depend on the connection table are implicitly truncated, as described later in this section. •Nonremoved rows are reset to count only current connections: For rows with  CURRENT_CONNECTIONS > 0 ,  TOTAL_CONNECTIONS  is reset to  CURRENT_CONNECTIONS . •Rows are removed for accounts, hosts, or users that have no current connections (rows with CURRENT_CONNECTIONS = 0 ). •The  users  table has two rows, for  user1  and  user2 , each row counting two connections per user name. •The  hosts  table has two rows, for  hosta  and  hostb , each row counting two connections per host name. •The  accounts  table has four rows, for the  user1 / hosta ,  user1 / hostb ,  user2 / hosta , and user2 / hostb  account values, each row counting one connection per account.