The Performance Schema provides statistics about connections to the server. When a client connects, it does so under a particular user name and from a particular host. The Performance Schema tracks connections per account (user name plus host name) and separately per user name and per host name, using these tables:
There are also summary tables that aggregate information about connections. See Section 220.127.116.11, “Connection Summary Tables”.
The meaning of “account” in the connection tables
is similar to its meaning in the MySQL grant tables in the
mysql database, in the sense that the term
refers to a combination of user and host values. Where they
differ is that in grant tables, the host part of an account can
be a pattern, whereas in Performance Schema tables, the host
value is always a specific nonpattern host name.
The connection tables all have
TOTAL_CONNECTIONS columns to track the
current and total number of connections per “tracking
value” on which statistics are based. The tables differ
in what they use for the tracking value. The
accounts table has
HOST columns to
track connections per user name plus host name combination. The
hosts tables have a
respectively, to track connections per user name and per host
Suppose that clients named
user2 each connect one time from
Performance Schema tracks the connections as follows:
accountstable will have four rows, for the
hostbaccount values, each row counting one connection per account.
userstable will have two rows, for
user2, each row counting two connections per user name.
hoststable will have two rows, for
hostb, each row counting two connections per host name.
When a client connects, the Performance Schema determines which
row in each connection table applies to the connection, 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
TOTAL_CONNECTIONS columns in that row.
When a client disconnects, the Performance Schema decrements by
CURRENT_CONNECTIONS column in the row
and leaves the
The Performance Schema also counts internal threads and threads
for user sessions that failed to authenticate. These are counted
in rows with
column values of
Each connection table can be truncated with
TRUNCATE TABLE, which has this
CURRENT_CONNECTIONS = 0are deleted.
For rows with
CURRENT_CONNECTIONS > 0,
TOTAL_CONNECTIONSis reset to
Connection summary tables that depend on the connection table are truncated implicitly (summary values are set to 0). For more information about implicit truncation, see Section 18.104.22.168, “Connection Summary Tables”.