25.12.8 Performance Schema Connection Tables
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:
The
accounts
table has four rows, for theuser1
/hosta
,user1
/hostb
,user2
/hosta
, anduser2
/hostb
account values, each row counting one connection per account.The
hosts
table has two rows, forhosta
andhostb
, each row counting two connections per host name.The
users
table has two rows, foruser1
anduser2
, each row counting two connections per user name.
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:
Rows are removed for accounts, hosts, or users that have no current connections (rows with
CURRENT_CONNECTIONS = 0
).Nonremoved rows are reset to count only current connections: For rows with
CURRENT_CONNECTIONS > 0
,TOTAL_CONNECTIONS
is reset toCURRENT_CONNECTIONS
.Summary tables that depend on the connection table are implicitly truncated, as described later in this section.
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:
Wait event summaries: Section 25.12.15.1, “Wait Event Summary Tables”
Stage event summaries: Section 25.12.15.2, “Stage Summary Tables”
Statement event summaries: Section 25.12.15.3, “Statement Summary Tables”
Transaction event summaries: Section 25.12.15.4, “Transaction Summary Tables”
Memory event summaries: Section 25.12.15.9, “Memory Summary Tables”
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.
Table 25.2 Implicit Effects of Connection Table Truncation
Truncated Connection Table | Implicitly Truncated Summary Tables |
---|---|
accounts |
Tables with names containing _summary_by_account , _summary_by_thread |
hosts |
Tables with names containing _summary_by_account , _summary_by_host , _summary_by_thread |
users |
Tables with names containing _summary_by_account , _summary_by_user , _summary_by_thread |
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.