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27.12.21.5 The processlist Table

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The MySQL process list indicates the operations currently being performed by the set of threads executing within the server. The processlist table is one source of process information. For a comparison of this table with other sources, see Sources of Process Information.

The processlist table can be queried directly. If you have the PROCESS privilege, you can see all threads, even those belonging to other users. Otherwise (without the PROCESS privilege), nonanonymous users have access to information about their own threads but not threads for other users, and anonymous users have no access to thread information.

Note

If the performance_schema_show_processlist system variable is enabled, the processlist table also serves as the basis for an alternative implementation underlying the SHOW PROCESSLIST statement. For details, see later in this section.

The processlist table contains a row for each server process:

mysql> SELECT * FROM performance_schema.processlist\G
*************************** 1. row ***************************
     ID: 5
   USER: event_scheduler
   HOST: localhost
     DB: NULL
COMMAND: Daemon
   TIME: 137
  STATE: Waiting on empty queue
   INFO: NULL
*************************** 2. row ***************************
     ID: 9
   USER: me
   HOST: localhost:58812
     DB: NULL
COMMAND: Sleep
   TIME: 95
  STATE:
   INFO: NULL
*************************** 3. row ***************************
     ID: 10
   USER: me
   HOST: localhost:58834
     DB: test
COMMAND: Query
   TIME: 0
  STATE: executing
   INFO: SELECT * FROM performance_schema.processlist
...

The processlist table has these columns:

ID
The connection identifier. This is the same value displayed in the Id column of the SHOW PROCESSLIST statement, displayed in the PROCESSLIST_ID column of the Performance Schema threads table, and returned by the CONNECTION_ID() function within the thread.
USER
The MySQL user who issued the statement. A value of system user refers to a nonclient thread spawned by the server to handle tasks internally, for example, a delayed-row handler thread or an I/O or SQL thread used on replica hosts. For system user, there is no host specified in the Host column. unauthenticated user refers to a thread that has become associated with a client connection but for which authentication of the client user has not yet occurred. event_scheduler refers to the thread that monitors scheduled events (see Section 25.4, “Using the Event Scheduler”).

Note

A USER value of system user is distinct from the SYSTEM_USER privilege. The former designates internal threads. The latter distinguishes the system user and regular user account categories (see Section 6.2.11, “Account Categories”).
HOST
The host name of the client issuing the statement (except for system user, for which there is no host). The host name for TCP/IP connections is reported in host_name:client_port format to make it easier to determine which client is doing what.
DB
The default database for the thread, or NULL if none has been selected.
COMMAND
The type of command the thread is executing on behalf of the client, or Sleep if the session is idle. For descriptions of thread commands, see Section 8.14, “Examining Server Thread (Process) Information”. The value of this column corresponds to the COM_xxx commands of the client/server protocol and Com_xxx status variables. See Section 5.1.10, “Server Status Variables”
TIME
The time in seconds that the thread has been in its current state. For a replica SQL thread, the value is the number of seconds between the timestamp of the last replicated event and the real time of the replica host. See Section 17.2.3, “Replication Threads”.
STATE
An action, event, or state that indicates what the thread is doing. For descriptions of STATE values, see Section 8.14, “Examining Server Thread (Process) Information”.
Most states correspond to very quick operations. If a thread stays in a given state for many seconds, there might be a problem that needs to be investigated.
INFO
The statement the thread is executing, or NULL if it is executing no statement. The statement might be the one sent to the server, or an innermost statement if the statement executes other statements. For example, if a CALL statement executes a stored procedure that is executing a SELECT statement, the INFO value shows the SELECT statement.

The processlist table has these indexes:

Primary key on (ID)

TRUNCATE TABLE is not permitted for the processlist table.

As mentioned previously, if the performance_schema_show_processlist system variable is enabled, the processlist table serves as the basis for an alternative implementation of other process information sources:

The SHOW PROCESSLIST statement.
The mysqladmin processlist command (which uses SHOW PROCESSLIST statement).

The default SHOW PROCESSLIST implementation iterates across active threads from within the thread manager while holding a global mutex. This has negative performance consequences, particularly on busy systems. The alternative SHOW PROCESSLIST implementation is based on the Performance Schema processlist table. This implementation queries active thread data from the Performance Schema rather than the thread manager and does not require a mutex.

MySQL configuration affects processlist table contents as follows:

Minimum required configuration:
- The MySQL server must be configured and built with thread instrumentation enabled. This is true by default; it is controlled using the DISABLE_PSI_THREAD CMake option.
- The Performance Schema must be enabled at server startup. This is true by default; it is controlled using the performance_schema system variable.
With that configuration satisfied, performance_schema_show_processlist enables or disables the alternative SHOW PROCESSLIST implementation. If the minimum configuration is not satisfied, the processlist table (and thus SHOW PROCESSLIST) may not return all data.
Recommended configuration:
- To avoid having some threads ignored:
  - Leave the performance_schema_max_thread_instances system variable set to its default or set it at least as great as the max_connections system variable.
  - Leave the performance_schema_max_thread_classes system variable set to its default.
- To avoid having some STATE column values be empty, leave the performance_schema_max_stage_classes system variable set to its default.
The default for those configuration parameters is -1, which causes the Performance Schema to autosize them at server startup. With the parameters set as indicated, the processlist table (and thus SHOW PROCESSLIST) produce complete process information.

The preceding configuration parameters affect the contents of the processlist table. For a given configuration, however, the processlist contents are unaffected by the performance_schema_show_processlist setting.

The alternative process list implementation does not apply to the INFORMATION_SCHEMA PROCESSLIST table or the COM_PROCESS_INFO command of the MySQL client/server protocol.

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Document created the 26/06/2006, last modified the 26/10/2018
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References

Manuel MySQL : https://dev.mysql.com/

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