MySQL :: MySQL 8.4 Reference Manual :: 29.12.22.7 The processlist Table (original) (raw)

29.12.22.7 The processlist Table

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, seeSources of Process Information.

The processlist table can be queried directly. If you have thePROCESS 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.

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 theSHOW PROCESSLIST statement, displayed in the PROCESSLIST_ID column of the Performance Schemathreads table, and returned by the CONNECTION_ID() function within the thread.
• USER
  The MySQL user who issued the statement. A value ofsystem 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 theHost 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 (seeSection 27.4, “Using the Event Scheduler”).
  Note
  A USER value of system user is distinct from theSYSTEM_USER privilege. The former designates internal threads. The latter distinguishes the system user and regular user account categories (see Section 8.2.11, “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_`hostname`_:_`clientport`_ format to make it easier to determine which client is doing what.
• DB
  The default database for the thread, orNULL 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, seeSection 10.14, “Examining Server Thread (Process) Information”. The value of this column corresponds to theCOM_ _`xxx`_ commands of the client/server protocol andCom_ _`xxx`_ status variables. SeeSection 7.1.10, “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 19.2.3, “Replication Threads”.
• STATE
  An action, event, or state that indicates what the thread is doing. For descriptions of STATE values, see Section 10.14, “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, orNULL 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 aSELECT statement, theINFO value shows theSELECT statement.
• EXECUTION_ENGINE
  The query execution engine. The value is eitherPRIMARY orSECONDARY. For use with HeatWave Service and HeatWave, where the PRIMARY engine isInnoDB and theSECONDARY engine is HeatWave (RAPID). For MySQL Community Edition Server, MySQL Enterprise Edition Server (on-premise), and HeatWave Service without HeatWave, the value is always PRIMARY.

The processlist table has these indexes:

• Primary key on (ID)

TRUNCATE TABLE is not permitted for the processlist table.

As mentioned previously, if theperformance_schema_show_processlist system variable is enabled, theprocesslist 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 Schemaprocesslist table. This implementation queries active thread data from the Performance Schema rather than the thread manager and does not require a mutex.

MySQL configuration affectsprocesslist 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 theDISABLE_PSI_THREAD CMake option.
  • The Performance Schema must be enabled at server startup. This is true by default; it is controlled using theperformance_schema system variable.
    With that configuration satisfied,performance_schema_show_processlist enables or disables the alternativeSHOW 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 theperformance_schema_max_thread_instances system variable set to its default or set it at least as great as themax_connections system variable.
    * Leave theperformance_schema_max_thread_classes system variable set to its default.
  • To avoid having some STATE column values be empty, leave theperformance_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, theprocesslist table (and thusSHOW PROCESSLIST) produce complete process information.

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

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