MySQL :: MySQL 8.0 Reference Manual :: 29.4.6 Pre-Filtering by Thread (original) (raw)

29.4.6 Pre-Filtering by Thread

The threads table contains a row for each server thread. Each row contains information about a thread and indicates whether monitoring is enabled for it. For the Performance Schema to monitor a thread, these things must be true:

• The thread_instrumentation consumer in the setup_consumers table must be YES.
• The threads.INSTRUMENTED column must beYES.
• Monitoring occurs only for those thread events produced from instruments that are enabled in thesetup_instruments table.

The threads table also indicates for each server thread whether to perform historical event logging. This includes wait, stage, statement, and transaction events and affects logging to these tables:

events_waits_history
events_waits_history_long
events_stages_history
events_stages_history_long
events_statements_history
events_statements_history_long
events_transactions_history
events_transactions_history_long

For historical event logging to occur, these things must be true:

• The appropriate history-related consumers in thesetup_consumers table must be enabled. For example, wait event logging in theevents_waits_history andevents_waits_history_long tables requires the correspondingevents_waits_history andevents_waits_history_long consumers to beYES.
• The threads.HISTORY column must beYES.
• Logging occurs only for those thread events produced from instruments that are enabled in thesetup_instruments table.

For foreground threads (resulting from client connections), the initial values of the INSTRUMENTED andHISTORY columns inthreads table rows are determined by whether the user account associated with a thread matches any row in the setup_actors table. The values come from the ENABLED andHISTORY columns of the matchingsetup_actors table row.

For background threads, there is no associated user.INSTRUMENTED and HISTORY are YES by default andsetup_actors is not consulted.

The initial setup_actors contents look like this:

mysql> SELECT * FROM performance_schema.setup_actors;
+------+------+------+---------+---------+
| HOST | USER | ROLE | ENABLED | HISTORY |
+------+------+------+---------+---------+
| %    | %    | %    | YES     | YES     |
+------+------+------+---------+---------+

The HOST and USER columns should contain a literal host or user name, or'%' to match any name.

The ENABLED and HISTORY columns indicate whether to enable instrumentation and historical event logging for matching threads, subject to the other conditions described previously.

When the Performance Schema checks for a match for each new foreground thread in setup_actors, it tries to find more specific matches first, using theUSER and HOST columns (ROLE is unused):

• Rows withUSER='_`literal`_' andHOST='_`literal`_'.
• Rows withUSER='_`literal`_' and HOST='%'.
• Rows with USER='%' andHOST='_`literal`_'.
• Rows with USER='%' andHOST='%'.

The order in which matching occurs matters because different matching setup_actors rows can have different USER and HOST values. This enables instrumenting and historical event logging to be applied selectively per host, user, or account (user and host combination), based on the ENABLED andHISTORY column values:

• When the best match is a row withENABLED=YES, theINSTRUMENTED value for the thread becomesYES. When the best match is a row withHISTORY=YES, theHISTORY value for the thread becomesYES.
• When the best match is a row withENABLED=NO, theINSTRUMENTED value for the thread becomesNO. When the best match is a row withHISTORY=NO, theHISTORY value for the thread becomesNO.
• When no match is found, the INSTRUMENTED and HISTORY values for the thread becomeNO.

The ENABLED and HISTORY columns in setup_actors rows can be set to YES or NO independent of one another. This means you can enable instrumentation separately from whether you collect historical events.

By default, monitoring and historical event collection are enabled for all new foreground threads because thesetup_actors table initially contains a row with '%' for bothHOST and USER. To perform more limited matching such as to enable monitoring only for some foreground threads, you must change this row because it matches any connection, and add rows for more specificHOST/USER combinations.

Suppose that you modifysetup_actors as follows:

UPDATE performance_schema.setup_actors
SET ENABLED = 'NO', HISTORY = 'NO'
WHERE HOST = '%' AND USER = '%';
INSERT INTO performance_schema.setup_actors
(HOST,USER,ROLE,ENABLED,HISTORY)
VALUES('localhost','joe','%','YES','YES');
INSERT INTO performance_schema.setup_actors
(HOST,USER,ROLE,ENABLED,HISTORY)
VALUES('hosta.example.com','joe','%','YES','NO');
INSERT INTO performance_schema.setup_actors
(HOST,USER,ROLE,ENABLED,HISTORY)
VALUES('%','sam','%','NO','YES');

The UPDATE statement changes the default match to disable instrumentation and historical event collection. The INSERT statements add rows for more specific matches.

Now the Performance Schema determines how to set theINSTRUMENTED and HISTORY values for new connection threads as follows:

• If joe connects from the local host, the connection matches the first inserted row. TheINSTRUMENTED andHISTORY values for the thread becomeYES.
• If joe connects fromhosta.example.com, the connection matches the second inserted row. The INSTRUMENTED value for the thread becomes YES and theHISTORY value becomesNO.
• If joe connects from any other host, there is no match. The INSTRUMENTED andHISTORY values for the thread becomeNO.
• If sam connects from any host, the connection matches the third inserted row. TheINSTRUMENTED value for the thread becomesNO and the HISTORY value becomes YES.
• For any other connection, the row withHOST and USER set to'%' matches. This row now hasENABLED and HISTORY set to NO, so theINSTRUMENTED andHISTORY values for the thread becomeNO.

Modifications to the setup_actors table affect only foreground threads created subsequent to the modification, not existing threads. To affect existing threads, modify the INSTRUMENTED andHISTORY columns ofthreads table rows.