### 27.4.2 Performance Schema Event Filtering

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 Events are processed in a producer/consumer fashion:

* Instrumented code is the source for events and produces events to be collected. The [`setup_instruments`](performance-schema-setup-instruments-table.html) table lists the instruments for which events can be collected, whether they are enabled, and (for enabled instruments) whether to collect timing information:

  ```
  mysql> SELECT NAME, ENABLED, TIMED
         FROM performance_schema.setup_instruments;
  +---------------------------------------------------+---------+-------+
  | NAME                                              | ENABLED | TIMED |
  +---------------------------------------------------+---------+-------+
  ...
  | wait/synch/mutex/sql/LOCK_global_read_lock        | YES     | YES   |
  | wait/synch/mutex/sql/LOCK_global_system_variables | YES     | YES   |
  | wait/synch/mutex/sql/LOCK_lock_db                 | YES     | YES   |
  | wait/synch/mutex/sql/LOCK_manager                 | YES     | YES   |
  ...
  ```

   The [`setup_instruments`](performance-schema-setup-instruments-table.html) table provides the most basic form of control over event production. To further refine event production based on the type of object or thread being monitored, other tables may be used as described in [Section 27.4.3, “Event Pre-Filtering”](performance-schema-pre-filtering.html).

* Performance Schema tables are the destinations for events and consume events. The [`setup_consumers`](performance-schema-setup-consumers-table.html) table lists the types of consumers to which event information can be sent and whether they are enabled:

  ```
  mysql> SELECT * FROM performance_schema.setup_consumers;
  +----------------------------------+---------+
  | NAME                             | ENABLED |
  +----------------------------------+---------+
  | events_stages_current            | NO      |
  | events_stages_history            | NO      |
  | events_stages_history_long       | NO      |
  | events_statements_cpu            | NO      |
  | events_statements_current        | YES     |
  | events_statements_history        | YES     |
  | events_statements_history_long   | NO      |
  | events_transactions_current      | YES     |
  | events_transactions_history      | YES     |
  | events_transactions_history_long | NO      |
  | events_waits_current             | NO      |
  | events_waits_history             | NO      |
  | events_waits_history_long        | NO      |
  | global_instrumentation           | YES     |
  | thread_instrumentation           | YES     |
  | statements_digest                | YES     |
  +----------------------------------+---------+
  ```

 Filtering can be done at different stages of performance monitoring:

* **Pre-filtering.**  This is done by modifying Performance Schema configuration so that only certain types of events are collected from producers, and collected events update only certain consumers. To do this, enable or disable instruments or consumers. Pre-filtering is done by the Performance Schema and has a global effect that applies to all users.

   Reasons to use pre-filtering:

  * To reduce overhead. Performance Schema overhead should be minimal even with all instruments enabled, but perhaps you want to reduce it further. Or you do not care about timing events and want to disable the timing code to eliminate timing overhead.

  * To avoid filling the current-events or history tables with events in which you have no interest. Pre-filtering leaves more “room” in these tables for instances of rows for enabled instrument types. If you enable only file instruments with pre-filtering, no rows are collected for nonfile instruments. With post-filtering, nonfile events are collected, leaving fewer rows for file events.

  * To avoid maintaining some kinds of event tables. If you disable a consumer, the server does not spend time maintaining destinations for that consumer. For example, if you do not care about event histories, you can disable the history table consumers to improve performance.

* **Post-filtering.**  This involves the use of `WHERE` clauses in queries that select information from Performance Schema tables, to specify which of the available events you want to see. Post-filtering is performed on a per-user basis because individual users select which of the available events are of interest.

   Reasons to use post-filtering:

  * To avoid making decisions for individual users about which event information is of interest.

  * To use the Performance Schema to investigate a performance issue when the restrictions to impose using pre-filtering are not known in advance.

 The following sections provide more detail about pre-filtering and provide guidelines for naming instruments or consumers in filtering operations. For information about writing queries to retrieve information (post-filtering), see [Section 27.5, “Performance Schema Queries”](performance-schema-queries.html).

