NAME

The instrument name. Instrument names may have multiple parts and form a hierarchy, as discussed in Section 26.6, “Performance Schema Instrument Naming Conventions”. Events produced from execution of an instrument have an EVENT_NAME value that is taken from the instrument NAME value. (Events do not really have a “name,” but this provides a way to associate events with instruments.)

ENABLED

Whether the instrument is enabled. The value is YES or NO. A disabled instrument produces no events. This column can be modified, although setting ENABLED has no effect for instruments that have already been created.

TIMED

Whether the instrument is timed. The value is YES, NO, or NULL. This column can be modified, although setting TIMED has no effect for instruments that have already been created.

A TIMED value of NULL indicates that the instrument does not support timing. For example, memory operations are not timed, so their TIMED column is NULL.

Setting TIMED to NULL for an instrument that supports timing has no effect, as does setting TIMED to non-NULL for an instrument that does not support timing.

If an enabled instrument is not timed, the instrument code is enabled, but the timer is not. Events produced by the instrument have NULL for the TIMER_START, TIMER_END, and TIMER_WAIT timer values. This in turn causes those values to be ignored when calculating the sum, minimum, maximum, and average time values in summary tables.

PROPERTIES

The instrument properties. This column uses the SET data type, so multiple flags from the following list can be set per instrument:

VOLATILITY

The instrument volatility. Volatility values range from low to high. The values correspond to the PSI_VOLATILITY_xxx constants defined in the mysql/psi/psi_base.h header file:

#define PSI_VOLATILITY_UNKNOWN 0
#define PSI_VOLATILITY_PERMANENT 1
#define PSI_VOLATILITY_PROVISIONING 2
#define PSI_VOLATILITY_DDL 3
#define PSI_VOLATILITY_CACHE 4
#define PSI_VOLATILITY_SESSION 5
#define PSI_VOLATILITY_TRANSACTION 6
#define PSI_VOLATILITY_QUERY 7
#define PSI_VOLATILITY_INTRA_QUERY 8

The VOLATILITY column is purely informational, to provide users (and the Performance Schema code) some hint about the instrument runtime behavior.

Instruments with a low volatility index (PERMANENT = 1) are created once at server startup, and never destroyed or re-created during normal server operation. They are destroyed only during server shutdown.

For example, the wait/synch/mutex/pfs/LOCK_pfs_share_list mutex is defined with a volatility of 1, which means it is created once. Possible overhead from the instrumentation itself (namely, mutex initialization) has no effect for this instrument then. Runtime overhead occurs only when locking or unlocking the mutex.

Instruments with a higher volatility index (for example, SESSION = 5) are created and destroyed for every user session. For example, the wait/synch/mutex/sql/THD::LOCK_query_plan mutex is created each time a session connects, and destroyed when the session disconnects.

This mutex is more sensitive to Performance Schema overhead, because overhead comes not only from the lock and unlock instrumentation, but also from mutex create and destroy instrumentation, which is executed more often.

Another aspect of volatility concerns whether and when an update to the ENABLED column actually has some effect:

For example, this statement does not affect the LOCK_query_plan mutex for existing sessions, but does have an effect on new sessions created subsequent to the update:

This statement actually has no effect at all:

This mutex is permanent, and was created already before the update is executed. The mutex is never created again, so the ENABLED value in setup_instruments is never used. To enable or disable this mutex, use the mutex_instances table instead.

DOCUMENTATION

A string describing the instrument purpose. The value is NULL if no description is available.

Primary key on (NAME)