Applications that monitor databases may make frequent use of
Performance Schema tables. To write queries for these tables
most efficiently, take advantage of their indexes. For example,
include a WHERE
clause that restricts
retrieved rows based on comparison to specific values in an
indexed column.
Most Performance Schema tables have indexes. Tables that do not
are those that normally contain few rows or are unlikely to be
queried frequently. Performance Schema indexes give the
optimizer access to execution plans other than full table scans.
These indexes also improve performance for related objects, such
as sys
schema views that use those
tables.
To see whether a given Performance Schema table has indexes and
what they are, use SHOW INDEX
or
SHOW CREATE TABLE
:
mysql> SHOW INDEX FROM performance_schema.accounts\G
*************************** 1. row ***************************
Table: accounts
Non_unique: 0
Key_name: ACCOUNT
Seq_in_index: 1
Column_name: USER
Collation: NULL
Cardinality: NULL
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: HASH
Comment:
Index_comment:
Visible: YES
*************************** 2. row ***************************
Table: accounts
Non_unique: 0
Key_name: ACCOUNT
Seq_in_index: 2
Column_name: HOST
Collation: NULL
Cardinality: NULL
Sub_part: NULL
Packed: NULL
Null: YES
Index_type: HASH
Comment:
Index_comment:
Visible: YES
mysql> SHOW CREATE TABLE performance_schema.rwlock_instances\G
*************************** 1. row ***************************
Table: rwlock_instances
Create Table: CREATE TABLE `rwlock_instances` (
`NAME` varchar(128) NOT NULL,
`OBJECT_INSTANCE_BEGIN` bigint(20) unsigned NOT NULL,
`WRITE_LOCKED_BY_THREAD_ID` bigint(20) unsigned DEFAULT NULL,
`READ_LOCKED_BY_COUNT` int(10) unsigned NOT NULL,
PRIMARY KEY (`OBJECT_INSTANCE_BEGIN`),
KEY `NAME` (`NAME`),
KEY `WRITE_LOCKED_BY_THREAD_ID` (`WRITE_LOCKED_BY_THREAD_ID`)
) ENGINE=PERFORMANCE_SCHEMA DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
To see the execution plan for a Performance Schema query and
whether it uses any indexes, use
EXPLAIN
:
mysql> EXPLAIN SELECT * FROM performance_schema.accounts
WHERE (USER,HOST) = ('root','localhost')\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: accounts
partitions: NULL
type: const
possible_keys: ACCOUNT
key: ACCOUNT
key_len: 278
ref: const,const
rows: 1
filtered: 100.00
Extra: NULL
The EXPLAIN
output indicates that
the optimizer uses the accounts
table ACCOUNT
index that comprises the
USER
and HOST
columns.
Performance Schema indexes are virtual: They are a construct of the Performance Schema storage engine and use no memory or disk storage. The Performance Schema reports index information to the optimizer so that it can construct efficient execution plans. The Performance Schema in turn uses optimizer information about what to look for (for example, a particular key value), so that it can perform efficient lookups without building actual index structures. This implementation provides two important benefits:
It entirely avoids the maintenance cost normally incurred for tables that undergo frequent updates.
It reduces at an early stage of query execution the amount of data retrieved. For conditions on the indexed columns, the Performance Schema efficiently returns only table rows that satisfy the query conditions. Without an index, the Performance Schema would return all rows in the table, requiring that the optimizer later evaluate the conditions against each row to produce the final result.
Performance Schema indexes are predefined and cannot be dropped, added, or altered.
Performance Schema indexes are similar to hash indexes. For example:
They are used only for equality comparisons that use the
=
or<=>
operators.They are unordered. If a query result must have specific row ordering characteristics, include an
ORDER BY
clause.
For additional information about hash indexes, see Section 10.3.9, “Comparison of B-Tree and Hash Indexes”.