10.9.4 索引提示
Index hints give the optimizer information about how to choose indexes during query processing. Index hints, described here, differ from optimizer hints, described in Section 10.9.3, “Optimizer Hints”. Index and optimizer hints may be used separately or together.
Index hints apply to SELECT
and UPDATE
statements. They also work with multi-table DELETE
statements, but not with single-table DELETE
, as shown later in this section.
Index hints are specified following a table name. (For the general syntax for specifying tables in a SELECT
statement, see Section 15.2.13.2, “JOIN Clause”.) The syntax for referring to an individual table, including index hints, looks like this:
tbl_name [[AS] alias] [index_hint_list]
index_hint_list:
index_hint [index_hint] ...
index_hint:
USE {INDEX|KEY}
[FOR {JOIN|ORDER BY|GROUP BY}] ([index_list])
| {IGNORE|FORCE} {INDEX|KEY}
[FOR {JOIN|ORDER BY|GROUP BY}] (index_list)
index_list:
index_name [, index_name] ...
The USE INDEX (
hint tells MySQL to use only one of the named indexes to find rows in the table. The alternative syntax index_list
)IGNORE INDEX (
tells MySQL to not use some particular index or indexes. These hints are useful if index_list
)EXPLAIN
shows that MySQL is using the wrong index from the list of possible indexes.
The FORCE INDEX
hint acts like USE INDEX (
, with the addition that a table scan is assumed to be very expensive. In other words, a table scan is used only if there is no way to use one of the named indexes to find rows in the table.index_list
)
MySQL 8.4 supports the index-level optimizer hints JOIN_INDEX
, GROUP_INDEX
, ORDER_INDEX
, and INDEX
, which are equivalent to and intended to supersede FORCE INDEX
index hints, as well as the NO_JOIN_INDEX
, NO_GROUP_INDEX
, NO_ORDER_INDEX
, and NO_INDEX
optimizer hints, which are equivalent to and intended to supersede IGNORE INDEX
index hints. Thus, you should expect USE INDEX
, FORCE INDEX
, and IGNORE INDEX
to be deprecated in a future release of MySQL, and at some time thereafter to be removed altogether.
These index-level optimizer hints are supported with both single-table and multi-table DELETE
statements.
For more information, see Index-Level Optimizer Hints.
Each hint requires index names, not column names. To refer to a primary key, use the name PRIMARY
. To see the index names for a table, use the SHOW INDEX
statement or the Information Schema STATISTICS
table.
An index_name
value need not be a full index name. It can be an unambiguous prefix of an index name. If a prefix is ambiguous, an error occurs.
Examples:
SELECT * FROM table1 USE INDEX (col1_index,col2_index)
WHERE col1=1 AND col2=2 AND col3=3;
SELECT * FROM table1 IGNORE INDEX (col3_index)
WHERE col1=1 AND col2=2 AND col3=3;
The syntax for index hints has the following characteristics:
-
It is syntactically valid to omit
index_list
forUSE INDEX
, which means “use no indexes.” Omittingindex_list
forFORCE INDEX
orIGNORE INDEX
is a syntax error. -
You can specify the scope of an index hint by adding a
FOR
clause to the hint. This provides more fine-grained control over optimizer selection of an execution plan for various phases of query processing. To affect only the indexes used when MySQL decides how to find rows in the table and how to process joins, useFOR JOIN
. To influence index usage for sorting or grouping rows, useFOR ORDER BY
orFOR GROUP BY
. -
You can specify multiple index hints:
SELECT * FROM t1 USE INDEX (i1) IGNORE INDEX FOR ORDER BY (i2) ORDER BY a;
It is not an error to name the same index in several hints (even within the same hint):
SELECT * FROM t1 USE INDEX (i1) USE INDEX (i1,i1);
However, it is an error to mix
USE INDEX
andFORCE INDEX
for the same table:SELECT * FROM t1 USE INDEX FOR JOIN (i1) FORCE INDEX FOR JOIN (i2);
If an index hint includes no FOR
clause, the scope of the hint is to apply to all parts of the statement. For example, this hint:
IGNORE INDEX (i1)
is equivalent to this combination of hints:
IGNORE INDEX FOR JOIN (i1)
IGNORE INDEX FOR ORDER BY (i1)
IGNORE INDEX FOR GROUP BY (i1)
When index hints are processed, they are collected in a single list by type (USE
, FORCE
, IGNORE
) and by scope (FOR JOIN
, FOR ORDER BY
, FOR GROUP BY
). For example:
SELECT * FROM t1
USE INDEX () IGNORE INDEX (i2) USE INDEX (i1) USE INDEX (i2);
is equivalent to:
SELECT * FROM t1
USE INDEX (i1,i2) IGNORE INDEX (i2);
The index hints then are applied for each scope in the following order:
-
{USE|FORCE} INDEX
is applied if present. (If not, the optimizer-determined set of indexes is used.) -
IGNORE INDEX
is applied over the result of the previous step. For example, the following two queries are equivalent:SELECT * FROM t1 USE INDEX (i1) IGNORE INDEX (i2) USE INDEX (i2); SELECT * FROM t1 USE INDEX (i1);
For FULLTEXT
searches, index hints work as follows:
-
For natural language mode searches, index hints are silently ignored. For example,
IGNORE INDEX(i1)
is ignored with no warning and the index is still used. -
For boolean mode searches, index hints with
FOR ORDER BY
orFOR GROUP BY
are silently ignored. Index hints withFOR JOIN
or noFOR
modifier are honored. In contrast to how hints apply for non-FULLTEXT
searches, the hint is used for all phases of query execution (finding rows and retrieval, grouping, and ordering). This is true even if the hint is given for a non-FULLTEXT
index.For example, the following two queries are equivalent:
SELECT * FROM t USE INDEX (index1) IGNORE INDEX FOR ORDER BY (index1) IGNORE INDEX FOR GROUP BY (index1) WHERE ... IN BOOLEAN MODE ... ; SELECT * FROM t USE INDEX (index1) WHERE ... IN BOOLEAN MODE ... ;
Index hints work with DELETE
statements, but only if you use multi-table DELETE
syntax, as shown here:
mysql> EXPLAIN DELETE FROM t1 USE INDEX(col2)
-> WHERE col1 BETWEEN 1 AND 100 AND COL2 BETWEEN 1 AND 100\G
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that
corresponds to your MySQL server version for the right syntax to use near 'use
index(col2) where col1 between 1 and 100 and col2 between 1 and 100' at line 1
mysql> EXPLAIN DELETE t1.* FROM t1 USE INDEX(col2)
-> WHERE col1 BETWEEN 1 AND 100 AND COL2 BETWEEN 1 AND 100\G
*************************** 1. row ***************************
id: 1
select_type: DELETE
table: t1
partitions: NULL
type: range
possible_keys: col2
key: col2
key_len: 5
ref: NULL
rows: 72
filtered: 11.11
Extra: Using where
1 row in set, 1 warning (0.00 sec)