CACHE INDEX tbl_index_list [, tbl_index_list] ... [PARTITION (partition_list | ALL)] IN key_cache_name tbl_index_list: tbl_name [[INDEX|KEY] (index_name[, index_name] ...)] partition_list: partition_name[, partition_name][, ...]
CACHE INDEX statement assigns
table indexes to a specific key cache. It is used only for
MyISAM tables. After the indexes have been
assigned, they can be preloaded into the cache if desired with
LOAD INDEX INTO
The following statement assigns indexes from the tables
t3 to the key cache named
mysql> CACHE INDEX t1, t2, t3 IN hot_cache; +---------+--------------------+----------+----------+ | Table | Op | Msg_type | Msg_text | +---------+--------------------+----------+----------+ | test.t1 | assign_to_keycache | status | OK | | test.t2 | assign_to_keycache | status | OK | | test.t3 | assign_to_keycache | status | OK | +---------+--------------------+----------+----------+
The syntax of
CACHE INDEX enables
you to specify that only particular indexes from a table should
be assigned to the cache. The current implementation assigns all
the table's indexes to the cache, so there is no reason to
specify anything other than the table name.
The key cache referred to in a
INDEX statement can be created by setting its size
with a parameter setting statement or in the server parameter
settings. For example:
mysql> SET GLOBAL keycache1.key_buffer_size=128*1024;
Key cache parameters can be accessed as members of a structured system variable. See Section 220.127.116.11, “Structured System Variables”.
A key cache must exist before you can assign indexes to it:
mysql> CACHE INDEX t1 IN non_existent_cache; ERROR 1284 (HY000): Unknown key cache 'non_existent_cache'
By default, table indexes are assigned to the main (default) key cache created at the server startup. When a key cache is destroyed, all indexes assigned to it become assigned to the default key cache again.
Index assignment affects the server globally: If one client assigns an index to a given cache, this cache is used for all queries involving the index, no matter which client issues the queries.
In MySQL 8.0, this statement is also supported for
MyISAM tables. You can assign one
or more indexes for one, several, or all partitions to a given
key cache. For example, you can do the following:
CREATE TABLE pt (c1 INT, c2 VARCHAR(50), INDEX i(c1)) ENGINE=MyISAM PARTITION BY HASH(c1) PARTITIONS 4; SET GLOBAL kc_fast.key_buffer_size = 128 * 1024; SET GLOBAL kc_slow.key_buffer_size = 128 * 1024; CACHE INDEX pt PARTITION (p0) IN kc_fast; CACHE INDEX pt PARTITION (p1, p3) IN kc_slow;
The previous set of statements performs the following actions:
Creates a partitioned table with 4 partitions; these partitions are automatically named
p3; this table has an index named
Creates 2 key caches named
Assigns the index for partition
kc_fastkey cache and the index for partitions
kc_slowkey cache; the index for the remaining partition (
p2) uses the server's default key cache.
If you wish instead to assign the indexes for all partitions in
pt to a single key cache named
kc_all, you can use either one of the
following 2 statements:
CACHE INDEX pt PARTITION (ALL) IN kc_all; CACHE INDEX pt IN kc_all;
The two statements just shown are equivalent, and issuing either
one of them has exactly the same effect. In other words, if you
wish to assign indexes for all partitions of a partitioned table
to the same key cache, then the
(ALL) clause is optional.
When assigning indexes for multiple partitions to a key cache, the partitions do not have to be contiguous, and you are not required to list their names in any particular order. Indexes for any partitions that are not explicitly assigned to a key cache automatically use the server's default key cache.
In MySQL 8.0, index preloading is also supported
MyISAM tables. For more
information, see Section 18.104.22.168, “LOAD INDEX INTO CACHE Syntax”.