#### 13.7.8.2 CACHE INDEX Statement

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```
CACHE INDEX {
      tbl_index_list [, tbl_index_list] ...
    | tbl_name PARTITION (partition_list)
  }
  IN key_cache_name

tbl_index_list:
  tbl_name [{INDEX|KEY} (index_name[, index_name] ...)]

partition_list: {
    partition_name[, partition_name] ...
  | ALL
}
```

 The [`CACHE INDEX`](cache-index.html) statement assigns table indexes to a specific key cache. It applies only to `MyISAM` tables, including partitioned `MyISAM` tables. After the indexes have been assigned, they can be preloaded into the cache if desired with [`LOAD INDEX INTO CACHE`](load-index.html).

 The following statement assigns indexes from the tables `t1`, `t2`, and `t3` to the key cache named `hot_cache`:

```
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`](cache-index.html) enables you to specify that only particular indexes from a table should be assigned to the cache. However, the 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 [`CACHE INDEX`](cache-index.html) statement can be created by setting its size with a parameter setting statement or in the server parameter settings. For example:

```
SET GLOBAL keycache1.key_buffer_size=128*1024;
```

 Key cache parameters are accessed as members of a structured system variable. See [Section 5.1.9.5, “Structured System Variables”](structured-system-variables.html).

 A key cache must exist before you assign indexes to it, or an error occurs:

```
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 are reassigned to the default key cache.

 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.

[`CACHE INDEX`](cache-index.html) is supported for partitioned `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 `p0`, ..., `p3`; this table has an index named `i` on column `c1`.

* Creates 2 key caches named `kc_fast` and `kc_slow`

* Assigns the index for partition `p0` to the `kc_fast` key cache and the index for partitions `p1` and `p3` to the `kc_slow` key 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 table `pt` to a single key cache named `kc_all`, you can use either of the following two 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 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, the `PARTITION (ALL)` clause is optional.

 When assigning indexes for multiple partitions to a key cache, the partitions need not be contiguous, and you need not list their names in any particular order. Indexes for any partitions not explicitly assigned to a key cache automatically use the server default key cache.

 Index preloading is also supported for partitioned `MyISAM` tables. For more information, see [Section 13.7.8.5, “LOAD INDEX INTO CACHE Statement”](load-index.html).

