#### 5.1.9.5 Structured System Variables

 A structured variable differs from a regular system variable in two respects:

* Its value is a structure with components that specify server parameters considered to be closely related.

* There might be several instances of a given type of structured variable. Each one has a different name and refers to a different resource maintained by the server.

 MySQL supports one structured variable type, which specifies parameters governing the operation of key caches. A key cache structured variable has these components:

* [`key_buffer_size`](server-system-variables.html#sysvar_key_buffer_size)

* [`key_cache_block_size`](server-system-variables.html#sysvar_key_cache_block_size)

* [`key_cache_division_limit`](server-system-variables.html#sysvar_key_cache_division_limit)

* [`key_cache_age_threshold`](server-system-variables.html#sysvar_key_cache_age_threshold)

 This section describes the syntax for referring to structured variables. Key cache variables are used for syntax examples, but specific details about how key caches operate are found elsewhere, in [Section 8.10.2, “The MyISAM Key Cache”](myisam-key-cache.html).

 To refer to a component of a structured variable instance, you can use a compound name in *`instance_name.component_name`* format. Examples:

```
hot_cache.key_buffer_size
hot_cache.key_cache_block_size
cold_cache.key_cache_block_size
```

 For each structured system variable, an instance with the name of `default` is always predefined. If you refer to a component of a structured variable without any instance name, the `default` instance is used. Thus, `default.key_buffer_size` and [`key_buffer_size`](server-system-variables.html#sysvar_key_buffer_size) both refer to the same system variable.

 Structured variable instances and components follow these naming rules:

* For a given type of structured variable, each instance must have a name that is unique *within* variables of that type. However, instance names need not be unique *across* structured variable types. For example, each structured variable has an instance named `default`, so `default` is not unique across variable types.

* The names of the components of each structured variable type must be unique across all system variable names. If this were not true (that is, if two different types of structured variables could share component member names), it would not be clear which default structured variable to use for references to member names that are not qualified by an instance name.

* If a structured variable instance name is not legal as an unquoted identifier, refer to it as a quoted identifier using backticks. For example, `hot-cache` is not legal, but ``hot-cache`` is.

* `global`, `session`, and `local` are not legal instance names. This avoids a conflict with notation such as `@@GLOBAL.*`var_name`*` for referring to nonstructured system variables.

 Currently, the first two rules have no possibility of being violated because the only structured variable type is the one for key caches. These rules may assume greater significance if some other type of structured variable is created in the future.

 With one exception, you can refer to structured variable components using compound names in any context where simple variable names can occur. For example, you can assign a value to a structured variable using a command-line option:

```
$> mysqld --hot_cache.key_buffer_size=64K
```

 In an option file, use this syntax:

```
[mysqld]
hot_cache.key_buffer_size=64K
```

 If you start the server with this option, it creates a key cache named `hot_cache` with a size of 64KB in addition to the default key cache that has a default size of 8MB.

 Suppose that you start the server as follows:

```
$> mysqld --key_buffer_size=256K \
         --extra_cache.key_buffer_size=128K \
         --extra_cache.key_cache_block_size=2048
```

 In this case, the server sets the size of the default key cache to 256KB. (You could also have written `--default.key_buffer_size=256K`.) In addition, the server creates a second key cache named `extra_cache` that has a size of 128KB, with the size of block buffers for caching table index blocks set to 2048 bytes.

 The following example starts the server with three different key caches having sizes in a 3:1:1 ratio:

```
$> mysqld --key_buffer_size=6M \
         --hot_cache.key_buffer_size=2M \
         --cold_cache.key_buffer_size=2M
```

 Structured variable values may be set and retrieved at runtime as well. For example, to set a key cache named `hot_cache` to a size of 10MB, use either of these statements:

```
mysql> SET GLOBAL hot_cache.key_buffer_size = 10*1024*1024;
mysql> SET @@GLOBAL.hot_cache.key_buffer_size = 10*1024*1024;
```

 To retrieve the cache size, do this:

```
mysql> SELECT @@GLOBAL.hot_cache.key_buffer_size;
```

 However, the following statement does not work. The variable is not interpreted as a compound name, but as a simple string for a [`LIKE`](string-comparison-functions.html#operator_like) pattern-matching operation:

```
mysql> SHOW GLOBAL VARIABLES LIKE 'hot_cache.key_buffer_size';
```

 This is the exception to being able to use structured variable names anywhere a simple variable name may occur.

