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able 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 refer 

ences 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 non structured system variables.

At the moment, the first two rules have no possibility of being violated because the only structured vari 

able type is the one for key caches. These rules assume greater significance if some other type of struc 

tured variable is created in the future.

With one exception, it is allowable to 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:

shell> mysqld   hot_cache.key_buffer_size=64K

In an option file, do this:

[mysqld]

hot_cache.key_buffer_size=64K

If you start the server with such an 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:

shell> 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:

shell> 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;

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