HOPEX V2R1U3 Doc

PACK_KEYS \ MySQL

PACK_KEYS takes effect only with MyISAM tables. Set this option to 1 if you want to have smaller

indexes. This usually makes updates slower and reads faster. Setting the option to 0 disables all

packing of keys. Setting it to DEFAULT tells the storage engine to pack only long CHAR or

VARCHAR columns.

If you do not use PACK_KEYS, the default is to pack strings, but not numbers. If you use

PACK_KEYS=1, numbers are packed as well.

When packing binary number keys, MySQL uses prefix compression:

- Every key needs one extra byte to indicate how many bytes of the previous key are the same for

the next key.

- The pointer to the row is stored in high-byte-first order directly after the key, to improve compression.

This means that if you have many equal keys on two consecutive rows, all following "same" keys

usually only take two bytes (including the pointer to the row). Compare this to the ordinary case

where the following keys takes storage_size_for_key + pointer_size (where the

pointer size is usually 4). Conversely, you get a significant benefit from prefix compression only if

you have many numbers that are the same. If all keys are totally different, you use one byte more per

key, if the key is not a key that can have NULL values. (In this case, the packed key length is stored

in the same byte that is used to mark if a key is NULL.)

Translations

Type

String (63) Enumeration

List of MetaClasses

Value

(0), (1), (2).