# MyISAM Index Storage Space

Regular [MyISAM](https://mariadb.com/docs/server/server-usage/storage-engines/myisam-storage-engine) tables make use of [B-tree indexes](https://mariadb.com/docs/server/ha-and-performance/optimization-and-tuning/optimization-and-indexes/storage-engine-index-types#b-tree-indexes).

String indexes are space-compressed, which reduces the size of [VARCHARs](https://mariadb.com/docs/server/reference/data-types/string-data-types/varchar) that don't use the full length, or a string that has trailing spaces. String indexes also make use of prefix-compression, where strings with identical prefixes are compressed.

Numeric indexes can also be prefix-compressed compressed if the [PACK\_KEYS=1](https://mariadb.com/docs/server/tables/create-table#table-options) option is used. Regardless, the high byte is always stored first, which allows a reduced index size.

In the worst case, with no strings being space-compressed, the total index storage space are (index\_length+4)/0.67 per index.

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