Compressed storage format is a read-only format that is generated with the myisampack tool. Compressed tables can be uncompressed with myisamchk.
Compressed tables have the following characteristics:
Compressed tables take very little disk space. This minimizes disk usage, which is helpful when using slow disks (such as CD-ROMs).
Each row is compressed separately, so there is very little access overhead. The header for a row takes up one to three bytes depending on the biggest row in the table. Each column is compressed differently. There is usually a different Huffman tree for each column. Some of the compression types are:
Suffix space compression.
Prefix space compression.
Numbers with a value of zero are stored using one bit.
If values in an integer column have a small range, the column is stored using the smallest possible type. For example, a
BIGINTcolumn (eight bytes) can be stored as aTINYINTcolumn (one byte) if all its values are in the range from-128to127.If a column has only a small set of possible values, the data type is converted to
ENUM.A column may use any combination of the preceding compression types.
Can be used for fixed-length or dynamic-length rows.
While a compressed table is read only, and you cannot
therefore update or add rows in the table, DDL (Data
Definition Language) operations are still valid. For example,
you may still use DROP to drop the table,
and TRUNCATE TABLE to empty the
table.