(mysql.info) compressed-format
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14.1.3.3 Compressed Table Characteristics
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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
`BIGINT' column (eight bytes) can be stored as a `TINYINT'
column (one byte) if all its values are in the range from
`-128' to `127'.
* 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.
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