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MySQL Reference Manual :: 14.1.3.3 Compressed Table Characteristics
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- 14.1.3 MyISAM Table Storage Formats
- 14.1.3.1 Static (Fixed-Length) Table Characteristics
- 14.1.3.2 Dynamic Table Characteristics
- 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.
All MySQL distributions as of version 3.23.19 include
myisampack by default. (This version is when
MySQL was placed under the GPL.) For earlier versions,
myisampack was included only with licenses or
support agreements, but the server still can read tables that were
compressed with myisampack. Compressed tables
can be uncompressed with myisamchk. (For the
ISAM storage engine, compressed tables can be
created with pack_isam and uncompressed with
isamchk.)
Compressed tables have the following characteristics:
Compressed tables take very little disk space. This minimizes disk usage, which is very nice when using slow disks (such as CD-ROMs).
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Each record is compressed separately, so there is very little access overhead. The header for a record is fixed (1-3 bytes) depending on the biggest record 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 column type is converted to
ENUM.A column may use a combination of the preceding compressions.
Can handle fixed-length or dynamic-length records.
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