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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 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).
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 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 column type is
converted to ENUM.
A column may use a combination of the preceding compressions.
Can handle fixed-length or dynamic-length records.
This document was generated on July, 26 2004 using texi2html 1.70.
