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Editorial Team
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Asked: 2026-05-31T12:15:52+00:00

In my MySQL database I see these statistics: Type Usage Data 16,384 Bytes Index

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In my MySQL database I see these statistics:

Type    Usage  
Data    16,384  Bytes  
Index   32,768  Bytes  
Overhead    405,0   MB  
Effective   -424,624,128    Bytes  
Total   49,152  Bytes

When I try the commands check table, ALTER TABLE 'mytable' ENGINE = INNODB, OPTIMIZE TABLE, nothing happens to the overhead.

Why does nothing happen, should I worry, and when should I worry? I’ve seen other questions that state ‘to worry when the overhead gets too large’. What is too large?

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1 Answer

  1. Editorial Team

    Overhead in PHPMyAdmin is calculated by ‘Data_free’ column returned by SHOW TABLE STATUS.
    It is actually explained in MySQL documentation:
    http://dev.mysql.com/doc/refman/5.1/en/show-table-status.html

    Data_free: The number of allocated but unused bytes.

    Beginning with MySQL 5.1.24, this information is also shown for InnoDB
    tables (previously, it was in the Comment value). InnoDB tables report
    the free space of the tablespace to which the table belongs. For a
    table located in the shared tablespace, this is the free space of the
    shared tablespace. If you are using multiple tablespaces and the table
    has its own tablespace, the free space is for only that table. Free
    space means the number of completely free 1MB extents minus a safety
    margin. Even if free space displays as 0, it may be possible to insert
    rows as long as new extents need not be allocated.

    However for InnoDB this is important “InnoDB tables report the free space of the tablespace to which the table belongs. For a table located in the shared tablespace, this is the free space of the shared tablespace.”. So with a typical InnoDB setup (‘innondb_file_per_table’ is not set) you will get the free space for all tables and not for a single table. That is probably why phpMyAdmin ignores/discards the information or on Optimize table nothing happens.

    You should read this post which clearly explains how to optimize a table in innodb.

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