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MySQL 5.7 Reference Manual
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8.6.4 MyISAM Table Optimization

To coalesce fragmented rows and eliminate wasted space that results from deleting or updating rows, run myisamchk in recovery mode:

shell> myisamchk -r tbl_name

You can optimize a table in the same way by using the OPTIMIZE TABLE SQL statement. OPTIMIZE TABLE does a table repair and a key analysis, and also sorts the index tree so that key lookups are faster. There is also no possibility of unwanted interaction between a utility and the server, because the server does all the work when you use OPTIMIZE TABLE. See Section 14.7.2.4, “OPTIMIZE TABLE Syntax”.

myisamchk has a number of other options that you can use to improve the performance of a table:

  • --analyze or -a: Perform key distribution analysis. This improves join performance by enabling the join optimizer to better choose the order in which to join the tables and which indexes it should use.

  • --sort-index or -S: Sort the index blocks. This optimizes seeks and makes table scans that use indexes faster.

  • --sort-records=index_num or -R index_num: Sort data rows according to a given index. This makes your data much more localized and may speed up range-based SELECT and ORDER BY operations that use this index.

For a full description of all available options, see Section 5.6.3, “myisamchk — MyISAM Table-Maintenance Utility”.


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