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myisamchk

database toolsLinux/Unix
The myisamchk command is one of the most frequently used commands in Linux/Unix-like operating systems. myisamchk Check, repair, or optimize MyISAM tables in MySQL databases
See ExamplesView Syntax

Quick Reference

Command Name:

myisamchk

Category:

database tools

Platform:

Linux/Unix

Basic Usage:

myisamchk [options] [arguments]

Common Use Cases

    Syntax

    myisamchk [options] tbl_name [...]

    Options

    Option Description
    -?, --help Display help and exit
    -a, --analyze Analyze distribution of keys
    -b, --backup Make a backup of the .MYD file as file-name-time.BAK
    -c, --check Check table for errors (default operation)
    -C, --check-only-changed Check only tables that have changed since last check
    -d, --description Print descriptive information about table
    -e, --extend-check Perform extended check
    -f, --force Restart with -r if there are any errors in the table
    -F, --fast Check only tables that haven't been closed properly
    -i, --information Print statistics information about table that is checked
    -k, --keys-used=# Define which indexes to update
    -l, --no-symlinks Do not follow symbolic links
    -m, --medium-check Faster than extended check
    -o, --optimize Optimize table
    -p, --parallel-recover Use parallel repair with threads
    -q, --quick Quick check; don't check rows to see if they link correctly
    -r, --recover Can fix almost anything except unique keys
    -s, --silent Silent mode; write output only when errors occur
    -S, --sort-index Sort index blocks
    -t, --sort-records=# Sort records according to a particular index
    -u, --unpack Unpack file packed with myisampack
    -v, --verbose Print more information
    -w, --wait Wait if table is locked
    --character-sets-dir=dir_name Directory where character sets are installed
    --correct-checksum Correct checksum information for table
    --safe-recover Use safe recover; slower than regular recover but can handle some additional cases
    --set-auto-increment[=value] Set AUTO_INCREMENT counter
    --tmpdir=path Path for temporary files

    Examples

    How to Use These Examples

    The examples below show common ways to use the myisamchk command. Try them in your terminal to see the results. You can copy any example by clicking on the code block.

    # Basic Examples Basic
    myisamchk /var/lib/mysql/database/table.MYI
    Check a specific MyISAM table for errors.
    myisamchk --check /var/lib/mysql/database/table.MYI
    Check a MyISAM table for errors (same as without options).
    # Advanced Examples Advanced
    myisamchk -r /var/lib/mysql/database/table.MYI Repair a MyISAM table that has errors. myisamchk -o /var/lib/mysql/database/table.MYI Optimize a MyISAM table to improve performance. myisamchk -e -i /var/lib/mysql/database/table.MYI Extended check with information about table. myisamchk -r -q /var/lib/mysql/database/table.MYI Quiet repair mode, showing only errors. myisamchk --safe-recover /var/lib/mysql/database/table.MYI Use slower but more reliable recovery method. myisamchk --sort-index /var/lib/mysql/database/table.MYI Sort the index in ascending order for faster access. myisamchk --analyze /var/lib/mysql/database/table.MYI Analyze distribution of keys to optimize joins. myisamchk -r -f /var/lib/mysql/database/table.MYI Force repair even if some rows would be deleted.

    Try It Yourself

    Practice makes perfect! The best way to learn is by trying these examples on your own system with real files.

    Understanding Syntax

    Pay attention to the syntax coloring: commands, options, and file paths are highlighted differently.

    Notes

    The 'myisamchk' command is a crucial maintenance utility for MySQL databases that use the MyISAM storage engine. It provides functionality to check, repair, and optimize MyISAM tables, helping database administrators maintain data integrity and performance. Although MyISAM is no longer the default storage engine in newer MySQL versions (which use InnoDB), many legacy systems still rely on MyISAM tables, making myisamchk an important tool in the MySQL ecosystem. Key features of the myisamchk command: 1. Table Checking: myisamchk can verify the integrity of MyISAM tables by checking for errors in the data file structure, index consistency, and data validity. It offers different levels of checks, from quick basic checks to thorough extended verifications. 2. Table Repair: When data corruption is detected, myisamchk can repair damaged MyISAM tables, recovering data and restoring structural integrity. It offers multiple repair modes, including standard recovery and a more thorough safe-recovery mode for handling complex corruption cases. 3. Performance Optimization: Beyond fixing errors, myisamchk can optimize tables to improve query performance by reorganizing data storage, rebuilding indexes, and reclaiming unused space. 4. Index Management: The command provides specific operations for managing indexes, including sorting index blocks for faster access and analyzing key distributions to optimize join operations. 5. Offline Operation: Unlike the CHECK TABLE and REPAIR TABLE SQL statements, myisamchk works directly with the table files while the MySQL server is not accessing them, which can be useful for maintenance during server downtime. 6. Detailed Diagnostics: myisamchk can provide verbose information about table structure, key distribution, and other statistics that help in diagnosing and resolving database issues. 7. Flexible Recovery Options: For severely damaged tables, myisamchk offers various recovery strategies, including options to recover as much data as possible even if some rows must be deleted. Common use cases for myisamchk include: - Scheduled maintenance of MyISAM tables during low-traffic periods - Recovery after system crashes that might have left tables in an inconsistent state - Diagnosing performance issues related to MyISAM tables - Optimizing tables that have become fragmented after many updates and deletes - Recovering data from corrupted tables when backups are unavailable or outdated - Analyzing table statistics to inform database design decisions It's important to note that myisamchk should generally be used when the MySQL server is not running, or at least when the specific tables being checked are not in use. This is because direct file manipulation while the server is accessing the same files can lead to further data corruption. For maintenance during server operation, the SQL statements CHECK TABLE, REPAIR TABLE, and OPTIMIZE TABLE are safer alternatives, though they may not offer all the capabilities of myisamchk. With the shift toward InnoDB as the default storage engine in modern MySQL installations, myisamchk's importance has diminished somewhat. However, it remains an essential tool for maintaining MySQL installations that still use MyISAM tables for specific use cases or legacy applications.

    Related Commands

    These commands are frequently used alongside myisamchk or serve similar purposes:

    mysql
    mysqlcheck
    myisamlog
    myisampack
    isamchk
    CHECK TABLE
    REPAIR TABLE
    OPTIMIZE TABLE
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    Use Cases

    Learn By Doing

    The best way to learn Linux commands is by practicing. Try out these examples in your terminal to build muscle memory and understand how the myisamchk command works in different scenarios.

    See ExamplesTry Lab Exercises
    $ myisamchk
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