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8.1. The MyISAM Storage Engine

MyISAM is the default storage engine. It is based on the older ISAM code but has many useful extensions. (Note that MySQL 5.0 does not support ISAM.)

Each MyISAM table is stored on disk in three files. The files have names that begin with the table name and have an extension to indicate the file type. An .frm file stores the table format. The data file has an .MYD (MYData) extension. The index file has an .MYI (MYIndex) extension.

To specify explicitly that you want a MyISAM table, indicate that with an ENGINE table option:


The older term TYPE is supported as a synonym for ENGINE for backward compatibility, but ENGINE is the preferred term and TYPE is deprecated.

Normally, it is unnecessary to use ENGINE to specify the MyISAM storage engine. MyISAM is the default engine unless the default has been changed. To ensure that MyISAM is used in situations where the default might have been changed, include the ENGINE option explicitly.

You can check or repair MyISAM tables with the mysqlcheck client or myisamchk utility. You can also compress MyISAM tables with myisampack to take up much less space. See Section, "Using myisamchk for Crash Recovery," Section 7.4, "myisampackGenerate Compressed, Read-Only MyISAM Tables," and Section 7.9, "mysqlcheckA Table Maintenance and Repair Program."

MyISAM tables have the following characteristics:

  • All data values are stored with the low byte first. This makes the data machine and operating system independent. The only requirements for binary portability are that the machine uses two's-complement signed integers and IEEE floating-point format. These requirements are widely used among mainstream machines. Binary compatibility might not be applicable to embedded systems, which sometimes have peculiar processors.

    There is no significant speed penalty for storing data low byte first; the bytes in a table row normally are unaligned and it takes little more processing to read an unaligned byte in order than in reverse order. Also, the code in the server that fetches column values is not time critical compared to other code.

  • All numeric key values are stored with the high byte first to allow better index compression.

  • Large files (up to 63-bit file length) are supported on filesystems and operating systems that support large files.

  • Dynamic-sized rows are much less fragmented when mixing deletes with updates and inserts. This is done by automatically combining adjacent deleted blocks and by extending blocks if the next block is deleted.

  • The maximum number of indexes per MyISAM table is 64. This can be changed by recompiling. Beginning with MySQL 5.0.18, you can configure the build by invoking configure with the --with-max-indexes=N option, where N is the maximum number of indexes to permit per MyISAM table. N must be less than or equal to 128. Before MySQL 5.0.18, you must change the source.

    The maximum number of columns per index is 16.

  • The maximum key length is 1000 bytes. This can also be changed by changing the source and recompiling. For the case of a key longer than 250 bytes, a larger key block size than the default of 1024 bytes is used.

  • When rows are inserted in sorted order (as when you are using an AUTO_INCREMENT column), the index tree is split so that the high node contains only one key. This improves space utilization in the index tree.

  • Internal handling of one AUTO_INCREMENT column per table is supported. MyISAM automatically updates this column for INSERT and UPDATE operations. This makes AUTO_INCREMENT columns faster (at least 10%). Values at the top of the sequence are not reused after being deleted. (When an AUTO_INCREMENT column is defined as the last column of a multiple-column index, reuse of values deleted from the top of a sequence does occur.) The AUTO_INCREMENT value can be reset with ALTER TABLE or myisamchk.

  • Dynamic-sized rows are much less fragmented when mixing deletes with updates and inserts. This is done by automatically combining adjacent deleted blocks and by extending blocks if the next block is deleted.

  • If a table has no free blocks in the middle of the data file, you can INSERT new rows into it at the same time that other threads are reading from the table. (These are known as "concurrent inserts.") A free block can occur as a result of deleting rows or an update of a dynamic length row with more data than its current contents. When all free blocks are used up (filled in), future inserts become concurrent again. See Section 6.3.3, "Concurrent Inserts."

  • You can put the data file and index file on different directories to get more speed with the DATA DIRECTORY and INDEX DIRECTORY table options to CREATE TABLE.

  • BLOB and TEXT columns can be indexed.

  • NULL values are allowed in indexed columns. This takes 01 bytes per key.

  • Each character column can have a different character set.

  • There is a flag in the MyISAM index file that indicates whether the table was closed correctly. If mysqld is started with the --myisam-recover option, MyISAM tables are automatically checked when opened, and are repaired if the table wasn't closed properly.

  • myisamchk marks tables as checked if you run it with the --update-state option.

    myisamchk --fast checks only those tables that don't have this mark.

  • myisamchk --analyze stores statistics for portions of keys, as well as for entire keys.

  • myisampack can pack BLOB and VARCHAR columns.

MyISAM also supports the following features:

  • Support for a true VARCHAR type; a VARCHAR column starts with a length stored in one or two bytes.

  • Tables with VARCHAR columns may have fixed or dynamic row length.

  • The sum of the lengths of the VARCHAR and CHAR columns in a table may be up to 64KB.

  • A hashed computed index can be used for UNIQUE. This allows you to have UNIQUE on any combination of columns in a table. (However, you cannot search on a UNIQUE computed index.)

Additional resources

8.1.1. MyISAM Startup Options

The following options to mysqld can be used to change the behavior of MyISAM tables. For additional information, see Section 4.2.1, "mysqld Command Options."

  • --myisam-recover=mode

    Set the mode for automatic recovery of crashed MyISAM tables.

  • --delay-key-write=ALL

    Don't flush key buffers between writes for any MyISAM table.

    Note: If you do this, you should not access MyISAM tables from another program (such as from another MySQL server or with myisamchk) when the tables are in use. Doing so risks index corruption. Using --external-locking does not eliminate this risk.

The following system variables affect the behavior of MyISAM tables. For additional information, see Section 4.2.2, "Server System Variables."

  • bulk_insert_buffer_size

    The size of the tree cache used in bulk insert optimization. Note: This is a limit per thread!

  • myisam_max_extra_sort_file_size

    Used to help MySQL to decide when to use the slow but safe key cache index creation method. Note: This parameter was given in bytes before MySQL 5.0.6, when it was removed.

  • myisam_max_sort_file_size

    Don't use the fast sort index method to create an index if the temporary file would become larger than this. Note: This parameter is given in bytes.

  • myisam_sort_buffer_size

    Set the size of the buffer used when recovering tables.

Automatic recovery is activated if you start mysqld with the --myisam-recover option. In this case, when the server opens a MyISAM table, it checks whether the table is marked as crashed or whether the open count variable for the table is not 0 and you are running the server with external locking disabled. If either of these conditions is true, the following happens:

  • The server checks the table for errors.

  • If the server finds an error, it tries to do a fast table repair (with sorting and without recreating the data file).

  • If the repair fails because of an error in the data file (for example, a duplicate-key error), the server tries again, this time re-creating the data file.

  • If the repair still fails, the server tries once more with the old repair option method (write row by row without sorting). This method should be able to repair any type of error and has low disk space requirements.

If the recovery wouldn't be able to recover all rows from previously completed statements and you didn't specify FORCE in the value of the --myisam-recover option, automatic repair aborts with an error message in the error log:

Error: Couldn't repair table: test.g00pages

If you specify FORCE, a warning like this is written instead:

Warning: Found 344 of 354 rows when repairing ./test/g00pages

Note that if the automatic recovery value includes BACKUP, the recovery process creates files with names of the form tbl_name-datetime.BAK. You should have a cron script that automatically moves these files from the database directories to backup media.

8.1.2. Space Needed for Keys

MyISAM tables use B-tree indexes. You can roughly calculate the size for the index file as (key_length+4)/0.67, summed over all keys. This is for the worst case when all keys are inserted in sorted order and the table doesn't have any compressed keys.

String indexes are space compressed. If the first index part is a string, it is also prefix compressed. Space compression makes the index file smaller than the worst-case figure if a string column has a lot of trailing space or is a VARCHAR column that is not always used to the full length. Prefix compression is used on keys that start with a string. Prefix compression helps if there are many strings with an identical prefix.

In MyISAM tables, you can also prefix compress numbers by specifying the PACK_KEYS=1 table option when you create the table. Numbers are stored with the high byte first, so this helps when you have many integer keys that have an identical prefix.

8.1.3. MyISAM Table Storage Formats

MyISAM supports three different storage formats. Two of them, fixed and dynamic format, are chosen automatically depending on the type of columns you are using. The third, compressed format, can be created only with the myisampack utility.

When you use CREATE TABLE or ALTER TABLE for a table that has no BLOB or TEXT columns, you can force the table format to FIXED or DYNAMIC with the ROW_FORMAT table option. This causes CHAR and VARCHAR columns to become CHAR for FIXED format, or VARCHAR for DYNAMIC format.

You can decompress tables by specifying ROW_FORMAT=DEFAULT with ALTER TABLE. Static (Fixed-Length) Table Characteristics

Static format is the default for MyISAM tables. It is used when the table contains no variable-length columns (VARCHAR, VARBINARY, BLOB, or TEXT). Each row is stored using a fixed number of bytes.

Of the three MyISAM storage formats, static format is the simplest and most secure (least subject to corruption). It is also the fastest of the on-disk formats due to the ease with which rows in the data file can be found on disk: To look up a row based on a row number in the index, multiply the row number by the row length to calculate the row position. Also, when scanning a table, it is very easy to read a constant number of rows with each disk read operation.

The security is evidenced if your computer crashes while the MySQL server is writing to a fixed-format MyISAM file. In this case, myisamchk can easily determine where each row starts and ends, so it can usually reclaim all rows except the partially written one. Note that MyISAM table indexes can always be reconstructed based on the data rows.

Static-format tables have these characteristics:

  • CHAR columns are space-padded to the column width. This is also true for NUMERIC and DECIMAL columns created before MySQL 5.0.3. BINARY columns are space-padded to the column width before MySQL 5.0.15. As of 5.0.15, BINARY columns are padded with 0x00 bytes.

  • Very quick.

  • Easy to cache.

  • Easy to reconstruct after a crash, because rows are located in fixed positions.

  • Reorganization is unnecessary unless you delete a huge number of rows and want to return free disk space to the operating system. To do this, use OPTIMIZE TABLE or myisamchk-r.

  • Usually require more disk space than dynamic-format tables. Dynamic Table Characteristics

Dynamic storage format is used if a MyISAM table contains any variable-length columns (VARCHAR, VARBINARY, BLOB, or TEXT), or if the table was created with the ROW_FORMAT=DYNAMIC table option.

Dynamic format is a little more complex than static format because each row has a header that indicates how long it is. A row can become fragmented (stored in non-contiguous pieces) when it is made longer as a result of an update.

You can use OPTIMIZE TABLE or myisamchk -r to defragment a table. If you have fixed-length columns that you access or change frequently in a table that also contains some variable-length columns, it might be a good idea to move the variable-length columns to other tables just to avoid fragmentation.

Dynamic-format tables have these characteristics:

  • All string columns are dynamic except those with a length less than four.

  • Each row is preceded by a bitmap that indicates which columns contain the empty string (for string columns) or zero (for numeric columns). Note that this does not include columns that contain NULL values. If a string column has a length of zero after trailing space removal, or a numeric column has a value of zero, it is marked in the bitmap and not saved to disk. Non-empty strings are saved as a length byte plus the string contents.

  • Much less disk space usually is required than for fixed-length tables.

  • Each row uses only as much space as is required. However, if a row becomes larger, it is split into as many pieces as are required, resulting in row fragmentation. For example, if you update a row with information that extends the row length, the row becomes fragmented. In this case, you may have to run OPTIMIZE TABLE or myisamchk -r from time to time to improve performance. Use myisamchk -ei to obtain table statistics.

  • More difficult than static-format tables to reconstruct after a crash, because rows may be fragmented into many pieces and links (fragments) may be missing.

  • The expected row length for dynamic-sized rows is calculated using the following expression:

    + (number of columns  + 7) / 8
    + (number of char columns)
    + (packed size of numeric columns)
    + (length of strings)
    + (number of NULL columns + 7) / 8

    There is a penalty of 6 bytes for each link. A dynamic row is linked whenever an update causes an enlargement of the row. Each new link is at least 20 bytes, so the next enlargement probably goes in the same link. If not, another link is created. You can find the number of links using myisamchk -ed. All links may be removed with OPTIMIZE TABLE or myisamchk -r. Compressed Table Characteristics

Compressed storage format is a read-only format that is generated with the myisampack tool. Compressed tables can be uncompressed with myisamchk.

Compressed tables have the following characteristics:

  • Compressed tables take very little disk space. This minimizes disk usage, which is helpful when using slow disks (such as CD-ROMs).

  • Each row is compressed separately, so there is very little access overhead. The header for a row takes up one to three bytes depending on the biggest row 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 data type is converted to ENUM.

    • A column may use any combination of the preceding compression types.

  • Can be used for fixed-length or dynamic-length rows.

8.1.4. MyISAM Table Problems

The file format that MySQL uses to store data has been extensively tested, but there are always circumstances that may cause database tables to become corrupted. The following discussion describes how this can happen and how to handle it. Corrupted MyISAM Tables

Even though the MyISAM table format is very reliable (all changes to a table made by an SQL statement are written before the statement returns), you can still get corrupted tables if any of the following events occur:

  • The mysqld process is killed in the middle of a write.

  • An unexpected computer shutdown occurs (for example, the computer is turned off).

  • Hardware failures.

  • You are using an external program (such as myisamchk) to modify a table that is being modified by the server at the same time.

  • A software bug in the MySQL or MyISAM code.

Typical symptoms of a corrupt table are:

  • You get the following error while selecting data from the table:

    Incorrect key file for table: '...'. Try to repair it
  • Queries don't find rows in the table or return incomplete results.

You can check the health of a MyISAM table using the CHECK TABLE statement, and repair a corrupted MyISAM table with REPAIR TABLE. When mysqld is not running, you can also check or repair a table with the myisamchk command.

If your tables become corrupted frequently, you should try to determine why this is happening. The most important thing to know is whether the table became corrupted as a result of a server crash. You can verify this easily by looking for a recent restarted mysqld message in the error log. If there is such a message, it is likely that table corruption is a result of the server dying. Otherwise, corruption may have occurred during normal operation. This is a bug. You should try to create a reproducible test case that demonstrates the problem. Problems from Tables Not Being Closed Properly

Each MyISAM index file (.MYI file) has a counter in the header that can be used to check whether a table has been closed properly. If you get the following warning from CHECK TABLE or myisamchk, it means that this counter has gone out of sync:

clients are using or haven't closed the table properly

This warning doesn't necessarily mean that the table is corrupted, but you should at least check the table.

The counter works as follows:

  • The first time a table is updated in MySQL, a counter in the header of the index files is incremented.

  • The counter is not changed during further updates.

  • When the last instance of a table is closed (because a FLUSH TABLES operation was performed or because there is no room in the table cache), the counter is decremented if the table has been updated at any point.

  • When you repair the table or check the table and it is found to be okay, the counter is reset to zero.

  • To avoid problems with interaction with other processes that might check the table, the counter is not decremented on close if it was zero.

In other words, the counter can become incorrect only under these conditions:

  • A MyISAM table is copied without first issuing LOCK TABLES and FLUSH TABLES.

  • MySQL has crashed between an update and the final close. (Note that the table may still be okay, because MySQL always issues writes for everything between each statement.)

  • A table was modified by myisamchk --recover or myisamchk --update-state at the same time that it was in use by mysqld.

  • Multiple mysqld servers are using the table and one server performed a REPAIR TABLE or CHECK TABLE on the table while it was in use by another server. In this setup, it is safe to use CHECK TABLE, although you might get the warning from other servers. However, REPAIR TABLE should be avoided because when one server replaces the data file with a new one, this is not known to the other servers.

  • In general, it is a bad idea to share a data directory among multiple servers. See Section 4.13, "Running Multiple MySQL Servers on the Same Machine," for additional discussion.

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