Spec-Zone .ru
спецификации, руководства, описания, API
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Spec-Zone .ru
спецификации, руководства, описания, API
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MyISAM is based on the older (and no longer available) ISAM storage engine but has many useful extensions.
Table 14.7. MyISAM Storage EngineFeatures
| Storage limits | 256TB | Transactions | No | Locking granularity | Table |
| MVCC | No | Geospatial data type support | Yes | Geospatial indexing support | Yes |
| B-tree indexes | Yes | T-tree indexes | No | Hash indexes | No |
| Full-text search indexes | Yes | Clustered indexes | No | Data caches | No |
| Index caches | Yes | Compressed data | Yes[a] | Encrypted data[b] | Yes |
| Cluster database support | No | Replication support[c] | Yes | Foreign key support | No |
| Backup / point-in-time recovery[d] | Yes | Query cache support | Yes | Update statistics for data dictionary | Yes |
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[a] Compressed MyISAM tables are supported only when using the compressed row format. Tables using the compressed row format with MyISAM are read only. [b] Implemented in the server (via encryption functions), rather than in the storage engine. [c] Implemented in the server, rather than in the storage engine. [d] Implemented in the server, rather than in the storage engine. |
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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:
CREATE TABLE t (i INT) ENGINE = MYISAM;
In MySQL 5.6, it is normally necessary to use ENGINE to specify the MyISAM storage engine because InnoDB is the default
engine.
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
4.5.3, "mysqlcheck — A Table Maintenance Program", Section
4.6.3, "myisamchk — MyISAM Table-Maintenance Utility", and
Section 4.6.5, "myisampack — Generate Compressed, Read-Only MyISAM Tables".
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 permit better index compression.
Large files (up to 63-bit file length) are supported on file systems and operating systems that support large files.
There is a limit of (232)2 (1.844E+19) rows in a MyISAM table.
The maximum number of indexes per MyISAM table is 64.
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 only
contains 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.
MyISAM supports concurrent inserts: 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. 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 8.10.3,
"Concurrent Inserts".
You can put the data file and index file in different directories on different
physical devices to get more speed with the DATA DIRECTORY and INDEX DIRECTORY table options to CREATE TABLE. See Section
13.1.17, "CREATE TABLE Syntax".
NULL values are permitted in indexed columns. This
takes 0 to 1 bytes per key.
Each character column can have a different character set. See Section 10.1, "Character Set Support".
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-options 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:
A forum dedicated to the MyISAM storage engine is
available at .