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MyISAM is the default storage engine for the MySQL relational database management system. It is based on the older ISAM code but has many useful extensions. In recent MySQL versions, the InnoDB engine has widely started to replace MyISAM due to its support for transactions, referential integrity constraints, and higher concurrency.
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. MySQL uses a .frm file to store the definition of the table, but this file is not a part of the MyISAM engine, but instead is a part of the server. The data file has a .MYD (MYData) extension. The index file has a .MYI (MYIndex) extension.
[edit] Comparison with InnoDB
- InnoDB recovers from a crash or other unexpected shutdown by replaying its logs. MyISAM must fully scan and repair or rebuild any indexes or possibly tables which had been updated but not fully flushed to disk. Since the InnoDB approach is approximately fixed time while the MyISAM time grows with the size of the data files, InnoDB offers greater perceived availability and reliability as database sizes grow.
- MyISAM relies on the operating system for caching reads and writes to the data rows while InnoDB does this within the engine itself, combining the row caches with the index caches. Dirty (changed) database pages are not immediately sent to the operating system to be written by InnoDB, which can make it substantially faster than MyISAM in some situations.
- InnoDB stores data rows physically in primary key order while MyISAM typically stores them mostly in the order in which they are added. Storing the rows in primary key order corresponds to the MS SQL Server feature of “Clustered Indexes” and the Oracle feature known as "index organized tables." When the primary key is selected to match the needs of common queries this can give a substantial performance benefit. For example, customer bank records might be grouped by customer in InnoDB but by transaction date with MyISAM, so InnoDB would likely require fewer disk seeks and less RAM to retrieve and cache a customer account history. On the other hand, inserting data in orders that differ substantially from primary key (PK) order will presumably require that InnoDB do a lot of reordering of data in order to get it into PK order. This places InnoDB at a slight disadvantage in that it does not permit insertion order based table structuring.
- InnoDB currently does not provide the compression and terse row formats provided by MyISAM, so both the disk and cache RAM required may be larger. A lower overhead format is available for MySQL 5.0, reducing overhead by about 20% and use of page compression is planned for a future version.
- When operating in fully ACID-compliant modes, InnoDB must do a flush to disk at least once per transaction, though it will combine flushes for inserts from multiple connections. For typical hard drives or arrays, this will impose a limit of about 200 update transactions per second. If you require higher transaction rates, disk controllers with write caching and battery backup will be required in order to maintain transactional integrity. InnoDB also offers several modes which reduce this effect, naturally leading to a loss of transactional integrity. MyISAM has none of this overhead, but only because it does not support transactions.
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