## MySQL Glossary 

These terms are commonly used in information about the MySQL database server. This glossary originated as a reference for terminology about the InnoDB storage engine, and the majority of definitions are InnoDB-related. 

## A 

.ARM file 

 Metadata for ARCHIVE  tables. Contrast with .ARZ file . Files with this extension are always included in backups produced by the mysqlbackup  command of the MySQL Enterprise Backup  product. See Also .ARZ file, MySQL Enterprise Backup, mysqlbackup command. 

.ARZ file 

 Data for ARCHIVE tables. Contrast with .ARM file . Files with this extension are always included in backups produced by the mysqlbackup  command of the MySQL Enterprise Backup  product. See Also .ARM file, MySQL Enterprise Backup, mysqlbackup command. 

ACID 

 An acronym standing for atomicity, consistency, isolation, and durability. These properties are all desirable in a database system, and are all closely tied to the notion of a transaction . The transactional features of InnoDB adhere to the ACID principles. 

Transactions are atomic  units of work that can be committed  or rolled back . When a transaction makes multiple changes to the database, either all the changes succeed when the transaction is committed, or all the changes are undone when the transaction is rolled back. 

The database remains in a consistent state at all times — after each commit or rollback, and while transactions are in progress. If related data is being updated across multiple tables, queries see either all old values or all new values, not a mix of old and new values. 

Transactions are protected (isolated) from each other while they are in progress; they cannot interfere with each other or see each other’s uncommitted data. This isolation is achieved through the locking  mechanism. Experienced users can adjust the isolation level , trading off less protection in favor of increased performance and concurrency , when they can be sure that the transactions really do not interfere with each other. 

The results of transactions are durable: once a commit operation succeeds, the changes made by that transaction are safe from power failures, system crashes, race conditions, or other potential dangers that many non-database applications are vulnerable to. Durability typically involves writing to disk storage, with a certain amount of redundancy to protect against power failures or software crashes during write operations. (In InnoDB , the doublewrite buffer  assists with durability.) 

See Also atomic, commit, concurrency, doublewrite buffer, isolation level, locking, rollback, transaction. 

adaptive flushing 

 An algorithm for InnoDB  tables that smooths out the I/O overhead introduced by checkpoints . Instead of flushing  all modified pages  from the buffer pool  to the data files  at once, MySQL periodically flushes small sets of modified pages. The adaptive flushing algorithm extends this process by estimating the optimal rate to perform these periodic flushes, based on the rate of flushing and how fast redo  information is generated. See Also buffer pool, checkpoint, data files, flush, InnoDB, page, redo log. 

adaptive hash index 

 An optimization for InnoDB  tables that can speed up lookups using =  and IN  operators, by constructing a hash index  in memory. MySQL monitors index searches for InnoDB  tables, and if queries could benefit from a hash index, it builds one automatically for index pages  that are frequently accessed. In a sense, the adaptive hash index configures MySQL at runtime to take advantage of ample main memory, coming closer to the architecture 

