that column. For the same reason, unique indexes do not prevent NULL  values; those values simply are not represented in the index. Declaring a NOT NULL  constraint on a column provides reassurance that there are no rows left out of the index, allowing for better query optimization (accurate counting of rows and estimation of whether to use the index). 

Because the primary key  must be able to uniquely identify every row in the table, a single-column primary key cannot contain any NULL  values, and a multi-column primary key cannot contain any rows with NULL  values in all columns. 

Although the Oracle database allows a NULL  value to be concatenated with a string, InnoDB  treats the result of such an operation as NULL . 

See Also index, primary key, SQL. 

## O 

.OPT file 

 A file containing database configuration information. Files with this extension are included in backups produced by the mysqlbackup  command of the MySQL Enterprise Backup  product. 

See Also MySQL Enterprise Backup, mysqlbackup command. 

ODBC 

 Acronym for Open Database Connectivity, an industry-standard API. Typically used with Windows-based servers, or applications that require ODBC to communicate with MySQL. The MySQL ODBC driver is called Connector/ODBC . 

See Also Connector/ODBC. 

off-page column 

 A column containing variable-length data (such as BLOB  and VARCHAR ) that is too long to fit on a B-tree  page. The data is stored in overflow pages . The DYNAMIC  row format is more efficient for such storage than the older COMPACT  row format. 

See Also B-tree, compact row format, dynamic row format, overflow page. 

OLTP 

 Acronym for “Online Transaction Processing” . A database system, or a database application, that runs a workload with many transactions , with frequent writes as well as reads, typically affecting small amounts of data at a time. For example, an airline reservation system or an application that processes bank deposits. The data might be organized in normalized  form for a balance between DML  (insert/update/delete) efficiency and query efficiency. Contrast with data warehouse . 

With its row-level locking  and transactional  capability, InnoDB  is the ideal storage engine for MySQL tables used in OLTP applications. 

See Also data warehouse, DML, InnoDB, query, row lock, transaction. 

online 

 A type of operation that involves no downtime, blocking, or restricted operation for the database. Typically applied to DDL . Operations that shorten the periods of restricted operation, such as fast index creation , have evolved into a wider set of online DDL  operations in MySQL 5.6. 

In the context of backups, a hot backup  is an online operation and a warm backup  is partially an online operation. 

See Also DDL, Fast Index Creation, hot backup, online DDL, warm backup. 

online DDL 

 A feature that improves the performance, concurrency, and availability of InnoDB  tables during DDL  (primarily ALTER TABLE ) operations. See Section 15.12, “InnoDB and Online DDL”  for details. 

