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  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. 6172 See Also DDL, Fast Index Creation, hot backup, online DDL, warm backup. In the context of backups, a  hot backup  is an online operation and a  warm backup  is partially an online operation.  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. 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.  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 . See Also B-tree, compact row format, dynamic row format, overflow page.  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 Connector/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 MySQL Enterprise Backup, mysqlbackup command.  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. .OPT file ODBC off-page column OLTP online online DDL