See Also commit, rollback, transaction, XA.  Data that is maintained throughout the life of a  transaction , recording all changes so that they can be undone in case of a  rollback  operation. It is stored in  undo logs  either within the  system tablespace  (in MySQL 5.7 or earlier) or in separate  undo tablespaces . As of MySQL 8.0, undo logs reside in undo tablespaces by default. See Also rollback, rollback segment, system tablespace, transaction, undo log, undo tablespace. See undo log.  A storage area that holds copies of data modified by active  transactions . If another transaction needs to see the original data (as part of a  consistent read  operation), the unmodified data is retrieved from this storage area. In MySQL 5.6 and MySQL 5.7, you can use the  innodb_undo_tablespaces  variable have undo logs reside in  undo tablespaces , which can be placed on another storage device such as an  SSD . In MySQL 8.0, undo logs reside in two default undo tablespaces that are created when MySQL is initialized, and additional undo tablespaces can be created using  CREATE UNDO TABLESPACE  syntax. The undo log is split into separate portions, the  insert undo buffer  and the  update undo buffer . See Also consistent read, rollback segment, SSD, system tablespace, transaction, undo tablespace.  A collection of  undo logs . Undo log segments exists within  rollback segments . An undo log segment might contain undo logs from multiple transactions. An undo log segment can only be used by one transaction at a time but can be reused after it is released at transaction  commit  or  rollback . May also be referred to as an “undo segment” . See Also commit, rollback, rollback segment, undo log.  An undo tablespace contains  undo logs . Undo logs exist within  undo log segments , which are contained within  rollback segments . Rollback segments have traditionally resided in the system tablespace. As of MySQL 5.6, rollback segments can reside in undo tablespaces. In MySQL 5.6 and MySQL 5.7, the number of undo tablespaces is controlled by the  innodb_undo_tablespaces  configuration option. In MySQL 8.0, two default undo tablespaces are created when the MySQL instance is initialized, and additional undo tablespaces can be created using  CREATE UNDO TABLESPACE  syntax. For more information, see Section 15.6.3.4, “Undo Tablespaces” . See Also rollback segment, system tablespace, undo log, undo log segment.  A system for supporting national characters, character sets, code pages, and other internationalization aspects in a flexible and standardized way. Unicode support is an important aspect of the  ODBC  standard.  Connector/ODBC 5.1 is a Unicode driver, as opposed to Connector/ODBC 3.51, which is an  ANSI  driver. See Also ANSI, Connector/ODBC, ODBC.  A kind of  constraint  that asserts that a column cannot contain any duplicate values. In terms of  relational algebra, it is used to specify 1-to-1 relationships. For efficiency in checking whether a value can be inserted (that unique constraint Unicode undo tablespace undo log segment undo U undo buffer undo log 6195