is, the value does not already exist in the column), a unique constraint is supported by an underlying  unique index . See Also constraint, relational, unique index. unique index  An index on a column or set of columns that have a  unique constraint . Because the index is known not to contain any duplicate values, certain kinds of lookups and count operations are more efficient than in the normal kind of index. Most of the lookups against this type of index are simply to determine if a certain value exists or not. The number of values in the index is the same as the number of rows in the table, or at least the number of rows with non-null values for the associated columns. Change buffering  optimization does not apply to unique indexes. As a workaround, you can temporarily set unique_checks=0  while doing a bulk data load into an  InnoDB  table. See Also cardinality, change buffering, unique constraint, unique key. unique key  The set of columns (one or more) comprising a  unique index . When you can define a  WHERE  condition that matches exactly one row, and the query can use an associated unique index, the lookup and error handling can be performed very efficiently. See Also cardinality, unique constraint, unique index. variable-length type  A data type of variable length.  VARCHAR ,  VARBINARY , and  BLOB  and  TEXT  types are variable-length types. InnoDB  treats fixed-length fields greater than or equal to 768 bytes in length as variable-length fields, which can be stored  off-page . For example, a  CHAR(255)  column can exceed 768 bytes if the maximum byte length of the character set is greater than 3, as it is with  utf8mb4 . See Also off-page column, overflow page. victim  The  transaction  that is automatically chosen to be  rolled back  when a  deadlock  is detected.  InnoDB  rolls back the transaction that has updated the fewest rows. Deadlock detection  can be disabled using the  innodb_deadlock_detect  configuration option. See Also deadlock, deadlock detection, innodb_lock_wait_timeout, transaction.  A stored query that when invoked produces a result set. A view acts as a virtual table. view virtual column See virtual generated column. virtual generated column  A column whose values are computed from an expression included in the column definition. Column values are not stored, but are evaluated when rows are read, immediately after any  BEFORE  triggers. A virtual generated column takes no storage.  InnoDB  supports secondary indexes on virtual generated columns. Contrast with  stored generated column . See Also base column, generated column, stored generated column. virtual index  A virtual index is a  secondary index  on one or more   virtual generated columns  or on a combination of virtual generated columns and regular columns or stored generated columns. For more information, see Section 13.1.20.9, “Secondary Indexes and Generated Columns” . 6196 V