See Also crash recovery, data files, ib_logfile, log buffer, LSN, redo, shutdown, transaction. For more information, see Section 15.10, “InnoDB Row Formats” . See Also compact row format, dynamic row format, row format.  An important aspect of modern database systems. The database server encodes and enforces relationships such as one-to-one, one-to-many, many-to-one, and uniqueness. For example, a person might have zero, one, or many phone numbers in an address database; a single phone number might be associated with several family members. In a financial database, a person might be required to have exactly one taxpayer ID, and any taxpayer ID could only be associated with one person. The database server can use these relationships to prevent bad data from being inserted, and to find efficient ways to look up information. For example, if a value is declared to be unique, the server can stop searching as soon as the first match is found, and it can reject attempts to insert a second copy of the same value. At the database level, these relationships are expressed through SQL features such as  columns  within a table, unique and  NOT NULL constraints ,  foreign keys , and different kinds of join operations. Complex relationships typically involve data split between more than one table. Often, the data is  normalized , so that duplicate values in one-to-many relationships are stored only once. In a mathematical context, the relations within a database are derived from set theory. For example, the  OR  and AND  operators of a  WHERE  clause represent the notions of union and intersection. See Also ACID, column, constraint, foreign key, normalized.  In the  full-text search  feature, a number signifying the similarity between the search string and the data in the FULLTEXT index . For example, when you search for a single word, that word is typically more relevant for a row where it occurs several times in the text than a row where it appears only once.  An  InnoDB  feature that, when enabled, sequentially writes redo log records to an archive file to avoid potential loss of data than can occur when a backup utility fails to keep pace with redo log generation while a backup operation is in progress. For more information, see Redo Log Archiving. See Also redo log.  The oldest  InnoDB row format . Prior to MySQL 5.0.3, it was the only row format available in  InnoDB . From MySQL 5.0.3 to MySQL 5.7.8, the default row format is  COMPACT . As of MySQL 5.7.9, the default row format is defined by the  innodb_default_row_format  configuration option, which has a default setting of  DYNAMIC . You can still specify the  REDUNDANT  row format for compatibility with older  InnoDB  tables.  The technique of maintaining data always in a consistent format, part of the  ACID  philosophy. In particular, data in different tables is kept consistent through the use of  foreign key constraints , which can prevent changes from happening or automatically propagate those changes to all related tables. Related mechanisms include the  unique constraint , which prevents duplicate values from being inserted by mistake, and the  NOT NULL constraint , which prevents blank values from being inserted by mistake. See Also ACID, FOREIGN KEY constraint, NOT NULL constraint, unique constraint. See Also full-text search, FULLTEXT index.  The default  isolation level  for  InnoDB . It prevents any rows that are queried from being changed by other transactions , thus blocking  non-repeatable reads  but not  phantom  reads. It uses a moderately strict  locking strategy so that all queries within a transaction see data from the same snapshot, that is, the data as it was at the time the transaction started. REPEATABLE READ relevance relational referential integrity redundant row format redo log archiving 6181