See Also connection, host, SSL. See index prefix.  A set of backup files, produced by the  MySQL Enterprise Backup  product, after all the stages of applying binary logs  and  incremental backups  are finished. The resulting files are ready to be  restored . Prior to the apply steps, the files are known as a  raw backup .  An SQL statement that is analyzed in advance to determine an efficient execution plan. It can be executed multiple times, without the overhead for parsing and analysis each time. Different values can be substituted for literals in the  WHERE  clause each time, through the use of placeholders. This substitution technique improves security, protecting against some kinds of SQL injection attacks. You can also reduce the overhead for converting and copying return values to program variables. Although you can use prepared statements directly through SQL syntax, the various  Connectors  have programming interfaces for manipulating prepared statements, and these APIs are more efficient than going through SQL.  A set of columns—and by implication, the index based on this set of columns—that can uniquely identify every row in a table. As such, it must be a unique index that does not contain any  NULL  values. InnoDB  requires that every table has such an index (also called the  clustered index  or  cluster index ), and organizes the table storage based on the column values of the primary key. When choosing primary key values, consider using arbitrary values (a  synthetic key ) rather than relying on values derived from some other source (a  natural key ).  An instance of an executing program. The operating system switches between multiple running processes, allowing for a certain degree of  concurrency . On most operating systems, processes can contain multiple threads  of execution that share resources. Context-switching between threads is faster than the equivalent switching between processes.  An artificial record in an index, used for  locking  key values or ranges that do not currently exist.  The POSIX threads standard, which defines an API for threading and locking operations on Unix and Linux systems. On Unix and Linux systems,  InnoDB  uses this implementation for  mutexes .  A type of garbage collection performed by one or more separate background threads (controlled by innodb_purge_threads ) that runs on a periodic schedule. Purge parses and processes  undo log  pages from the  history list  for the purpose of removing clustered and secondary index records that were marked for deletion (by previous  DELETE  statements) and are no longer required for  MVCC  or  rollback . Purge frees undo log pages from the history list after processing them. 6177 purge Pthreads pseudo-record process primary key prepared statement prepared backup prefix See Also binary log, hot backup, incremental backup, MySQL Enterprise Backup, raw backup, restore. See Also client-side prepared statement, connector, server-side prepared statement. See Also clustered index, index, natural key, synthetic key. See Also concurrency, thread. See Also infimum record, locking, supremum record. See Also mutex.