 A  temporary tablespace  that stores  rollback segments  for changes made to user-created temporary tables. See Also temporary tablespace.  A type of  transaction  involved in  XA  operations. It consists of several actions that are transactional in themselves, but that all must either complete successfully as a group, or all be rolled back as a group. In essence, this extends  ACID  properties “up a level”  so that multiple ACID transactions can be executed in concert as components of a global operation that also has ACID properties. See Also ACID, transaction, XA.  An  InnoDB  optimization that performs some low-level I/O operations (log write) once for a set of  commit operations, rather than flushing and syncing separately for each commit. See Also binary log, commit. H  Acronym for “globally unique identifier” , an ID value that can be used to associate data across different databases, languages, operating systems, and so on. (As an alternative to using sequential integers, where the same values could appear in different tables, databases, and so on referring to different data.) Older MySQL versions represented it as  BINARY(16) . Currently, it is represented as  CHAR(36) . MySQL has a  UUID() function that returns GUID values in character format, and a  UUID_SHORT()  function that returns GUID values in integer format. Because successive GUID values are not necessarily in ascending sort order, it is not an efficient value to use as a primary key for large InnoDB tables.  A type of  index  intended for queries that use equality operators, rather than range operators such as greater- than or  BETWEEN . It is available for  MEMORY  tables. Although hash indexes are the default for  MEMORY  tables for historic reasons, that storage engine also supports  B-tree  indexes, which are often a better choice for general- purpose queries. MySQL includes a variant of this index type, the  adaptive hash index , that is constructed automatically for InnoDB  tables if needed based on runtime conditions. See Also adaptive hash index, B-tree, index, InnoDB.  Acronym for “hard disk drive” . Refers to storage media using spinning platters, usually when comparing and contrasting with  SSD . Its performance characteristics can influence the throughput of a  disk-based  workload. See Also disk-based, SSD.  A periodic message that is sent to indicate that a system is functioning properly. In a  replication  context, if the source  stops sending such messages, one of the  replicas  can take its place. Similar techniques can be used between the servers in a cluster environment, to confirm that all of them are operating properly. See Also replication, source.  A value representing an upper limit, either a hard limit that should not be exceeded at runtime, or a record of the maximum value that was actually reached. Contrast with  low-water mark . See Also low-water mark.  A list of  transactions  with delete-marked records scheduled to be processed by the  InnoDB purge  operation. Recorded in the  undo log . The length of the history list is reported by the command  SHOW ENGINE INNODB 6155 global temporary tablespace global transaction group commit GUID hash index HDD heartbeat high-water mark history list