## global temporary tablespace 

 A temporary tablespace  that stores rollback segments  for changes made to user-created temporary tables. See Also temporary tablespace. 

## global transaction 

 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. 

## group commit 

 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. 

## GUID 

 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. 

## H 

## hash index 

 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. 

## HDD 

 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. 

## heartbeat 

 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. 

## high-water mark 

 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. 

## history list 

 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 

