See Also history list, MVCC, rollback, undo log.  The technique of storing changes to secondary index pages, resulting from  DELETE  operations, in the  change buffer  rather than writing the changes immediately, so that the physical writes can be performed to minimize random I/O. (Because delete operations are a two-step process, this operation buffers the write that normally purges an index record that was previously marked for deletion.) It is one of the types of  change buffering ; the others are  insert buffering  and  delete buffering . See Also change buffer, change buffering, delete buffering, insert buffer, insert buffering.  Another name for the  InnoDB history list . Related to the  innodb_max_purge_lag  configuration option. See Also history list, purge.  A  thread  within the  InnoDB  process that is dedicated to performing the periodic  purge  operation. In MySQL 5.6 and higher, multiple purge threads are enabled by the  innodb_purge_threads  configuration option. See Also purge, thread.  A programming language used in a broad range of fields, from Unix scripting to large-scale applications. Includes runtime typing, built-in high-level data types, object-oriented features, and an extensive standard library. Often used as a “glue”  language between components written in other languages. The MySQL  Python API  is the open- source  MySQLdb  module. See Also MySQLdb, Python API. See Also API, Python.  In  SQL , an operation that reads information from one or more  tables . Depending on the organization of data and the parameters of the query, the lookup might be optimized by consulting an  index . If multiple tables are involved, the query is known as a  join . For historical reasons, sometimes discussions of internal processing for statements use “query”  in a broader sense, including other types of MySQL statements such as  DDL  and  DML  statements. See Also DDL, DML, index, join, SQL, table.  The set of decisions made by the optimizer about how to perform a  query  most efficiently, including which  index or indexes to use, and the order in which to  join  tables.  Plan stability  involves the same choices being made consistently for a given query. See Also index, join, plan stability, query. See general query log.  To reduce the amount of database activity, often in preparation for an operation such as an  ALTER TABLE , a backup , or a  shutdown . Might or might not involve doing as much  flushing  as possible, so that  InnoDB  does not continue doing background I/O. 6178 quiesce query log query execution plan query Q Python API Python purge thread purge lag purge buffering