See Also history list, MVCC, rollback, undo log. 

## purge buffering 

 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. 

## purge lag 

 Another name for the InnoDB history list . Related to the innodb_max_purge_lag  configuration option. See Also history list, purge. 

## purge thread 

 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. 

## Python 

 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. 

## Python API 

See Also API, Python. 

## Q 

## query 

 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. 

## query execution plan 

 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. 

## query log 

See general query log. 

## quiesce 

 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. 

