Temporary list
The temporary list is a temporary object that allows the optimizer to store intermediate results of a query. The list is an unsorted data structure that is used to simplify the operation of the query. Since the list does not have any keys, the rows within the list can only be retrieved by a sequential scan operation.
The temporary list can be used for a variety of reasons, some of which include an overly complex view or derived table, Symmetric Multiprocessing (SMP) or simply to prevent a portion of the query from being processed multiple times.
A temporary list is an internal data structure and can only be created by the database manager.
Visual explain icon:
- List scan
The list scan operation is used when a portion of the query will be processed multiple times, but no key columns can be identified. In these cases, that portion of the query is processed once and its results are stored within the temporary list. The list can then be scanned for only those rows that satisfy any selection or processing contained within the temporary object.