Database Reference
In-Depth Information
The database is changed, modifying the current value of data in
tables.
Rollback entries are created, storing a list of rollback entries to be
applied in case a transaction is rolled back.
What happens when a COMMIT command is executed?
Rollback entries are deleted. Changes are already made. Nothing
else is required.
What happens when a ROLLBACK command is executed?
Rollback entries are written to the redo logs, namely “undo” redo
log entries.
Rollback is applied to the database undoing previously made
changes.
Rollback entries are deleted.
Note:
Redo log entries are always written before any database changes to
ensure recoverability.
What happens when data is changed? Different DML commands have
different types of redo log entries made for them. These different log entries
are logical when one thinks about what needs to be stored in order to re-
create a change, if need be in recovery.
INSERT will log the ROWID (pointer) plus the data content of the
entire row.
UPDATE only needs to log the ROWID and individual column
changes.
DELETE simply logs the ROWID.
It is not essential to know these facts, but the information is interesting
anyway in helping to understand how it all works.
Note:
Precise syntax for COMMIT, ROLLBACK, SET TRANSACTION,
and SAVEPOINT commands are covered in Chapter 15.
3.3.4
Temporary Sort Space
A temporary tablespace is an overflow for sorting onto disk when in-mem-
ory sort space limits for an executing sort have been exceeded. Sorting is
