Index Create and Rebuild Locking Improvements in 11g (Ch Ch Ch Changes) February 11, 2008Posted by Richard Foote in Oracle General, Oracle Indexes, Index Rebuild, Locking Issues, 11g.
Although the CREATE INDEX … ONLINE and ALTER INDEX … REBUILD ONLINE options have been available for a long while, they can still introduce locking issues in highly active databases.
Oracle requires a table lock on the index base table at the start of the CREATE or REBUILD process (to guarantee DD information) and a lock at the end of the process (to merge index changes made during the rebuild into the final index structure).
These locks have two implications. Firstly, if there’s an active transaction on the base table of the index being created or rebuilt at the time one of these locks is required, the indexing process will hang. This will of course impact the time it takes to complete the indexing process. However the second far more serious issue is that any other active transactions on the base table starting after the indexing process hangs will likewise be locked and be prevented from continuing, until the indexing process obtains and releases its locks. In highly concurrent environments with many transactions, this can cause serious disruptions to the response times of these impacted transactions. Of course, depending on the time the initial locking transactions take to commit or rollback, this backlog of locked transactions can be quite significant.
Oracle11g has made some improvements in the locking implications regarding creating or rebuilding indexes online.
During the creation or rebuilding of an index online, Oracle still requires two associated table locks on the base table at the start and end of indexing process. If there’s an active transaction on the base table at the time one of these locks is required, the indexing process will still hang as its done previously until all these prior active transactions have completed. No change so far.
However, if the indexing process has been locked out and subsequent transactions relating to the base table start afterwards, these transactions will no longer in turn be locked out by the indexing table locks and are able to complete successfully. The indexing process no longer impacts other concurrent transactions on the base table, it will be the only process potentially left hanging while waiting to acquire its associated lock resource.
This means it may not be quite so “risky” to urgently introduce that new index or rebuild that troublesome index during core business hours due to the reduced locking implications introduced in 11g.
See this demo for Index Rebuild Locking Issues in 10g and the 11g Improvements.
Does this means we can now simply rebuild all our indexes, whenever ? Ummmm, no ;)