Oracle Database - Internal Locks

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Internal locks are higher-level, more complex mechanisms than latches and mutexes and serve various purposes.


The database uses the following types of internal locks:

Dictionary cache locks

These locks are of very short duration and are held on entries in dictionary caches while the entries are being modified or used. They guarantee that statements being parsed do not see inconsistent object definitions. Dictionary cache locks can be shared or exclusive. Shared locks are released when the parse is complete, whereas exclusive locks are released when the DDL operation is complete.

File and log management locks

These locks protect various files.

For example:

Because file and log locks indicate the status of files, these locks are necessarily held for a long time.

Tablespace and undo segment locks

These locks protect tablespaces and undo segments.

For example:

  • all instances accessing a database must agree on whether a tablespace is online or offline.
  • Undo segments are locked so that only one database instance can write to a segment.

Documentation / Reference

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Oracle Database - System Lock

The system lock protect internal database structures such as data files. Oracle Database uses various types of system locks to protect internal database and memory structures. These mechanisms are inaccessible...

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