LOCK TABLE Statement

Manually lock one or more tables.

Syntax:

   LOCK TABLE [schema.] table [options] IN lockmode MODE [NOWAIT]

   LOCK TABLE [schema.] view [options] IN lockmode MODE [NOWAIT]

options:
      PARTITION partition
      SUBPARTITION subpartition
      @dblink

lockmodes:
      EXCLUSIVE
      SHARE
      ROW EXCLUSIVE
      SHARE ROW EXCLUSIVE
      ROW SHARE* | SHARE UPDATE*

If NOWAIT is omitted Oracle will wait until the table is available.

Default Locking Behaviour

A pure SELECT will not lock any rows.

INSERT, UPDATE or DELETE's will place a ROW EXCLUSIVE lock.

SELECT...FROM...FOR UPDATE Will place a ROW SHARE lock.

Multiple Locks on the same rows with LOCK TABLE

Even when a row is locked you can always perform a SELECT (because SELECT does not lock any rows) in addition to this, each type of lock will allow additional locks to be granted as follows.

ROW SHARE = Allow ROW EXCLUSIVE or ROW SHARE or SHARE locks to be granted to the locked rows.

ROW EXCLUSIVE = Allow ROW EXCLUSIVE or ROW SHARE locks to be granted to the locked rows.

SHARE ROW EXCLUSIVE = Allow ROW SHARE locks to be granted to the locked rows.

SHARE = Allow ROW SHARE or SHARE locks to be granted to the locked rows.

EXCLUSIVE = Allow SELECT queries only

Although it is valid to place more than one lock on a row, UPDATES and DELETE's may still cause a wait if a conflicting row lock is held by another transaction.

* = Oracle 6 option included for compatibility

Related Commands:

COMMIT
DELETE

INSERT
ROLLBACK
SELECT

UPDATE
DBMS_LOCK

Related Views:

  DBA_ALL_TABLES       ALL_ALL_TABLES       USER_ALL_TABLES
  DBA_TABLES           ALL_TABLES           USER_TABLES         TAB
                                                                DICTIONARY
                                                                DICT_COLUMNS
  V$LOCK
  V$_LOCK
  V$LOCKED_OBJECT
  V$LOCKS_WITH_COLLISIONS
  V$LOCK_ACTIVITY
  V$LOCK_ELEMENT 



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Simon Sheppard
SS64.com