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Answer Posted / mahesh reddy g
lockmodes:
EXCLUSIVE
SHARE
ROW EXCLUSIVE
SHARE ROW EXCLUSIVE
ROW SHARE* | SHARE UPDATE*If NOWAIT is omitted Oracle
will wait until the table is available.
Several tables can be locked with a single command -
separate with commas
e.g. LOCK TABLE table1,table2,table3 IN ROW EXCLUSIVE MODE;
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 NOWAIT - will place a ROW
EXCLUSIVE 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
| Is This Answer Correct ? | 30 Yes | 14 No |
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