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

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