LOCK — lock a table
LOCK [ TABLE ] [ ONLY ]name
[ * ] [, ...] [ INlockmode
MODE ] [ NOWAIT ] wherelockmode
is one of: ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE | SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE
LOCK TABLE
obtains a table-level lock, waiting
if necessary for any conflicting locks to be released. If
NOWAIT
is specified, LOCK
TABLE
does not wait to acquire the desired lock: if it
cannot be acquired immediately, the command is aborted and an
error is emitted. Once obtained, the lock is held for the
remainder of the current transaction. (There is no UNLOCK
TABLE
command; locks are always released at transaction
end.)
When a view is locked, all relations appearing in the view definition query are also locked recursively with the same lock mode.
When acquiring locks automatically for commands that reference
tables, LightDB always uses the least
restrictive lock mode possible. LOCK TABLE
provides for cases when you might need more restrictive locking.
For example, suppose an application runs a transaction at the
READ COMMITTED
isolation level and needs to ensure that
data in a table remains stable for the duration of the transaction.
To achieve this you could obtain SHARE
lock mode over the
table before querying. This will prevent concurrent data changes
and ensure subsequent reads of the table see a stable view of
committed data, because SHARE
lock mode conflicts with
the ROW EXCLUSIVE
lock acquired by writers, and your
LOCK TABLE
statement will wait until any concurrent holders of name
IN SHARE MODEROW
EXCLUSIVE
mode locks commit or roll back. Thus, once you
obtain the lock, there are no uncommitted writes outstanding;
furthermore none can begin until you release the lock.
To achieve a similar effect when running a transaction at the
REPEATABLE READ
or SERIALIZABLE
isolation level, you have to execute the LOCK TABLE
statement
before executing any SELECT
or data modification statement.
A REPEATABLE READ
or SERIALIZABLE
transaction's
view of data will be frozen when its first
SELECT
or data modification statement begins. A LOCK
TABLE
later in the transaction will still prevent concurrent writes
— but it won't ensure that what the transaction reads corresponds to
the latest committed values.
If a transaction of this sort is going to change the data in the
table, then it should use SHARE ROW EXCLUSIVE
lock mode
instead of SHARE
mode. This ensures that only one
transaction of this type runs at a time. Without this, a deadlock
is possible: two transactions might both acquire SHARE
mode, and then be unable to also acquire ROW EXCLUSIVE
mode to actually perform their updates. (Note that a transaction's
own locks never conflict, so a transaction can acquire ROW
EXCLUSIVE
mode when it holds SHARE
mode — but not
if anyone else holds SHARE
mode.) To avoid deadlocks,
make sure all transactions acquire locks on the same objects in the
same order, and if multiple lock modes are involved for a single
object, then transactions should always acquire the most
restrictive mode first.
More information about the lock modes and locking strategies can be found in Section 13.3.
name
The name (optionally schema-qualified) of an existing table to
lock. If ONLY
is specified before the table name, only that
table is locked. If ONLY
is not specified, the table and all
its descendant tables (if any) are locked. Optionally, *
can be specified after the table name to explicitly indicate that
descendant tables are included.
The command LOCK TABLE a, b;
is equivalent to
LOCK TABLE a; LOCK TABLE b;
. The tables are locked
one-by-one in the order specified in the LOCK
TABLE
command.
lockmode
The lock mode specifies which locks this lock conflicts with. Lock modes are described in Section 13.3.
If no lock mode is specified, then ACCESS
EXCLUSIVE
, the most restrictive mode, is used.
NOWAIT
Specifies that LOCK TABLE
should not wait for
any conflicting locks to be released: if the specified lock(s)
cannot be acquired immediately without waiting, the transaction
is aborted.
LOCK TABLE ... IN ACCESS SHARE MODE
requires SELECT
privileges on the target table. LOCK TABLE ... IN ROW EXCLUSIVE
MODE
requires INSERT
, UPDATE
, DELETE
,
or TRUNCATE
privileges on the target table. All other forms of
LOCK
require table-level UPDATE
, DELETE
,
or TRUNCATE
privileges.
The user performing the lock on the view must have the corresponding privilege on the view. In addition the view's owner must have the relevant privileges on the underlying base relations, but the user performing the lock does not need any permissions on the underlying base relations.
LOCK TABLE
is useless outside a transaction block: the lock
would remain held only to the completion of the statement. Therefore
LightDB reports an error if LOCK
is used outside a transaction block.
Use
BEGIN and
COMMIT
(or ROLLBACK)
to define a transaction block.
LOCK TABLE
only deals with table-level locks, and so
the mode names involving ROW
are all misnomers. These
mode names should generally be read as indicating the intention of
the user to acquire row-level locks within the locked table. Also,
ROW EXCLUSIVE
mode is a shareable table lock. Keep in
mind that all the lock modes have identical semantics so far as
LOCK TABLE
is concerned, differing only in the rules
about which modes conflict with which. For information on how to
acquire an actual row-level lock, see Section 13.3.2
and The Locking Clause
in the SELECT documentation.
Obtain a SHARE
lock on a primary key table when going to perform
inserts into a foreign key table:
BEGIN WORK; LOCK TABLE films IN SHARE MODE; SELECT id FROM films WHERE name = 'Star Wars: Episode I - The Phantom Menace'; -- Do ROLLBACK if record was not returned INSERT INTO films_user_comments VALUES (_id_, 'GREAT! I was waiting for it for so long!'); COMMIT WORK;
Take a SHARE ROW EXCLUSIVE
lock on a primary key table when going to perform
a delete operation:
BEGIN WORK; LOCK TABLE films IN SHARE ROW EXCLUSIVE MODE; DELETE FROM films_user_comments WHERE id IN (SELECT id FROM films WHERE rating < 5); DELETE FROM films WHERE rating < 5; COMMIT WORK;
There is no LOCK TABLE
in the SQL standard,
which instead uses SET TRANSACTION
to specify
concurrency levels on transactions. LightDB supports that too;
see SET TRANSACTION for details.
Except for ACCESS SHARE
, ACCESS EXCLUSIVE
,
and SHARE UPDATE EXCLUSIVE
lock modes, the
LightDB lock modes and the
LOCK TABLE
syntax are compatible with those
present in Oracle.