LOCK(l) SQL - Language Statements (2002-11-22) LOCK(l)
NAME
LOCK - explicitly lock a table
SYNOPSIS
LOCK [ TABLE ] name [, ...]
LOCK [ TABLE ] name [, ...] IN lockmode MODE
where lockmode is one of:
ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE |
SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE
INPUTS
name The name (optionally schema-qualified) of an existing
table to lock.
ACCESS SHARE MODE
This is the least restrictive lock mode. It conflicts
only with ACCESS EXCLUSIVE mode. It is used to protect
a table from being modified by concurrent ALTER TABLE,
DROP TABLE and VACUUM FULL commands.
Note: The SELECT command acquires a lock of this mode
on referenced tables. In general, any query that only
reads a table and does not modify it will acquire this
lock mode.
ROW SHARE MODE
Conflicts with EXCLUSIVE and ACCESS EXCLUSIVE lock
modes.
Note: The SELECT FOR UPDATE command acquires a lock of
this mode on the target table(s) (in addition to ACCESS
SHARE locks on any other tables that are referenced but
not selected FOR UPDATE).
ROW EXCLUSIVE MODE
Conflicts with SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE
and ACCESS EXCLUSIVE modes.
Note: The commands UPDATE, DELETE, and INSERT acquire
this lock mode on the target table (in addition to
ACCESS SHARE locks on any other referenced tables). In
general, this lock mode will be acquired by any query
that modifies the data in a table.
SHARE UPDATE EXCLUSIVE MODE
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Conflicts with SHARE UPDATE EXCLUSIVE, SHARE, SHARE ROW
EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE modes. This
mode protects a table against concurrent schema changes
and VACUUM runs.
Note: Acquired by VACUUM (without FULL).
SHARE MODE
Conflicts with ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE,
SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE
modes. This mode protects a table against concurrent
data changes.
Note: Acquired by CREATE INDEX.
SHARE ROW EXCLUSIVE MODE
Conflicts with ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE,
SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS
EXCLUSIVE modes.
Note: This lock mode is not automatically acquired by
any PostgreSQL command.
EXCLUSIVE MODE
Conflicts with ROW SHARE, ROW EXCLUSIVE, SHARE UPDATE
EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and
ACCESS EXCLUSIVE modes. This mode allows only
concurrent ACCESS SHARE, i.e., only reads from the
table can proceed in parallel with a transaction
holding this lock mode.
Note: This lock mode is not automatically acquired by
any PostgreSQL command.
ACCESS EXCLUSIVE MODE
Conflicts with all lock modes. This mode guarantees
that the holder is the only transaction accessing the
table in any way.
Note: Acquired by ALTER TABLE, DROP TABLE, and VACUUM
FULL statements. This is also the default lock mode
for LOCK TABLE statements that do not specify a mode
explicitly.
OUTPUTS
LOCK TABLE
The lock was successfully acquired.
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ERROR name: Table does not exist.
Message returned if name does not exist.
DESCRIPTION
LOCK TABLE obtains a table-level lock, waiting if necessary
for any conflicting locks to be released. 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 acquiring locks automatically for commands that
reference tables, PostgreSQL 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
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 name IN SHARE MODE statement
will wait until any concurrent holders of ROW EXCLUSIVE mode
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.
Note: To achieve a similar effect when running a
transaction at the SERIALIZABLE isolation level, you
have to execute the LOCK TABLE statement before
executing any DML statement. A serializable
transaction's view of data will be frozen when its
first DML statement begins. A later LOCK 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.)
Two general rules may be followed to prevent deadlock
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conditions:
o Transactions have to acquire locks on the same objects in
the same order.
For example, if one application updates row R1 and than
updates row R2 (in the same transaction) then the second
application shouldn't update row R2 if it's going to
update row R1 later (in a single transaction). Instead, it
should update rows R1 and R2 in the same order as the
first application.
o If multiple lock modes are involved for a single object,
then transactions should always acquire the most
restrictive mode first.
An example for this rule was given previously when
discussing the use of SHARE ROW EXCLUSIVE mode rather than
SHARE mode.
PostgreSQL does detect deadlocks and will rollback at least
one waiting transaction to resolve the deadlock. If it is
not practical to code an application to follow the above
rules strictly, an alternative solution is to be prepared to
retry transactions when they are aborted by deadlocks.
When locking multiple tables, the command LOCK a, b; is
equivalent to LOCK a; LOCK b;. The tables are locked one-
by-one in the order specified in the LOCK command.
NOTES
LOCK ... IN ACCESS SHARE MODE requires SELECT privileges on
the target table. All other forms of LOCK require UPDATE
and/or DELETE privileges.
LOCK is useful only inside a transaction block
(BEGIN...COMMIT), since the lock is dropped as soon as the
transaction ends. A LOCK command appearing outside any
transaction block forms a self-contained transaction, so the
lock will be dropped as soon as it is obtained.
RDBMS locking uses the following standard terminology:
EXCLUSIVE
An exclusive lock prevents other locks of the same type
from being granted.
SHARE
A shared lock allows others to also hold the same type
of lock, but prevents the corresponding EXCLUSIVE lock
from being granted.
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LOCK(l) SQL - Language Statements (2002-11-22) LOCK(l)
ACCESS
Locks table schema.
ROW Locks individual rows.
PostgreSQL does not follow this terminology exactly. 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 does not follow this naming
convention accurately, since it is a sharable 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.
USAGE
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;
COMPATIBILITY
SQL92
There is no LOCK TABLE in SQL92, which instead uses SET
TRANSACTION to specify concurrency levels on transactions.
We support that too; see SET TRANSACTION
[set_transaction(l)] for details.
Except for ACCESS SHARE, ACCESS EXCLUSIVE, and SHARE UPDATE
EXCLUSIVE lock modes, the PostgreSQL lock modes and the LOCK
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TABLE syntax are compatible with those present in
Oracle(TM).
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