LOCK

LOCK

7

SQL - Language Statements

LOCK

lock a table

LOCK \[ TABLE \] \[ ONLY \]

name

\[ \* \] \[, \...\] \[ IN

lockmode

MODE \] \[ NOWAIT \]

where

lockmode

is one of:

ACCESS SHARE \| ROW SHARE \| ROW EXCLUSIVE \| SHARE UPDATE EXCLUSIVE \|
SHARE \| SHARE ROW EXCLUSIVE \| EXCLUSIVE \| ACCESS EXCLUSIVE

Description
===========

`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,
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 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 name IN SHARE MODE` statement will wait until any concurrent
holders of `ROW
   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
MDASH 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 MDASH 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 [???](#explicit-locking).

Parameters
==========

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 [???](#explicit-locking).

    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.

Notes
=====

`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, by default, the view\'s owner must
have the relevant privileges on the underlying base relations, whereas
the user performing the lock does not need any permissions on the
underlying base relations. However, if the view has `security_invoker`
set to `true` (see [`CREATE VIEW`](#sql-createview)), the user
performing the lock, rather than the view owner, must have the relevant
privileges 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
PostgreSQL reports an error if `LOCK` is used outside a transaction
block. Use [`BEGIN`](#sql-begin) and [`COMMIT`](#sql-commit) (or
[`ROLLBACK`](#sql-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 [???](#locking-rows) and
[???](#sql-for-update-share) in the [???](#sql-select) documentation.

Examples
========

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
=============

There is no `LOCK TABLE` in the SQL standard, which instead uses
`SET TRANSACTION` to specify concurrency levels on transactions.
PostgreSQL supports that too; see [???](#sql-set-transaction) for
details.

Except for `ACCESS SHARE`, `ACCESS EXCLUSIVE`, and
`SHARE UPDATE EXCLUSIVE` lock modes, the PostgreSQL lock modes and the
`LOCK TABLE` syntax are compatible with those present in Oracle.
