LOCK — lock a table
LOCK [ TABLE ] [ ONLY ]name[ * ] [, ...] [ INlockmodeMODE ] [ NOWAIT ] wherelockmodeis 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, 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 
   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.
     
lockmodeThe 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
    PostgreSQL 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.  PostgreSQL supports that too;
   see 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.