1LOCK(7) PostgreSQL 9.2.24 Documentation LOCK(7)
2
6 LOCK - lock a table
7
9 LOCK [ TABLE ] [ ONLY ] name [ * ] [, ...] [ IN lockmode MODE ] [ NOWAIT ]
10 where lockmode is one of:
12 ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE
14 | SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE
15
17 LOCK TABLE obtains a table-level lock, waiting if necessary for any
18 conflicting locks to be released. If NOWAIT is specified, LOCK TABLE
19 does not wait to acquire the desired lock: if it cannot be acquired
20 immediately, the command is aborted and an error is emitted. Once
21 obtained, the lock is held for the remainder of the current
22 transaction. (There is no UNLOCK TABLE command; locks are always
23 released at transaction end.)
24 When acquiring locks automatically for commands that reference tables,
26 PostgreSQL always uses the least restrictive lock mode possible. LOCK
27 TABLE provides for cases when you might need more restrictive locking.
28 For example, suppose an application runs a transaction at the Read
29 Committed isolation level and needs to ensure that data in a table
30 remains stable for the duration of the transaction. To achieve this you
31 could obtain SHARE lock mode over the table before querying. This will
32 prevent concurrent data changes and ensure subsequent reads of the
33 table see a stable view of committed data, because SHARE lock mode
34 conflicts with the ROW EXCLUSIVE lock acquired by writers, and your
35 LOCK TABLE name IN SHARE MODE statement will wait until any concurrent
36 holders of ROW EXCLUSIVE mode locks commit or roll back. Thus, once you
37 obtain the lock, there are no uncommitted writes outstanding;
38 furthermore none can begin until you release the lock.
39 To achieve a similar effect when running a transaction at the
41 REPEATABLE READ or SERIALIZABLE isolation level, you have to execute
42 the LOCK TABLE statement before executing any SELECT or data
43 modification statement. A REPEATABLE READ or SERIALIZABLE transaction's
44 view of data will be frozen when its first SELECT or data modification
45 statement begins. A LOCK TABLE later in the transaction will still
46 prevent concurrent writes — but it won't ensure that what the
47 transaction reads corresponds to the latest committed values.
48 If a transaction of this sort is going to change the data in the table,
50 then it should use SHARE ROW EXCLUSIVE lock mode instead of SHARE mode.
51 This ensures that only one transaction of this type runs at a time.
52 Without this, a deadlock is possible: two transactions might both
53 acquire SHARE mode, and then be unable to also acquire ROW EXCLUSIVE
54 mode to actually perform their updates. (Note that a transaction's own
55 locks never conflict, so a transaction can acquire ROW EXCLUSIVE mode
56 when it holds SHARE mode — but not if anyone else holds SHARE mode.) To
57 avoid deadlocks, make sure all transactions acquire locks on the same
58 objects in the same order, and if multiple lock modes are involved for
59 a single object, then transactions should always acquire the most
60 restrictive mode first.
61 More information about the lock modes and locking strategies can be
63 found in Section 13.3, “Explicit Locking”, in the documentation.
64
66 name
67 The name (optionally schema-qualified) of an existing table to
68 lock. If ONLY is specified before the table name, only that table
69 is locked. If ONLY is not specified, the table and all its
70 descendant tables (if any) are locked. Optionally, * can be
71 specified after the table name to explicitly indicate that
72 descendant tables are included.
73 The command LOCK TABLE a, b; is equivalent to LOCK TABLE a; LOCK
75 TABLE b;. The tables are locked one-by-one in the order specified
76 in the LOCK TABLE command.
77 lockmode
79 The lock mode specifies which locks this lock conflicts with. Lock
80 modes are described in Section 13.3, “Explicit Locking”, in the
81 documentation.
82 If no lock mode is specified, then ACCESS EXCLUSIVE, the most
84 restrictive mode, is used.
85 NOWAIT
87 Specifies that LOCK TABLE should not wait for any conflicting locks
88 to be released: if the specified lock(s) cannot be acquired
89 immediately without waiting, the transaction is aborted.
90
92 LOCK TABLE ... IN ACCESS SHARE MODE requires SELECT privileges on the
93 target table. All other forms of LOCK require table-level UPDATE,
94 DELETE, or TRUNCATE privileges.
95 LOCK TABLE is useless outside a transaction block: the lock would
97 remain held only to the completion of the statement. Therefore
98 PostgreSQL reports an error if LOCK is used outside a transaction
99 block. Use BEGIN(7) and COMMIT(7) (or ROLLBACK(7)) to define a
100 transaction block.
101 LOCK TABLE only deals with table-level locks, and so the mode names
103 involving ROW are all misnomers. These mode names should generally be
104 read as indicating the intention of the user to acquire row-level locks
105 within the locked table. Also, ROW EXCLUSIVE mode is a sharable table
106 lock. Keep in mind that all the lock modes have identical semantics so
107 far as LOCK TABLE is concerned, differing only in the rules about which
108 modes conflict with which. For information on how to acquire an actual
109 row-level lock, see Section 13.3.2, “Row-level Locks”, in the
110 documentation and the FOR UPDATE/FOR SHARE Clause in the SELECT
111 reference documentation.
112
114 Obtain a SHARE lock on a primary key table when going to perform
115 inserts into a foreign key table:
116 BEGIN WORK;
118 LOCK TABLE films IN SHARE MODE;
119 SELECT id FROM films
120 WHERE name = 'Star Wars: Episode I - The Phantom Menace';
121 -- Do ROLLBACK if record was not returned
122 INSERT INTO films_user_comments VALUES
123 (_id_, 'GREAT! I was waiting for it for so long!');
124 COMMIT WORK;
125 Take a SHARE ROW EXCLUSIVE lock on a primary key table when going to
127 perform a delete operation:
128 BEGIN WORK;
130 LOCK TABLE films IN SHARE ROW EXCLUSIVE MODE;
131 DELETE FROM films_user_comments WHERE id IN
132 (SELECT id FROM films WHERE rating < 5);
133 DELETE FROM films WHERE rating < 5;
134 COMMIT WORK;
135
137 There is no LOCK TABLE in the SQL standard, which instead uses SET
138 TRANSACTION to specify concurrency levels on transactions. PostgreSQL
139 supports that too; see SET TRANSACTION (SET_TRANSACTION(7)) for
140 details.
141 Except for ACCESS SHARE, ACCESS EXCLUSIVE, and SHARE UPDATE EXCLUSIVE
143 lock modes, the PostgreSQL lock modes and the LOCK TABLE syntax are
144 compatible with those present in Oracle.
145PostgreSQL 9.2.24 2017-11-06 LOCK(7)