LOCK -- lock a table.
lockmode is one of:
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 acquiring locks automatically for commands that reference tables, EDB Postgres Advanced Server 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 isolation level read committed 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 isolation level serializable, you have to execute the
LOCK TABLE statement before executing any data modification statement. A serializable transaction’s view of data will be frozen when its first data modification statement begins. A later
LOCK TABLE 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
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.
The name (optionally schema-qualified) of an existing table to lock.
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.
The lock mode specifies which locks this lock conflicts with.
If no lock mode is specified, then the server uses the most restrictive mode,
ACCESS EXCLUSIVE. (
ACCESS EXCLUSIVE is not compatible with Oracle databases. In EDB Postgres Advanced Server, this configuration mode ensures that no other transaction can access the locked table in any manner.)
LOCK TABLE should not wait for any conflicting locks to be released: if the specified lock cannot be immediately acquired without waiting, the transaction is aborted.
All forms of
LOCK TABLE is useful only inside a transaction block since the lock is dropped as soon as the transaction ends. A
LOCK TABLE command appearing outside any transaction block forms a self-contained transaction, so the lock will be dropped as soon as it is obtained.
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 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.