LOCK

where 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 stopped 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, Greengage DB 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 the 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 the SHARE lock mode conflicts with the ROW EXCLUSIVE lock acquired by writers, and your LOCK TABLE <table_name> IN SHARE MODE statement will wait until any concurrent holders of the 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 — 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 the SHARE ROW EXCLUSIVE lock mode instead of the 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 the SHARE mode, and then be unable to also acquire the ROW EXCLUSIVE mode to actually perform their updates. Note that a transaction’s own locks never conflict, so a transaction can acquire the ROW EXCLUSIVE mode when it holds the SHARE mode — but not if anyone else holds the 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.

LOCK TABLE …​ IN ACCESS SHARE MODE requires SELECT privileges on the target table. All other forms of LOCK require table-level UPDATE, DELETE, or TRUNCATE privileges.

LOCK TABLE is useless outside a transaction block: the lock would remain held only to the completion of the statement. Therefore, Greengage DB reports an error if LOCK is used outside a transaction block. Use BEGIN and END 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, the 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 clauses in the SELECT reference documentation.

Obtain a SHARE lock on the films table when going to perform inserts into the films_user_comments table:

Take a SHARE ROW EXCLUSIVE lock on a table when performing a delete operation:

There is no LOCK TABLE in the SQL standard, which instead uses SET TRANSACTION to specify concurrency levels on transactions. Greengage DB supports that too.

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