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LOCK TABLE statement

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Derby Reference Manual

LOCK TABLE statement

The LOCK TABLE statement allows you to explicitly acquire a shared or exclusive table
lock on the specified table. The table lock lasts until the end of the current transaction.

To lock a table, you must either be the

database owner

or the table owner.

Explicitly locking a table is useful to:

· Avoid the overhead of multiple row locks on a table (in other words, user-initiated

lock escalation)

· Avoid deadlocks

You cannot lock system tables with this statement.

Syntax

LOCK TABLE

table-Name

IN { SHARE | EXCLUSIVE } MODE

After a table is locked in either mode, a transaction does not acquire any subsequent
row-level locks on a table. For example, if a transaction locks the entire

Flights

table in

share mode in order to read data, a particular statement might need to lock a particular
row in exclusive mode in order to update the row. However, the previous table-level lock
on the

Flights

table forces the exclusive lock to be table-level as well.

If the specified lock cannot be acquired because another connection already holds a lock
on the table, a statement-level exception is raised (SQLState X0X02) after the deadlock
timeout period.

Examples
To lock the entire

Flights

table in share mode to avoid a large number of row locks,

use the following statement:

LOCK TABLE Flights IN SHARE MODE;
SELECT *
FROM Flights
WHERE orig_airport > 'OOO';

You have a transaction with multiple UPDATE statements. Since each of the individual
statements acquires only a few row-level locks, the transaction will not automatically
upgrade the locks to a table-level lock. However, collectively the UPDATE statements
acquire and release a large number of locks, which might result in deadlocks. For this
type of transaction, you can acquire an exclusive table-level lock at the beginning of the
transaction. For example:

LOCK TABLE FlightAvailability IN EXCLUSIVE MODE;
UPDATE FlightAvailability
SET economy_seats_taken = (economy_seats_taken + 2)
WHERE flight_id = 'AA1265' AND flight_date = DATE('2004-03-31');

UPDATE FlightAvailability
SET economy_seats_taken = (economy_seats_taken + 2)
WHERE flight_id = 'AA1265' AND flight_date = DATE('2004-04-11');

UPDATE FlightAvailability
SET economy_seats_taken = (economy_seats_taken + 2)
WHERE flight_id = 'AA1265' AND flight_date = DATE('2004-04-12');

UPDATE FlightAvailability
SET economy_seats_taken = (economy_seats_taken + 2)
WHERE flight_id = 'AA1265' AND flight_date = DATE('2004-04-15');

If a transaction needs to look at a table before updating the table, acquire an exclusive
lock before selecting to avoid deadlocks. For example: