Openedge Data Management: SQL Reference - Product ...

LOCK TABLE
Without a table lock, the first SELECT statement in the following example could exceed the limits
of the record lock table, while the LOCK TABLE statement prevents the subsequent SELECT
statement from consuming the record lock table:
-- Without a table lock, this SELECT statement creates an
-- entry in the record lock table for every row in teratab.
SELECT COUNT (*) FROM teratab ;
-- The LOCK TABLE IN SHARE MODE operation preserves the
-- record lock table resource.
LOCK TABLE teratab IN SHARE MODE ;
SELECT COUNT (*) FROM teratab ;
Notes • The LOCK TABLE statement might encounter a locking conflict with another transaction.
• The SHARE MODE option detects a locking conflict if another transaction:
– Locked the table in EXCLUSIVE MODE and has not issued a COMMIT or ROLLBACK
– Inserted, updated, or deleted rows in the table and has not issued a COMMIT or
ROLLBACK
• The EXCLUSIVE MODE option detects a locking conflict if another transaction:
– Locked the table in SHARE MODE or EXCLUSIVE MODE and has not issued a COMMIT or
ROLLBACK
– Read from, inserted, updated, or deleted rows and has not issued a COMMIT or
ROLLBACK
• When there is a locking conflict, the transaction is suspended and the database returns an
error. You might configure the time at which the transaction is suspended. The default is
five seconds.
• You can use explicit table locking to improve the performance of a single transaction, at
the cost of decreasing the concurrency of the system and potentially blocking other
transactions. It is more efficient to lock a table explicitly if you know that the transaction
will be updating a substantial part of a table. You gain efficiency by decreasing the
overhead of the implicit locking mechanism, and by decreasing any potential wait time for
acquiring individual record locks on the table.
• You can use explicit table locking to minimize potential deadlocks in situations where a
transaction is modifying a substantial part of a table. Before making a choice between
explicit or implicit locking, compare the benefits of table locking with the disadvantages
of losing concurrency.
• The database releases explicit and implicit locks only when the transaction ends with a
COMMIT or ROLLBACK operation.
Authorization
Must have DBA privilege or SELECT privilege on the table.
41