Databases with transaction logging

• You cannot lock system catalog tables.
• You cannot switch between shared and exclusive table locking within a transaction. For example, once you lock the table in shared mode, you cannot upgrade the lock mode to exclusive.
• If you issue a LOCK TABLE statement before you access a row in the table, and PDQ is not in effect, no row locks are set for the table. In this way, you can override row-level locking and avoid exceeding the maximum number of locks that are defined in the database server configuration. (But if PDQ is in effect, you might run out of locks with error -134 unless the LOCKS parameter of your ONCONFIG file specifies a large enough number of locks.)
• All row and table locks release automatically after a transaction is completed. The UNLOCK TABLE statement fails in a database that uses transaction logging.
• The same user can explicitly use LOCK TABLE to lock up to 32 tables concurrently. (Use SET ISOLATION to specify an appropriate isolation level, such as Repeatable Read, if you need to lock rows from more than 32 tables during a single transaction.)

BEGIN WORK;
LOCK TABLE orders IN EXCLUSIVE MODE;
 ...
COMMIT WORK;
BEGIN WORK;
LOCK TABLE orders IN SHARE MODE;
 ...
COMMIT WORK;