Keywords: Lock granularity, how optimizer chooses, Lock escalation, chosen by optimizer

# About the system's selection of lock granularity

When a system is configured for row-level locking, it decides whether to use table-level locking or row-level locking for each table in each DML statement. The system bases this decision on the number of rows read or written for each table, and on whether a full conglomerate scan is done for each table.
Note: When you have turned off row-level locking for your system, Derby always uses table-level locking.
The first goal of the system's decision is concurrency; wherever possible, the system chooses row-level locking. However, row-level locking uses a lot of resources and might have a negative impact on performance. Sometimes row-level locking does not provide much more concurrency than table-level locking. In those situations, the system might choose to escalate the locking scheme from row-level locking to table-level locking to improve performance. For example, if a connection is configured for TRANSACTION_SERIALIZABLE isolation, the system chooses table-level locking for the following statement:

    SELECT *
    FROM FlightAvailability AS fa, Flights AS fts
    WHERE fts.flight_id = fa.flight_id
    AND fts.segment_number = fa.segment_number

To satisfy the isolation requirements, Derby would have to lock all the rows in both the FlightAvailability and the Flights tables. Locking both the tables would be cheaper, would provide the same isolation, and would allow the same concurrency.
Note: You can force lock escalation for specific tables when you alter them with the LOCKSIZE clause. For these tables, Derby always chooses table-level locking. For more information, see the Java DB Reference Manual.

- How the system makes its decision if it has a choice
- Lock escalation threshold

Parent topic: Derby's cost-based optimization
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