Saturday, July 06, 2013

DB2 Locking, Part 14: Using the LOCK TABLE Statement

We continue the series on DB2 locking today with a look at the SQL statement: LOCK TABLE .

You can issue the LOCK TABLE statement in your application programs to raise the lock granularity to the table (actually, table space) level. Doing so means that you will not need to take lower level locks (at the page or row level, whichever is in effect for the table space in question). Issuing a LOCK TABLE in a program can make sense in certain, specific circumstances. Consider using the LOCK TABLE statement to control the efficiency of locking in programs that will issue many page/row lock requests when there are no concurrent requests for the same data. Issuing a LOCK TABLE, at times, can be a reasonable alternative to using an ISOLATION level of RR or RS when a large percentage of a table's rows or pages will be modified.

If your environment can withstand the concurrency hit caused by LOCK TABLE you can gain performance by reducing locking activity. Taking a larger granularity lock at the table(space) level instead of multiple page or row locks will improve the performance of your program, albeit at the expense of concurrent activity to the data. If you wish to avoid modifying the program you can START the table space is read only mode (RO) and achieve similar results (no locks are taken because the data cannot be modified). This can be accomplished using a command like so:


Of course, if you go that route you will have to make sure that you restart the table space for read write activity (RW) after the program finishes. This can be unwieldy to implement.

If, instead, you wish to use the LOCK TABLE approach, there are two types of LOCK TABLE requests. The LOCK TABLE...IN SHARE MODE statement acquires an S-lock on the table specified in the statement. This locking strategy effectively eliminates the possibility of concurrent modification programs running while the LOCK TABLE is in effect. The S-lock is obtained on the table space for tables contained in non-segmented table spaces. This is important to understand, especially if you have multi-table table spaces.

The LOCK TABLE...IN EXCLUSIVE MODE statement acquires an X-lock on the table specified in the statement. All concurrent processing is suspended until the X-lock is released. Again, for non-seqmented table spaces, the X-lock is obtained on the table space not the table.

In both cases, you can specify the PART parameter to indicate that only a specific partition is to be locked. For example, to lock only the third partition of a partitioned table space, you can issue: LOCK TABLE...PART 3 IN EXCLUSIVE MODE.
The table locks acquired as a result of the LOCK TABLE statement are held until the next COMMIT point unless RELEASE(DEALLOCATE) was specified for the plan issuing the LOCK TABLE statement. In that situation, the lock is held until the program terminates. That means, for RELEASE(COMMIT) programs, you will need to issue the LOCK TABLE again after each COMMIT or processing will revert to page/row locking.

Also, keep in mind that the lock will not take effect until the statement executes, even if ACQUIRE(ALLOCATE) was coded at BIND time. 

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