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scott%ORA11GR2> create table demo ( x int primary key ); Table created. scott%ORA11GR2> create or replace trigger demo_bifer 2 before insert on demo 3 for each row 4 declare 5 l_lock_id number; 6 resource_busy exception; 7 pragma exception_init( resource_busy, -54 ); 8 begin 9 l_lock_id := 10 dbms_utility.get_hash_value( to_char( :new.x ), 0, 1024 ); 11 if ( dbms_lock.request 12 ( id => l_lock_id, 13 lockmode => dbms_lock.x_mode, 14 timeout => 0, 15 release_on_commit => TRUE ) not in (0,4) ) 16 then 17 raise resource_busy; 18 end if; 19 end; 20 / Trigger created. scott%ORA11GR2> insert into demo(x) values (1); 1 row created. Now, to demonstrate us catching this blocking INSERT problem in a single session, we ll use an AUTONOMOUS_TRANSACTION so that it seems as if this next block of code was executed in another SQL*Plus session. In fact, if you use another session, the behavior will be the same. Here we go:
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CHAPTER 6 LOCKING AND LATCHING
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scott%ORA11GR2> declare 2 pragma autonomous_transaction; 3 begin 4 insert into demo(x) values (1); 5 commit; 6 end; 7 / declare * ERROR at line 1: ORA-00054: resource busy and acquire with NOWAIT specified or timeout expired ORA-06512: at "SCOTT.DEMO_BIFER", line 14 ORA-04088: error during execution of trigger 'SCOTT.DEMO_BIFER' ORA-06512: at line 4 The concept here is to take the supplied primary key value of the table protected by the trigger and put it in a character string. We can then use DBMS_UTILITY.GET_HASH_VALUE to come up with a mostly unique hash value for the string. As long as we use a hash table smaller than 1,073,741,823, we can lock that value exclusively using DBMS_LOCK. After hashing, we take that value and use DBMS_LOCK to request that lock ID to be exclusively locked with a timeout of ZERO (this returns immediately if someone else has locked that value). If we timeout or fail for any reason, we raise ORA-54 Resource Busy. Otherwise, we do nothing it is OK to insert, we won t block. Upon committing our transaction, all locks, including those allocated by this DBMS_LOCK call, will be released. Of course, if the primary key of your table is an INTEGER and you don t expect the key to go over 1 billion, you can skip the hash and just use the number as the lock ID. You ll need to play with the size of the hash table (1,024 in this example) to avoid artificial resource busy messages due to different strings hashing to the same number. The size of the hash table will be application (data)-specific, and it will be influenced by the number of concurrent insertions as well. You might also add a flag to the trigger to allow people to turn the check on and off. If I were going to insert hundreds or thousands of records, for example, I might not want this check enabled.
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Blocked Merges, Updates, and Deletes
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In an interactive application one where you query some data out of the database, allow an end user to manipulate it, and then put it back into the database a blocked UPDATE or DELETE indicates that you probably have a lost update problem in your code. (I ll call it a bug in your code if you do.) You are attempting to UPDATE a row that someone else is already updating (in other words, one that someone else already has locked). You can avoid the blocking issue by using the SELECT FOR UPDATE NOWAIT query to Verify the data has not changed since you queried it out (preventing lost updates). Lock the row (preventing the UPDATE or DELETE from blocking).
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As discussed earlier, you can do this regardless of the locking approach you take. Both pessimistic and optimistic locking may employ the SELECT FOR UPDATE NOWAIT query to verify the row has not changed. Pessimistic locking would use that SELECT FOR UPDATE NOWAIT statement the instant the user indicated her intention to modify the data. Optimistic locking would use that statement immediately prior to updating the data in the database. Not only will this resolve the blocking issue in your application, but it ll also correct the data integrity issue. Since a MERGE is simply an INSERT and UPDATE (and in 10g and above, with the enhanced MERGE syntax, it s a DELETE as well), you would use both techniques simultaneously.
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