In software development, quite often the problem you encounter has virtually nothing to do with the thing you’re trying to do.
I’m creating some views which will provide an interface to a database. The views will have Instead-of triggers so that all DML will be passed to my own (packaged) procedures.
I’ve got a particular view which is a join between a base table, filtered by some criteria, and a code/description mapping view. I’ve implemented instead-of-insert, update and delete triggers that do the appropriate DML on the base table.
Inserts and updates work fine, but when deleting from the view I’m getting “TM contention” wait events. When deleting across a database link the session gets “ORA-02049: timeout: distributed transaction waiting for lock” after a long delay. If the delete is issued within the same database as the view, the session waits indefinitely for the lock.
I had a look at the view and noticed that it is a key-preserved view, so I theorised that maybe the session was locking the row before running my instead-of-delete trigger that does the delete; the trigger tries to delete the row that has already been locked. Doesn’t make sense. When I remove the instead-of-delete trigger however, I get “ORA-01752: cannot delete from view without exactly one key-preserved table”. So I need the trigger to handle the delete.
Looked up the Oracle doc “Instance Tuning Using Performance Views” about “TM enqueue”: “The most common reason for waits on TM locks tend to involve foreign key constraints where the constrained columns are not indexed. Index the foreign key columns to avoid this problem.” I saw this ref come up in a search early on but I initially dismissed it because my problem wasn’t related to performance. I’m glad I took a second look.
Righty-o. Looked at the table I was deleting from, and lo and behold there is another table with a fk to my table with no index on the fk column (mind you, there was an index that contained that column along with another column, obviously the database wasn’t going to use that to avoid the table lock).
When I created an index on the fk column, the problem was resolved. Brilliant!
The application never needed to delete from that table before, so this had never been encountered. In this case adding the index solved the problem. It’s on a very small, rarely-used table so the index won’t have a negative impact. Took about half an hour to work out, but I learned a bit more about Oracle, so that’s good.