Diagnosing Oracle Wait for TX Enqueue in Mode 4–Bitmap Index Entry

on April 7, 2013

A wait for the TX enqueue in mode 4 can also occur when multiple sessions try to update or delete different rows that are covered by the same bitmap entry. Of course, this does not apply if the application does not use bitmap indexes.

Unlike the B-tree index entry, which contains a single ROWID, a bitmap entry can potentially cover a range of ROWIDs. So when a bitmap index entry is locked, all the ROWIDs that are covered by the entry are also locked.

The following listing shows an example of a TX enqueue wait in mode 4 as seen in the V$LOCK view due to bitmap entry. What is the difference between this listing and the preceding one in the unique key enforcement? Can you tell if you are dealing with a bitmap index entry or unique key enforcement issue by looking at the V$LOCK view output? No, you can’t. The object ID in the TM lock doesn’t help either, as it is the object ID of the table and not the index. That is why it is very important for you to capture the SQL statement of the waiting and blocking sessions. If the waiting session is attempting an insert, you are dealing with a unique key enforcement issue. If the waiting session is attempting an update or delete, most likely you are dealing with a bitmap entry issue.

Fig 6-18






In order to resolve the contention, you have to hunt down the offending user. However, the offending user is not always the user who holds the lock. That user was there first, for crying out loud. If the user has a legitimate reason to hold the lock, the waiters should back out of their transactions.

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