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Re: enq: TX - index contention

From: K Gopalakrishnan <>
Date: Tue, 22 Mar 2005 20:39:40 +0530
Message-ID: <>


TX enqueue waits with LMODE 4 can happen for the following reasons. Quoting from 'Oracle Wait Interface'

"A wait for the TX enqueue in mode 4 is normally due to one of the following reasons:
ITL (interested transaction list) shortage Unique key enforcement
Bitmap index entry

Here, we will talk about the ITL, which is a transaction slot in a data block. The initial number of ITL slots is defined by the INITRANS clause and is limited by the MAXTRANS clause. By default, a table has 1 ITL and an index has 2 ITLs. Each ITL takes up 24 bytes and contains the transaction ID in the format of USN.SLOT#.WRAP#. Every DML transaction needs to acquire its own ITL space within a block before data can be manipulated. Contention for ITL occurs when all the available ITLs within a block are currently in use and there is not enough space in the PCTFREE area for Oracle to dynamically allocate a new ITL slot. In this case, the session will wait until one of the transactions is committed or rolled back, and it will reuse that ITL slot. ITL is like a building parking space. Everyone who drives to the building needs a parking space. If the parking lot is full, you have to circle the lot until someone leaves the building. "

So I would expect any of the above conditions in your case? You may want to query the V$segment_statistics for ITL waits to eliminate the ITL issue. If you have bitmap indexes, that should be one of the casues for those excesive waits. The wait time is close to 3 seconds and I suspect that could be because of the bitmap index issue,


Best Regards,
K Gopalakrishnan 
Co-Author: Oracle Wait Interface, Oracle Press 2004
Received on Tue Mar 22 2005 - 10:13:15 CST

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