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Message Posted: Wed, 28 Mar 2007 @ 10:19:49 GMT

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Subj:   Re: Pseudo Tables in EXPLAIN
From:   Anomy Anom

<-- Anonymously Posted: Tuesday, March 27, 2007 18:51 -->

  Could some one explain me what is Pseudo table. When ever I run explain, I see this entry.  

To avoid deadlocks initially Teradata obtains requested lock on a pseudo table then applies the actual lock when it executes that particular step.

For example if we have two different transactions which are trying to access the same table t1.

     Txn 1 --- read lock on t1
     Txn 2 --- write lock on t1

In older system (before pseudo lock feature) assume that couple of amps will obtain read lock on t1 and remaining amps obtain write lock on t1. In such situtaion deadlock will occur and one of the transaction needs to be aborted to complete the other. To avoid such situations lock on pseudo table has to be applied before locking the actual table.


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