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Message Posted: Wed, 03 Dec 2003 @ 13:35:27 GMT


     
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Subj:   Help on Explain
 
From:   Narayan Murthy

Can some explain what is happening in this explain!

- What does two-AMP JOIN step mean - with those conditions?

- Why does the join appear again - "using a nested join"?

- What does "unique index # 4" - mean?


I have a USI created on t3 with the column b(int).

explain
sel A.b,B.b from t3 A, t3 B where A.b=B.b and A.b=1;

Explanation
--------------------------------------------------------------------
  1) First, we do a two-AMP JOIN step from CHK.B by way of unique index
     # 4 "CHK.B.b = 1", which is joined to CHK.A by way of unique index
     # 4 "CHK.A.b = CHK.B.b" with an additional condition of ("CHK.A.b
     = 1").  CHK.B and CHK.A are joined using a nested join, with a
     join condition of ("CHK.A.b = CHK.B.b").  The result goes into
     Spool 1 (one-amp), which is built locally on the AMPs.  The size
     of Spool 1 is estimated with high confidence to be 1 row.  The
     estimated time for this step is 0.05 seconds.
  -> The contents of Spool 1 are sent back to the user as the result of
     statement 1.  The total estimated time is 0.05 seconds.

Narayan



     
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