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Message Posted: Wed, 03 Dec 2003 @ 14:15:34 GMT


     
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Subj:   Re: Help on Explain
 
From:   Vivek Pandey

Hi Narayan,

  Can some explain what is happening in this explain!  


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


It is a 2 AMP step as it is a USI access method. AMP 1 to access the USI subtable with USI = 1 specified. (This USI subtable will then point to the base table row.) AMP 2 to access the base table row then.


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


That says Teradata is using the 'nested join' internally to join the rows. This internal join could be nested,merge or a product join.

The condition for nested join is.

1.An equality value for a unique index on Table 1 (this will retrieve a single row).

2.A Join on a column of that single row to any index on Table 2


Condition 1 is satisfied by A.b=1 (A.b being the USI)
Condition 1 is satisfied by A.b=B.b (B.b being the USI and an index again)


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


unique index # 4 means the USI t3.b is being used.

You can get all the indexes defined on the table by quering the dictionary view dbc.indices.

The IndexNumber column is often 4 for USI & 8 for a NUSI.

You 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.

Thanks & Regards,

Vivek.



     
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