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Message Posted: Tue, 23 Nov 2004 @ 18:43:00 GMT


     
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Subj:   Explain - low vs high confidence
 
From:   akleema_talukder

Hi all,

I have an SQL and the explain for this SQL is below. The column cust_id, and sbu_id has statistics and it is up-to-date. Then why it it is showing low confidence? To get the high confidence what should I do?

Any hints will be greatly appreciated.

SQL:

     explain
     sel cust_id, sum(sbu_id) from gdyr_edw.cust group by 1;

Explain:

Explanation
--------------------------------------------------
 
  1)First, we lock a distinct gdyr_edw."pseudo table" for read on a RowHash to prevent global deadlock for gdyr_edw.cust.  
  2)Next, we lock gdyr_edw.cust for read.  
  3)We do a SUM step to aggregate from gdyr_edw.cust by way of an all-rows scan with no residual conditions, and the grouping identifier in field 1025. Aggregate Intermediate Results are computed locally, then placed in Spool 3. The size of Spool 3 is estimated with low confidence to be 380,547 rows.  
  4)We do an all-AMPs RETRIEVE step from Spool 3 (Last Use) by way of an all-rows scan into Spool 1, which is built locally on the AMPs. The size of Spool 1 is estimated with low confidence to be 380,547 rows. The estimated time for this step is 1.78 seconds.  
  5)Finally, we send out an END TRANSACTION step to all AMPs involved in processing the request.  
  ->The contents of Spool 1 are sent back to the user as the result of statement 1.  


thanks,

Akleema



     
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