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Message Posted: Mon, 15 Mar 2004 @ 17:07:08 GMT


     
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Subj:   Odd explain plan in a query using MINUS
 
From:   John Adams

Hi, folks,

We're doing some compression and have been asked to verify that the data in a compressed table is the same as the data in an uncompressed table.

The simplest way I could see to do this was symmetric differences, like so:

LOCKING original FOR ACCESS
LOCKING compressed FOR ACCESS
SELECT *
FROM original
MINUS
SELECT *
FROM compressed

and then vice-versa. Since the tables have identical PIs, I expected to see entirely AMP-local activity, so this part of the explain plan puzzles me:

       
  2)Next, we do an All-AMPs RETRIEVE step from original by way of an all-rows scan into Spool 36048, which is redistributed by hash code to all AMPs. This step begins a parallel block of steps.  
  3)We do an All-AMPs RETRIEVE step from compressed by way of an all-rows scan into Spool 36049, which is redistributed by hash code to all AMPs. This step ends a parallel block of steps.  


What sort of redistribution is involved in this? "None" would have been my answer before seeing the explain plan, and "None" is still my best guess.

Can someone tell me exactly what's going on with this?


All the best,

John A



     
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