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Message Posted: Thu, 11 Sep 2003 @ 12:16:12 GMT


     
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Subj:   Group by Spool estimation.
 
From:   David Clough

We have a query that estimates 1.8 Billion rows that will be written to Spool prior to a Group By statement (and that estimates about right).

Looking at the columns I'm bringing into the Spool, I estimate 33 bytes per Spool row (assuming Trimmed fields go through into Spool in Trimmed format, Integers go through as 4 bytes and DECIMAL(18,6) goes through as 8 bytes).

How much extra Spool would you estimate being necessary for a Group By on all columns except the one Decimal column which is the summed measure.

In the past (with other DBMS's) I've assumed 2.5 -3 times the temporary space being required.

Any rules of thumb I can use in order to be able to calculate how much Spool I'm going to need to allow my query to complete?


Regards,

Dave Clough
Database Designer
Database Design Group



     
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