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Message Posted: Fri, 05 Mar 2004 @ 16:15:02 GMT


     
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Subj:   Re: Do we lose available DiskSpace if we change the number of AMPs?
 
From:   Frank O Connor

Hi

There appears to be another consideration when it comes to skew.

If I understand Teradata properly, you can appear to lose effective space when your number of amps increaes. Please look at the simple example below.

Imagine, you have a table with 1.1m rows. 1 million of these have distinct PIs, and 100K have a constant PI. Naturally, this will result in skew.

So, if you have a 10 amp system, you would expect 100K rows to be written to 9 amps and 200K (1m/10 + 100K constant PI rows). This will take the space of 2m rows if the 2m rows were perfectly skewed.

However, if you have a 20 amp system, you would expect 50K rows to be written to 19 amps and 150K to 1 amp. This would take the space of 3m rows!

Therefore, it would appear that you do lose space. Also, it also shows the importance of eliminating skew at all times.


my 2 euro cents

Frank



     
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