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Message Posted: Thu, 06 Jan 2005 @ 13:34:41 GMT


     
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Subj:   Skew puzzle
 
From:   Sreeram Venkatasubramanian

Hello group,

I am a newbie to Teradata, so pls help me solve this one!

I had a table with a skew of about 17%.

I used the query in the bottom to calculate the skew.

I copied this entire table to a new table in a different database with the same primary index.

I ran the same query and it shows a skew of only 0.18% !!

My understanding was that the skew is based only on the data and the primary index.

I have no idea how the skew got reduced when the data and the primary index are same.

Any idea will be great !


Cheers,

Sreeram


Below is the query from the Teradata manual that I used for calculating the table skew.

     SELECT  di.databasename, di.tablename, di.indexname, di.indextype,
     di.uniqueflag, max(columnposition) Nbr_columns,
     SUM(cast(t.currentperm as decimal(18,0))/(1024*1024)) perm_mb,
      100*((cast(max(t.currentperm ) as decimal(18,3)) -
     cast(ave(t.currentperm ) as decimal(18,3))))
               /(nullif(cast(max(t.currentperm) as decimal(18,3)),0)) as
     tableskew,
      100*((cast(max(t.currentperm ) as decimal(18,3)) -
     cast(min(t.currentperm ) as decimal(18,3))))
               /(nullif(cast(min(t.currentperm) as decimal(18,3)),0) ) as
     tablevariance,
      max (case when columnposition = 1 then columnname else '' end) col1,
      max (case when columnposition = 2 then columnname else '' end) col2,
      max (case when columnposition = 3 then columnname else '' end) col3,
      max (case when columnposition = 4 then columnname else '' end) col4,
      max (case when columnposition = 5 then columnname else '' end) col5,
      max (case when columnposition = 6 then columnname else '' end) col6,
      max (case when columnposition = 7 then columnname else '' end) col7,
      max (case when columnposition = 8 then columnname else '' end) col8,
      max (case when columnposition = 9 then columnname else '' end) col9
     FROM dbc.indices di, dbc.tablesize t WHERE  di.databasename in ('myDB')
     and di.indextype = 'P'
      and di.databasename = t.databasename and di.tablename = t.tablename
     GROUP BY di.databasename, di.tablename, di.indextype, di.indexname,
     di.uniqueflag ORDER BY di.databasename, di.tablename, di.indextype,
     di.indexname


     
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