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Message Posted: Mon, 22 Oct 2007 @ 13:34:38 GMT

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Dave - Some things to think about:

1) Consider the Teradata space requirement for storing the compression values in one column. I believe that Teradata limits the size of the column that holds a column's compression values to 8100 bytes.

2) Are U considering the size of the table header? I recall a limitation of about 30K bytes for all columns to be compressed in a table.

3) We ran into a gotcha the other day:

when the NetSavings were calculated, 2 of the alternative, compression profiles for a column were tied. So when the items are ranked, U need to distinguish between the ties for NetSavings by using the Cost or RawSavings to break ties. (This occurred for a table of about 50 rows with a width of about 100 bytes. If a DBA is running the process interactively, a non-issue; but a problem for the automated solution.)

If U are using a statistical based approach (to avoid looking at all the data), U will find that compression profiles using 255 values may not be the best option. I found that automated solutions of samples at the 255 value level start to break down. The error is very small, but is present.

Good luck.

Rgrds,

Bill Gregg
Thomson Teradata Team

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