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Status Not under consideration
Workspace Db2
Created by Guest
Created on Dec 2, 2014

Allow exclusion of columns from row compression to reduce overflows on updates

We have many large tables in our data warehouse that hold historized data records. There are four date columns:
- begin of visibility based on processing time (BAT_BEGIN)
- end of visibility based on processing time (BAT_END)
- begin of visibility based on effective date (WIT_BEGIN)
- end of visibility based on effective date (WIT_END).
On insert, a record always gets BAT_END = 9999-12-31 and WIT_END = 9999-12-31. When a newer record arrives, we update the old record with the BAT_BEGIN of the new record (WIT_END is a bit more complicated to set but for this RFE we can see it as similar).
The majority of the columns (about 60%) have BAT_END = 9999-12-31 and WIT_END = 9999-12-31, all other dates are a lot less often. In our eyes this leads to the fact that the 9999-12-31 gets very well compressed and other date values in this column need more space.
With adaptive row compression enabled (also previously with the "normal" row compression), we now get quite high overflow rates when we do the update of the BAT_END/WIT_END columns. And that leads to many necessary reorgs. We can for sure now set PCT_FREE to higher values but according to our tests it would need to be at a very high value, rendering compression quite useless.

If we could tell DB2 to not compress values in BAT_END/WIT_END columns, we could circumvent these problems. So we suggest to put a feature in a future release to define columns as not compressable.