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HCC And Virtual Columns

This is just a short heads-up note to those dealing with HCC-enabled tables (so at present this applies only to Exadata customers).

As already outlined in a previous post about compression restrictions tables with HCC enabled do not support dropping columns - DROP COLUMN gets silently converted into SET UNUSED and DROP UNUSED COLUMNS throws an error to be unsupported.

I've recently come across an interesting variation of this restriction. Obviously Oracle treats virtual columns in this case the same: If you drop a virtual column of a HCC-enabled table it doesn't get dropped but is also silently turned into an unused column - which doesn't really make sense to me since dropping it doesn't require any physical modification to the underlying data structures.

Now you might wonder why this could be relevant? Well it can be important for several reasons: