Paul Reeves wrote:

> I have often wondered why the engine doesn't automatically recompute
> these values.

Probably because the computational cost to recompute the value is
significant enough that you generally don't want it recalculating too
often.

Also, I think it'd take a pretty drastic data change to alter the
selectivity by a large amount. I believe the selectivity is the ratio
of unique values in the index to total number of records indexed,
right? This ratio unlikely to change, even if you delete or import
large amounts of data in the table. Sure, you could do UPDATE T1 SET
PREVIOUSLY_UNIQUE_DATA = 42, but this is hardly a common occurrance.
The selectivity is almost always accurate enough for the optimizer's
purpose of decided whether to use an index or not.

Bill Karwin