Thread

Re: Selectivity of "=" (Re: [HACKERS] Index not used on simple se lect)

Zeugswetter Andreas <andreas.zeugswetter@telecom.at> — 1999-07-28T15:45:32Z

> BTW, this argument proves rigorously that the selectivity of a search
> for any value other than the MFOV is not more than 0.5, so there is some
> basis for my intuition that eqsel should not return a value above 0.5.
> So, in the cases where eqsel does not know the exact value being
> searched for, I'd still be inclined to cap its result at 0.5.
> 
Yes, this is imho an easy and efficient fix. I would even use a lower value,
like 0,3.
Good database design would not create an index for such bad selectivity
anyway.
So if you have a performance problem because of so bad selectivity,
the advice is to drop the index.

If you plan to store explicit key values, I would do this in an extra
statistic, 
that stores bunches of equally sized buckets, and distinct values for very
badly 
scewed values.

Example assuming int index column:
from	to	nrow_estimate
1	100	10005
101	20000	9997
20001	100000	10014

badly scewed values (excluded in above table):
val		nrow_estimate
1		100000
5		1000000

But imho this is an overkill, and seldom useful.

Andreas