Gavin Flower wrote:
> The standard deviation (sd) is proportional to the square root of
> the number in the sample in a Normal Distribution.
>
> In a Normal Distribution, about 2/3 the values will be within plus
> or minus one sd of the mean.
>
> There seems to be an implicit assumption that the distribution of
> values follows the Normal Distribution - has this been verified?
The whole problem here is precisely to determine what is the data
distribution -- one side of it is how to represent it for the planner
(which we do by storing a number of distinct values, a list of MCVs and
their respective frequencies, and a histogram representing values not in
the MCV list); the other side is how to figure out what data to put in
the MCV list and histogram (i.e. what to compute during ANALYZE).
If we knew the distribution was a normal, we wouldn't need any of these
things -- we'd just store the mean and standard deviation.
--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services