Idea for the statistics collector

Since it's currently all for collecting statistics on tables, why can't it
collect another type of statistic, like:

- How often the estimator gets it wrong?

At the end of an index scan, the executor could compare the number of rows
returned against what was estimated, and if it falls outside a certain
range, flag it.

Also, the average ratio of rows coming out of a distinct node vs the number
going in.

For a join clause, the amount of correlation between two columns (hard).

etc

Ideally, the planner could then use this info to make better plans.
Eventually, the whole system could become somewhat self-tuning.

Does anyone see any problems with this?
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> There are 10 kinds of people in the world, those that can do binary
> arithmetic and those that can't.