Re: Does "correlation" mislead the optimizer on large

On Fri, 24 Jan 2003, Tom Lane wrote:

> Ron Mayer <ron(at)intervideo(dot)com> writes:
> > In particular, if I have a large table t with columns 'a','b','c', etc,
> > and I cluster the table as follows:
> > create table t_ordered as select * from t order by a,b;
> > vacuum analyze t_ordered;
> > Column "b" will (correctly) get a very low "correlation" in
> > the pg_stats table -- but I think the optimizer would do better
> > assuming a high correlation because similar 'b' values are still
> > grouped closely on the same disk pages.
>
> How would that be? They'll be separated by the stride of 'a'.

I think it's a clumping effect.

For example, I made a table (ordered) with 20 values of a, 50 values of b
(each showing up in each a) and 100 values of c (not used, just means 100
rows for each (a,b) combination. It's got 541 pages it looks like. Analyze
sets the correlation to about 0.08 on the table and so a query like:
select * from test1 where b=1; prefers a sequence scan (1791 vs 2231)
while the index scan actually performs about 5 times better.

I guess the reason is that in general, the index scan *really* is reading
something on the order of 40 pages rather than the much larger estimate
(I'd guess something on the order of say 300-400? I'm not sure how to
find that except by trying to reverse engineer the estimate number),
because pretty much each value of a will probably have 1 or 2 pages with
b=1.

I'm not really sure how to measure that, however.