Re: pg_stats.correlation rule of thumb for re-clustering a table?

On Fri, 2025-09-12 at 10:46 -0400, Ron Johnson wrote:
> Purely OLTP tables (that are only accessed randomly) can of course live with 0% correlation,
> but lots of tables are mixed-use, and so benefit from physical ordering on a carefully chosen field..
>
> SELECT abs(correlation)::numeric(3,2) as correlation
> FROM pg_stats
> WHERE schemaname = 'foo' AND tablename = 'bar'
>   AND attname = 'blarge';
>  correlation
> -------------
>         0.84
> (1 row)
>
> Obviously 84% is no need to worry, but what about 60% or 40%? Currently, I use 60%, but would like to do better.

Either the difference is gradual, so that it there is no real cut-off point,
or there is a sudden plan change at some point that depends on the query the
data and the parameter settings.  I don't think it is possible to give reliable
numbers that cover all cases.

I suggest that you run a series of benchmarks with a copy of the table with
different correlation values and come up with numbers that are meaningful
for your individual case.

Yours,
Laurenz Albe