Thanks Tomas and David for hacking on this patch.
On 04.04.2017 20:19, Tomas Vondra wrote:
I'm not sure we still need the min_group_size, when evaluating
dependencies. It was meant to deal with 'noisy' data, but I think it
after switching to the 'degree' it might actually be a bad idea.
Consider this:
create table t (a int, b int);
insert into t select 1, 1 from generate_series(1, 10000) s(i);
insert into t select i, i from generate_series(2, 20000) s(i);
create statistics s with (dependencies) on (a,b) from t;
analyze t;
select stadependencies from pg_statistic_ext ;
stadependencies
--------------------------------------------
[{1 => 2 : 0.333344}, {2 => 1 : 0.333344}]
(1 row)
So the degree of the dependency is just ~0.333 although it's obviously
a perfect dependency, i.e. a knowledge of 'a' determines 'b'. The
reason is that we discard 2/3 of rows, because those groups are only a
single row each, except for the one large group (1/3 of rows).
Just for me to follow the comments better. Is "dependency" roughly the
same as when statisticians speak about " conditional probability"?
Sven
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