On 04/05/2017 08:41 AM, Sven R. Kunze wrote:
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 ;
     [{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"?

No, it's more 'functional dependency' from relational normal forms.


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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