[HACKERS] how to implement selectivity injection in postgresql

[Rajmohan C]

SELECT c1, c2, c3, FROM T1, T2, T3
        WHERE T1.x = T2.x AND
        T2.y=T3.y AND
        T1.x >= ? selectivity 0.00001 AND
        T2.y > ? selectivity 0.5 AND
        T3.z = ? selectivity 0.2 AND
        T3.w = ?

I need to implement Selectivity injection as shown in above query in
PostgreSQL by which we can inject selectivity of each predicate or at least
selectivity at relation level directly as part of query. Is there any
on-going work on this front? If there is no ongoing work on this, How
should I start implementing this feature?