Improve choose_custom_plan for initial partition prune case

[Andy Fan]

Given the plan example:

CREATE TABLE measurement (
    city_id         int not null,
    logdate         date not null,
    peaktemp        int,
    unitsales       int
) PARTITION BY RANGE (logdate);

CREATE TABLE measurement_y2006m02 PARTITION OF measurement
    FOR VALUES FROM ('2006-02-01') TO ('2006-03-01');

CREATE TABLE measurement_y2006m03 PARTITION OF measurement
    FOR VALUES FROM ('2006-03-01') TO ('2006-04-01');

prepare s as select * from measurement where logdate = $1;
execute s('2006-02-01').

The generic plan will probably not be chosen because it doesn't reduce the
cost
which can be reduced at initial_prune while the custom plan reduces such
cost
at  planning time. which makes the cost comparison not fair.  I'm thinking
if we can
get an estimated cost reduction of initial_prunne for generic plan based on
the
partition pruned at plan time from custom plan and then reducing
such costs from the generic plan.  I just went through the related code but
didn't write anything now.  I'd like to see if this is a correct direction
to go.

-- 
Best Regards
Andy Fan