planner: consider index path when the needed columns only contain handleCol

[francis0407]

Feature Request

Is your feature request related to a problem? Please describe:

When we only want to get primary key, we can consider index path if there is no condition on the PK. For example:

create table t1 (a int primary key, b int, .... many columns, index idx_b(b));
create table t2 (a int, b int);

In this query, we can use IndexScan for t1.a:

select t2.a, t2.b
from t1, t2
where t1.a = t2.a;

We can also use a IndexScan for this one:

select count(1) from t1;