Consider two tables defined as

create table t1 (a int primary key, b int, check b < 5); 

create table t2 (a int primary key);

and a query

select * from t1, t2 where t1.b=t2.a and t2.a > 2;

If to take into account the check constraint on t1.b then the query will be looking as

select * from t1, t2 where t1.b=t2.a and t2.a > 2 and t1.b < 5;

As t1.b is equal to t2.a a much better condition on the column t2.a can be deduced:
t2.a > 2 and t2.a < 5

Consider another query

select * from t1, t2 where t1.a=t2.b and t2.a > 10;

As in the previous one, a new condition for t2.a can be deduced: t2.a > 10 and t2.a < 5
This condition is always false, so there is no need to evaluate the query.