recreate table t (id int primary key, f1 int, f2 int, f3 int);

insert into t
select val, mod(val, 100), mod(val, 200), mod(val, 300)
from (
  select row_number() over() as val
  from rdb$types, rdb$types, rdb$types
  rows 1000000);

commit;

create index it1 on t(f1, f2);
create index it2 on t(f1, f3);

-- Test #1: two conditions mapped to a compound index IT2
-- correct cardinality estimation

select count(*)
from t
where f1 = 1 and f3 = 3;

Select Expression
    -> Aggregate [rows: 1]
        -> Filter [rows: 3337]
            -> Table "T" Access By ID [rows: 3337]
                -> Bitmap
                    -> Index "IT2" Range Scan (full match)

-- Test #2: three conditions, two of them are mapped to the compound index IT2, the third one is not mapped to an index
-- incorrect cardinality estimation, third condition is not accounted

select count(*)
from t
where f1 = 1 and f2 = 2 and f3 = 3;

Select Expression
    -> Aggregate [rows: 1]
        -> Filter [rows: 3337]
            -> Table "T" Access By ID [rows: 3337]
                -> Bitmap
                    -> Index "IT2" Range Scan (full match)

-- Test #3: three conditions, two of them are mapped to the compound index IT2, the third one has index usage forcibly disabled
-- correct cardinality estimation, third condition reduces cardinality

select count(*)
from t
where f1 = 1 and f2+0 = 2 and f3 = 3;

Select Expression
    -> Aggregate [rows: 1]
        -> Filter [rows: 334]
            -> Table "T" Access By ID [rows: 3337]
                -> Bitmap
                    -> Index "IT2" Range Scan (full match)