> [Query1]
> select id from TEST where id2 in (select id2 from subquerytable);
>
> [Query2]
> select id from TEST where id2 in (1,2);

The subselect in query1 cannot benefit from any index since it doesn't have a WHERE clause. More importantly Query1 is not a good query to use in Firebird. The subselect is logically executed once for every record in TEST. This is done because similar queries like

select id from TEST where id2 in (select id2 from subquerytable where TEST.Name = subquerytable.Name)

would have to execute the subquery for every row in TEST.

Rewrite your query to

select id from TEST T where exists(select * from subquerytable sq where sq.id2 = t.id2)

This query would use an index for subquerytable.

HTH,
Set