Several months ago I wrote about avoiding inlist iterator, but this post about how to force inlist iterator in queries like that:
select * from xt_test1 t1 ,xt_test2 t2 where t1.a = :a and t2.id in (a,b)
i.e. when we need to get rows from big table using index by list of values from another table.
A couple of workarounds