:) create table mt (id Int64, A Nullable(Int64), X Int64 materialized coalesce(A, -1)) engine=MergeTree order by id
:) insert into mt values (1, 42)  -- X will be 42 (as expected)
:) insert into mt select 1, 42    -- X will be -1
:) select *, X from mt order by id
┌─id─┬──A─┬──X─┐
│  1 │ 42 │ 42 │
└────┴────┴────┘
┌─id─┬──A─┬──X─┐
│  1 │ 42 │ -1 │
└────┴────┴────┘

:) select *, X from mt order by id
┌─id─┬──A─┬──X─┐
│  1 │ 42 │ 42 │
└────┴────┴────┘
┌─id─┬──A─┬──X─┐
│  1 │ 42 │ 42 │
└────┴────┴────┘