Acero and Substrait: How to select struct field from a struct column?

[Li Jin]

Hi,

I am recently trying to do
(1) assign a struct type column s<v1, v2>
(2) flatten the struct columns (by assign v1=s[v1], v2=s[v2] and drop the s
column)

via Substrait and Acero.

However, I ran into the problem where I don't know the proper substrait
message to encode this (for (2))

Normally, if I select a column from the origin table, it would look like
this (e.g, select column index 1 from the original table):

selection {
  direct_reference {
    struct_field {
        1
    }
  }
}

But I can't figure out how to express "select struct field 0 from field 2
of the original table where field 2 is a struct column"

Any idea how the substrait message should look like for the above?