Support structs in Adbc.Column.to_list

[aspett]

Adds support for :struct & {:struct, columns} types in Adbc.Column.to_list

Fixes an issue where reading data through Table.to_rows() would return %Adbc.Column{} values.

[josevalim]

According to the typespec above, it can only be type: :struct, never {:struct, _columns}. Is this clause necessary?

[aspett]

I'd have to update the typespec, but yes I only implemented this after coming across it 😅. You can reproduce it with a duckdb query like

CREATE TABLE t (col1 text, col2 text)
INSERT INTO t VALUES ('val1', 'val2');

SELECT struct_pack(col1 := col1, col2 := col2) FROM t

iex(6)> Adbc.Connection.query(conn, "SELECT struct_pack(col1 := col1, col2 := col2) FROM t")
{:ok,
 %Adbc.Result{
   num_rows: 0,
   data: [
     %Adbc.Column{
       name: "struct_pack(col1 := col1, col2 := col2)",
       type: {:struct,
        [
          %Adbc.Column{
            name: "col1",
            type: :string,
            nullable: true,
            metadata: nil,
            data: nil,
            length: nil,
            offset: nil
          },
          %Adbc.Column{
            name: "col2",
            type: :string,
            nullable: true,
            metadata: nil,
            data: nil,
            length: nil,
            offset: nil
          }
        ]},
       nullable: true,
       metadata: nil,
       data: [#Reference<0.2903375071.2569404447.12019>],
       length: nil,
       offset: nil
     }
   ]
 }}

[josevalim]

It would be nice to standardize in one or the other. Perhaps the type: :struct is the one that is not actually used and the typespecs are out of date? But in general we should not have two representations for structs. :)

[josevalim]

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