Support index acess for Row Type

Today we only support access by field for Row Type. For row types without field types, we have to first cast it into a Row with field names:

presto > select CAST(v AS ROW(f1 BIGINT, f2 VARCHAR)).f1 
from (VALUES ROW(ROW(1, 'a')), ROW(ROW(2, 'b'))) AS t(v);

 _col0
-------
     1
     2
(2 rows)

It will be nice to support index access, i.e. v[1] will return the first field of the row.

[electrum]

Index access seems strange given that it must be constant. It’s really a tuple, not an array. What about generating well-known synthetic field names? For example, f1, f2, or c1, c2, etc. This makes it obvious they are constant field accesses and not array accesses.

[geraint0923]

Good idea! But I think we should only allow constant index, otherwise it will be hard to infer the field type.