Arrangements created through casts duplicate data

[frankmcsherry]

Feature request

An index that uses an expression that casts the data, for example here from varchar to text, will maintain the payload twice: in both the key (as text) and in the value (as varchar).

CREATE INDEX t1_a_cast ON t1(CAST(a AS text));

Our mechanisms for going from row to (key, val) and back to row only allow projections for row -> val and [key, val] -> row, preventing the use of more general MirScalarExpr variants, including casts. More generally there are other crazy inverses one could apply (if the key is x/2, you could keep only the bit sheared off as the residual value), but realistically the casts are the clearest example of current problem and opportunity.

[teskje]

@mgree For your awareness. Could the mirtype design help with this issue as well?

[mgree]

mirtype would let us eliminate the cast wholesale, so... yes!