# Unwrap

Inverse of wrap

## Signature

unwrap(operand: Relation, tva: AttrName) -> Relation

## Examples

unwrap(wrap(suppliers, [:city, :status], :extra), :extra)

## Description

Computes the relation obtained by unwrapping a tuple-valued attribute `tva`.
Unwrapping of `tva` on an input tuple `t` consists in removing `tva` from
`t` and extending the result with `tva`'s own attributes:

Tuple(x: 1, tva: Tuple(y: 2, z: 3)) -> Tuple(x: 1, y: 2, z: 3)

`tva` must be a tuple-valued attribute. It should not itself contain any
attribute whose name clashes with an attribute of `operand`. In the examples
above, `tva` may not contain an attribute called `x`.

## Implementation notes

This operator does not compile to SQL so far. Contributions are welcome to
provide it with a SQL compilation for SQL DBMSs that support this kind of
feature (e.g. PostgreSQL with JSON data type)