Table Function Syntax

[mhayes853]

I currently have a small helper package that supports SQLite Vec functions, and I would like to add support for vec_each which is a table function in SQLite Vec.

Another discussion a couple months ago brought up json_each which is also in the same boat, though the discussion didn't really propose the kind of general solution that I think would be ideal.

In particular, the limiting factor seems to stem from the fact that the Table protocol is quite static in nature, and can't have it's tableFragment altered dynamically since it's a static requirement which prevents any sort of dynamic value to be bound.

As a result, creating this statement from Swift.

SELECT * FROM vec_each(<some vector here>);

Without resorting to escaping via #sql isn't really possible.

Ideally, a general solution here would be non-static and also be able to be usable for similar concepts in other SQL dialects (eg. Table Value Functions in SQL Server). Something like:

public protocol TableFragmentExpression<QueryValue>: QueryExpression where QueryValue: Table {
  var tableFragment: QueryFragment { get }
}

Then for vec_each:

@Table
@Selection
public struct VecEachRow {
  public let rowId: Int
  public let value: Float

  // ...
}

public struct VecEachExpression: TableFragmentExpression {
  public typealias QueryValue = VecEachRow
  public let vector: [Float]

  public var tableFragment: QueryFragment {
    "vec_each(\(vector))"
  }
}

public func vecEach(_ vector: [Float]) -> VecEachExpression {
  // ...
}

// Would need dedicated overloads on TableFragmentSelection to support all the select syntax that `Table` gets.
let query = vecEach(vector).all
  .where { /* ... */ }
  .select { /* ... */ }