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Type Aliases

In a Substrait plan, types are spelled out whenever and wherever they are needed. For parameterized types, all type parameters are spelled out per type reference. For a parameterized type with a large number of parameters, complex nested type parameters, or string parameters, this can significantly bloat the size of the plan proportional to the number of such type references in the plan as such type parameters are repeatedly spelled out.

To alleviate the problem, Substrait offers a type alias mechanism.

Type aliases allow a plan to declare a type once and reference it multiple times within a plan. A type alias can be used wherever a type is expected.

Type Alias

A type alias is a mapping from an anchor to a concrete Substrait type. A valid type alias is described below.

  • All type parameters must be specified.
  • Cannot directly be another alias.
  • Type parameters can reference other aliased types as long as no circular dependencies are introduced.
  • Nullability of aliased type is ignored. Nullability must be specified when the aliased type is referenced.
  • Type variation may be specified in the aliased type.

Type Alias Reference

A type alias reference is a Substrait type and can appear wherever a Substrait type is expected. The reference must specify the nullability of the aliased type.

Examples

type alias 1 --> VARCHAR<100> // OK to alias Substrait VARCHAR.
type alias 2 --> UserDefined<i8, type alias ref (1, NULLABLE)> // OK to reference other type alias 1. UserDefined<i8, VARCHAR<100>?>.
type alias 3 --> UserDefined<i8, type alias ref (2, REQUIRED)> // OK to reference other type alias 1. UserDefined<i8, VARCHAR<100>>.

type alias 4 --> type alias ref (1, NULLABLE)  // NOT OK to alias another alias directly.
type alias 5 --> STRUCT<i8, type alias ref (0, NULLABLE)> // NOT OK to reference an undefined type alias 0.
type alias 6 --> STRUCT<type alias ref (6, REQUIRED) // NOT OK to reference itself.
type alias 7 --> STRUCT<i8, type alias ref (8, REQUIRED)>
type alias 8 --> STRUCT<type alias ref (7, REQUIRED)> // NOT OK because type alias 7 and 8 have a circular dependency.