Support external type systems in DDL

Many connectors and formats require supporting external data types. Postgres users request UUID support, Avro users require enum support, etc.

FLINK-19869 implemented support for Postgres UUIDs poorly and even impacts performance with regular strings.

The long-term solution should be user-defined types in Flink. This is however a bigger effort that requires a FLIP and a bigger amount of resources.

As a mid-term solution, we should offer a consistent approach based on DDL options that allows to define a mapping from Flink type system to the external type system. I suggest the following:

CREATE TABLE MyTable (
...
) WITH(
  'mapping.data-types' = '<Flink field name>: <External field data type>'
)

The mapping defines a map from Flink data type to external data type. The external data type should be string parsable. This works for most connectors and formats (e.g. Avro schema string).

Examples:

CREATE TABLE MyTable (
  regular_col STRING,
  uuid_col STRING,
  point_col ARRAY<DOUBLE>,
  box_col ARRAY<ARRAY<DOUBLE>>
) WITH(
  'mapping.data-types' = 'uuid_col: uuid, point_col: point, box_col: box'
)

We provide a table of supported mapping data types. E.g. the point type is always maped to ARRAY<DOUBLE>. In general we choose a data type in Flink that comes closest to the required functionality.

Future work:

In theory, we can also offer mapping of field names. It might be a requirement that Flink's column name is different from the external system's one.

CREATE TABLE MyTable (
...
) WITH(
  'mapping.names' = '<Flink field name>: <External field name>'
)