Schema Inference On Non-existent Attributes in Strict Typing Mode

[am357]

According to the findings by the experimental implementation work for Schema Inferencer In PartiQL Rust the following questions need to get answered by the PartiQL Type System spec. work:

Case 1

Let customers be a closed Schema. In Permissive typing mode what should be the output schema of the following query?

-- customers: <<{'id': INT, 'name': STRING>>
SELECT customers.id, customers.name, customers.age FROM customers;

Some Intuitive options:

1. User MISSING as in <<{'id': INT, 'name': STRING, 'age': MISSING}>>—this appears to be wrong because it conflicts with Tuple constructors as it has been defined by PartiQL Specification — Section 6.1.1, 2019
2. Use NULL as in <<{'id': INT, 'name': STRING, 'age': NULL}>>—this can be a viable option; this allows to further perform the schema inference if the derived schema is dependent on this schema (E.g., in a sub-query).
3. Elide the attribute as in <<{'id': INT, 'name': STRING}>>—this can also be a valid option but it may result in some information loss if this schema is being used to further inferring the schema of the query.

For now, we decide to go with the NULL option pending RFC specification.

See @jpschorr comment: partiql/partiql-lang-rust#399