External collections, via Calcite

[julianhyde]

Currently lists are always ML lists, but this feature would allow lists via a "table" API.

For example, a list could be based on the Emps table in a JDBC data source. Evaluating that list in ML would entail preparing a JDBC query.

We will use Apache Calcite's Table API to represent an external collection, and RelBuilder to build relational expressions.

A schema (in the JDBC sense of the word) would be an ML record. For example, hr would be a value with emps and depts fields, and you could write

from e in (#emps hr), d in (#depts hr) where #deptno e = #deptno d

We would need some API to create the hr value.

[julianhyde]

Fixed in 2acb5ca. When you start the shell, there are now collections called scott and foodmart backed by JDBC databases. And you can add your own collection based on anything in Calcite. For example:

= scott;
val it = {bonus=<relation>,dept=<relation>,emp=<relation>,salgrade=<relation>} :
{bonus:{comm:real, ename:string, job:string, sal:real} list,
dept:{deptno:int, dname:string, loc:string} list,
emp:{comm:real, deptno:int, empno:int, ename:string, hiredate:string, job:string, mgr:int, sal:real} list,
salgrade:{grade:int, hisal:real, losal:real} list}
= from e in #emp scott where #deptno e = 10 yield #ename e;
val it = ["CLARK","KING","MILLER"] : string list