In 'from' clause, allow initializers to reference previous variables

[julianhyde]

In 'from' clause, allow initializers to reference previous variables. For example:

from word in ["hello", "world"],
    c in String_explode word
  yield c;

The initializer of c is String_explode word, which references the word variable defined in the outer loop.

Comparison with SQL

In SQL, this is a called a lateral join.

SQL example 1. Use the LATERAL keyword to signal that a sub-query in the FROM clause can reference other variables:

SELECT *
FROM orders AS o,
 LATERAL (SELECT * FROM lineItems AS i WHERE i.orderId = o.id)

Note that the sub-query references o. Without LATERAL, that would be illegal.

SQL example 2. The UNNEST function is implicitly lateral:

SELECT *
FROM orders AS o,
  LATERAL UNNEST(o.lineItems) AS i

SQL example 3. Use the CROSS APPLY syntax to call a table function, again, implicitly lateral:

SELECT *
FROM emps AS e
  CROSS APPLY findDependents(e.empno)

Efficient implementation?

Should we use an efficient implementation, so that if the initializer does not reference previous variables, it is evaluated at the start?

No. If the user wants that efficiency, they can assign the initializer to a variable. For example, in

from i in String_explode "abc",
    j in String_explode "xy";

i's initializer String_explode "abc" should be called once, and j's initializer String_explode "xy" should be called 3 times. But in

from i in String_explode "",
    j in String_explode "xy";

i's initializer String_explode "" should be called once, and j's initializer String_explode "xy" should be called 0 times. In

let
  val i0 = String_explode "abc"
  val j0 = String_explode "xy"
in
  from i in i0,
      j in j0
end

each String_explode is evaluated once.

[julianhyde]

Fixed in 7397c39.