External table with complex order expression not allowed

[pepijnve]

Describe the bug

At https://datafusion.apache.org/user-guide/sql/ddl.html#example-with-order-clause the documentation states

This applies even if the expression used for sorting is complex, allowing for greater flexibility.

and then an example is given using

WITH ORDER (c2 ASC, c5 + c8 DESC NULL FIRST)

If you actually try to run this you get an error stating

Expected single column reference in sort_order[0][0], got c5 + c8

I've traced the origin of this check all the way back to a change by @alamb for #4170

To Reproduce

Run a query on the table from the example at https://datafusion.apache.org/user-guide/sql/ddl.html#example-with-order-clause

Expected behavior

The example works

Additional context

No response

[alamb]

I don't remember any particular reason the expressions are limited to constants -- our sort tracking infrastructure maybe wasn't sophisticated enough back then.

Or maybe we just didn't write the necessary tests.

[pepijnve]

Ok, if there's no intrinsic reason for the restriction then I'll have a go at relaxing the constraint.

[pepijnve]

take