Support `UNNEST` as table function (UDTF)

[waynexia]

Is your feature request related to a problem or challenge?

UNNEST is implemented in two ways, SELECT UNNEST(...) is handled with other functions

datafusion/datafusion/sql/src/expr/function.rs

Lines 246 to 255 in 84232d8

	// Build Unnest expression
	if name.eq("unnest") {
	let mut exprs = self.function_args_to_expr(args, schema, planner_context)?;
	if exprs.len() != 1 {
	return plan_err!("unnest() requires exactly one argument");
	}
	let expr = exprs.swap_remove(0);
	Self::check_unnest_arg(&expr, schema)?;
	return Ok(Expr::Unnest(Unnest::new(expr)));
	}

And SELECT ... FROM UNNEST(...) is provided as TableFactory

datafusion/datafusion/sql/src/relation/mod.rs

Lines 117 to 123 in 84232d8

	TableFactor::UNNEST {
	alias,
	array_exprs,
	with_offset: false,
	with_offset_alias: None,
	with_ordinality,
	} => {

But those two ways are both non-generic, cases like substrait can't handle it properly, as the decoder will try to look for table function:

async fn roundtrip_unnest() -> Result<()> {
    roundtrip("SELECT * FROM unnest([1, 2, 3])").await
}

Describe the solution you'd like

Implement UNNEST as a table function, making it more general and reduce the complexity (and branches) of implementation

Describe alternatives you've considered

No response

Additional context

No response

[jonahgao]

That's a great proposal; I plan to try it.

[waynexia]

Thank you @jonahgao 🚀