Unify schema usage in Datafusion

[comphead]

Sometimes DF uses 2 schema sources, which is schema from previous node and current like in example(s) below.

Would be nice to check if we can use only 1 schema, multiple schemas are usually confusing

          I agree it doesn't make sense -- however, it is the same logic as in `with_new_exprs`:

datafusion/datafusion/expr/src/logical_plan/plan.rs

Lines 913 to 925 in fad16e7

	LogicalPlan::Union(Union { schema, .. }) => {
	let input_schema = inputs[0].schema();
	// If inputs are not pruned do not change schema.
	let schema = if schema.fields().len() == input_schema.fields().len() {
	schema.clone()
	} else {
	input_schema.clone()
	};
	Ok(LogicalPlan::Union(Union {
	inputs: inputs.into_iter().map(Arc::new).collect(),
	schema,
	}))
	}

Originally posted by @alamb in #10405 (comment)

[yyy1000]

When I try to fix this, I found that there may be a latent issue.
I want to open #10494 to discuss it.