Move SortExec partition check to constructor

[alamb]

Note: migrated from original JIRA: https://issues.apache.org/jira/browse/ARROW-11625

SortExec has the following error check at execution time and this could be moved into the try_new constructor so the error check happens at planning time instead.

 
{code:java}
if 1 != self.input.output_partitioning().partition_count() {
return Err(DataFusionError::Internal(
"SortExec requires a single input partition".to_owned(),
));
} {code}

[alamb]

Comment from Hendrik Makait(hendrik.makait) @ 2021-02-14T17:46:12.067+0000:

I'd love to check that out tomorrow.

Comment from Hendrik Makait(hendrik.makait) @ 2021-02-15T20:05:17.782+0000:

Moving this check into the constructor leads to failing tests. As far as I can see, this is because the planner does an optimization step that inserts a merge for children that contain multiple partitions.
{code:java}
match plan.required_child_distribution() {
    Distribution::UnspecifiedDistribution => plan.with_new_children(children),
    Distribution::SinglePartition => plan.with_new_children(
        children
            .iter()
            .map(|child| {
                if child.output_partitioning().partition_count() == 1 {
                    child.clone()
                } else {
                    Arc::new(MergeExec::new(child.clone()))
                }
            })
            .collect(),
    ),
}
{code}
 What's the reason for moving this check into planning time? How should I proceed?

Comment from Andy Grove(andygrove) @ 2021-02-16T14:44:11.773+0000:

I see. Ok, maybe we can't do this at planning time then.