Which issue does this PR close?

The DataFusion SQL Unparser does not correctly roundtrip SQL queries that use UNION rather than UNION ALL. For example the following query:

SELECT col1 FROM footable
UNION
SELECT col1 FROM bartable

Should result in a query that filters out duplicate rows from the final result. DataFusion handles this by adding a LogicalPlan::Distinct node as the parent of the LogicalPlan::Union node.

Distinct:
  Union:
    TableScan
    TableScan

However, this is currently unparsed to the following SQL:

SELECT col1 FROM footable
UNION ALL
SELECT col1 FROM bartable

That will cause incorrect results when executed, because the duplicate rows will not be filtered out.

• Closes #.

Rationale for this change

Adds support for correctly round-tripping SQL queries that rely on the distinct properties of the UNION set operator.

What changes are included in this PR?

Keeps track of whether a Distinct node is the direct parent of a Union node in the Unparser's QueryBuilder. If so, then when the Union node is being processed the SetModified is set to None rather than All. That will cause the sqlparser AST to correctly emit the UNION SQL operator.

Are these changes tested?

Yes, I've added a new SQL roundtrip test that would have failed previously.

Are there any user-facing changes?

No