Support unparsing UNION for distinct results (#75)

UPSTREAM NOTE: This was submitted upstream:
apache#15814 and will be in DF 48

Author: phillipleblanc

datafusion/sql/src/unparser/ast.rs

@@ -32,6 +32,8 @@ pub struct QueryBuilder {
     fetch: Option<ast::Fetch>,
     locks: Vec<ast::LockClause>,
     for_clause: Option<ast::ForClause>,
+    // If true, we need to unparse LogicalPlan::Union as a SQL `UNION` rather than a `UNION ALL`.
+    distinct_union: bool,
 }
 
 #[allow(dead_code)]
@@ -78,6 +80,13 @@ impl QueryBuilder {
         self.for_clause = value;
         self
     }
+    pub fn distinct_union(&mut self) -> &mut Self {
+        self.distinct_union = true;
+        self
+    }
+    pub fn is_distinct_union(&self) -> bool {
+        self.distinct_union
+    }
     pub fn build(&self) -> Result<ast::Query, BuilderError> {
         let order_by = self
             .order_by_kind
@@ -115,6 +124,7 @@ impl QueryBuilder {
             fetch: Default::default(),
             locks: Default::default(),
             for_clause: Default::default(),
+            distinct_union: false,
         }
     }
 }

datafusion/sql/src/unparser/plan.rs

@@ -605,6 +605,23 @@ impl Unparser<'_> {
                         false,
                     );
                 }
+
+                // If this distinct is the parent of a Union and we're in a query context,
+                // then we need to unparse as a `UNION` rather than a `UNION ALL`.
+                if let Distinct::All(input) = distinct {
+                    if matches!(input.as_ref(), LogicalPlan::Union(_)) {
+                        if let Some(query_mut) = query.as_mut() {
+                            query_mut.distinct_union();
+                            return self.select_to_sql_recursively(
+                                input.as_ref(),
+                                query,
+                                select,
+                                relation,
+                            );
+                        }
+                    }
+                }
+
                 let (select_distinct, input) = match distinct {
                     Distinct::All(input) => (ast::Distinct::Distinct, input.as_ref()),
                     Distinct::On(on) => {
@@ -889,14 +906,23 @@ impl Unparser<'_> {
                     return internal_err!("UNION operator requires at least 2 inputs");
                 }
 
+                let set_quantifier =
+                    if query.as_ref().is_some_and(|q| q.is_distinct_union()) {
+                        // Setting the SetQuantifier to None will unparse as a `UNION`
+                        // rather than a `UNION ALL`.
+                        ast::SetQuantifier::None
+                    } else {
+                        ast::SetQuantifier::All
+                    };
+
                 // Build the union expression tree bottom-up by reversing the order
                 // note that we are also swapping left and right inputs because of the rev
                 let union_expr = input_exprs
                     .into_iter()
                     .rev()
                     .reduce(|a, b| SetExpr::SetOperation {
                         op: ast::SetOperator::Union,
-                        set_quantifier: ast::SetQuantifier::All,
+                        set_quantifier,
                         left: Box::new(b),
                         right: Box::new(a),
                     })

datafusion/sql/tests/cases/plan_to_sql.rs

@@ -172,6 +172,13 @@ fn roundtrip_statement() -> Result<()> {
                 UNION ALL
                 SELECT j3_string AS col1, j3_id AS id FROM j3
             ) AS subquery GROUP BY col1, id ORDER BY col1 ASC, id ASC"#,
+            r#"SELECT col1, id FROM (
+                SELECT j1_string AS col1, j1_id AS id FROM j1
+                UNION
+                SELECT j2_string AS col1, j2_id AS id FROM j2
+                UNION
+                SELECT j3_string AS col1, j3_id AS id FROM j3
+            ) AS subquery ORDER BY col1 ASC, id ASC"#,
             "SELECT id, count(*) over (PARTITION BY first_name ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING),
             last_name, sum(id) over (PARTITION BY first_name ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING),
             first_name from person",