Reuse alias if possible

datafusion/expr/src/expr.rs

@@ -490,6 +490,20 @@ impl Alias {
         }
     }
 
+    /// Create an alias with an optional schema/field qualifier.
+    pub fn new_boxed(
+        expr: Box<Expr>,
+        relation: Option<impl Into<TableReference>>,
+        name: impl Into<String>,
+    ) -> Self {
+        Self {
+            expr,
+            relation: relation.map(|r| r.into()),
+            name: name.into(),
+            metadata: None,
+        }
+    }
+
     pub fn with_metadata(
         mut self,
         metadata: Option<std::collections::HashMap<String, String>>,
@@ -1385,8 +1399,23 @@ impl Expr {
     }
 
     /// Return `self AS name` alias expression
+    ///
+    /// # Example
+    /// ```
+    /// # use datafusion_expr::col;
+    /// let expr = col("foo").alias("bar");
+    /// assert_eq!(expr.to_string(), "foo AS bar");
+    ///
+    /// // when aliasing over the exising alias, the previous one is removed
+    /// let expr = col("foo").alias("bar").alias("baz");
+    /// assert_eq!(expr.to_string(), "foo AS baz");
     pub fn alias(self, name: impl Into<String>) -> Expr {
-        Expr::Alias(Alias::new(self, None::<&str>, name.into()))
+        if let Expr::Alias(Alias { expr, .. }) = self {
+            // reuse the existing layer if possible
+            Expr::Alias(Alias::new_boxed(expr, None::<&str>, name.into()))
+        } else {
+            Expr::Alias(Alias::new(self, None::<&str>, name.into()))
+        }
     }
 
     /// Return `self AS name` alias expression with metadata
@@ -1416,7 +1445,12 @@ impl Expr {
         relation: Option<impl Into<TableReference>>,
         name: impl Into<String>,
     ) -> Expr {
-        Expr::Alias(Alias::new(self, relation, name.into()))
+        if let Expr::Alias(Alias { expr, .. }) = self {
+            // reuse the existing layer if possible
+            Expr::Alias(Alias::new_boxed(expr, relation, name.into()))
+        } else {
+            Expr::Alias(Alias::new(self, relation, name.into()))
+        }
     }
 
     /// Return `self AS name` alias expression with a specific qualifier and metadata

datafusion/optimizer/tests/optimizer_integration.rs

@@ -310,7 +310,7 @@ fn eliminate_redundant_null_check_on_count() {
         GROUP BY col_int32
         HAVING c IS NOT NULL";
     let plan = test_sql(sql).unwrap();
-    let expected = "Projection: test.col_int32, count(Int64(1)) AS count(*) AS c\
+    let expected = "Projection: test.col_int32, count(Int64(1)) AS c\
     \n  Aggregate: groupBy=[[test.col_int32]], aggr=[[count(Int64(1))]]\
     \n    TableScan: test projection=[col_int32]";
     assert_eq!(expected, format!("{plan}"));

datafusion/sql/src/unparser/plan.rs

@@ -371,29 +371,33 @@ impl Unparser<'_> {
                 // Projection can be top-level plan for unnest relation
                 // The projection generated by the `RecursiveUnnestRewriter` from a UNNEST relation will have
                 // only one expression, which is the placeholder column generated by the rewriter.
-                let unnest_input_type = if p.expr.len() == 1 {
+                let unnest_params = if p.expr.len() == 1 {
                     Self::check_unnest_placeholder_with_outer_ref(&p.expr[0])
                 } else {
                     None
                 };
-                if self.dialect.unnest_as_table_factor() && unnest_input_type.is_some() {
-                    if let LogicalPlan::Unnest(unnest) = &p.input.as_ref() {
-                        if let Some(unnest_relation) =
-                            self.try_unnest_to_table_factor_sql(unnest)?
-                        {
-                            relation.unnest(unnest_relation);
-                            return self.select_to_sql_recursively(
-                                p.input.as_ref(),
-                                query,
-                                select,
-                                relation,
-                            );
+                if self.dialect.unnest_as_table_factor() {
+                    if let Some((_, unnest_alias)) = &unnest_params {
+                        if let LogicalPlan::Unnest(unnest) = &p.input.as_ref() {
+                            if let Some(unnest_relation) =
+                                self.try_unnest_to_table_factor_sql(unnest, unnest_alias)?
+                            {
+                                relation.unnest(unnest_relation);
+                                return self.select_to_sql_recursively(
+                                    p.input.as_ref(),
+                                    query,
+                                    select,
+                                    relation,
+                                );
+                            }
                         }
                     }
                 }
 
                 // Projection can be top-level plan for derived table
                 if select.already_projected() {
+                    let unnest_input_type = unnest_params.map(|(t, _)| t);
+
                     return self.derive_with_dialect_alias(
                         "derived_projection",
                         plan,
@@ -906,15 +910,22 @@ impl Unparser<'_> {
     /// - If the column is not a placeholder column, return [None].
     ///
     /// `outer_ref` is the display result of [Expr::OuterReferenceColumn]
-    fn check_unnest_placeholder_with_outer_ref(expr: &Expr) -> Option<UnnestInputType> {
-        if let Expr::Alias(Alias { expr, .. }) = expr {
+    fn check_unnest_placeholder_with_outer_ref(
+        expr: &Expr,
+    ) -> Option<(UnnestInputType, &str)> {
+        if let Expr::Alias(Alias {
+            expr,
+            name: alias_name,
+            ..
+        }) = expr
+        {
             if let Expr::Column(Column { name, .. }) = expr.as_ref() {
                 if let Some(prefix) = name.strip_prefix(UNNEST_PLACEHOLDER) {
                     if prefix.starts_with(&format!("({}(", OUTER_REFERENCE_COLUMN_PREFIX))
                     {
-                        return Some(UnnestInputType::OuterReference);
+                        return Some((UnnestInputType::OuterReference, alias_name));
                     }
-                    return Some(UnnestInputType::Scalar);
+                    return Some((UnnestInputType::Scalar, alias_name));
                 }
             }
         }
@@ -924,12 +935,15 @@ impl Unparser<'_> {
     fn try_unnest_to_table_factor_sql(
         &self,
         unnest: &Unnest,
+        alias: &str,
     ) -> Result<Option<UnnestRelationBuilder>> {
         let mut unnest_relation = UnnestRelationBuilder::default();
         let LogicalPlan::Projection(projection) = unnest.input.as_ref() else {
             return Ok(None);
         };
 
+        unnest_relation.alias(Some(self.new_table_alias(alias.to_string(), vec![])));
+
         if !matches!(projection.input.as_ref(), LogicalPlan::EmptyRelation(_)) {
             // It may be possible that UNNEST is used as a source for the query.
             // However, at this point, we don't yet know if it is just a single expression

datafusion/sql/tests/cases/plan_to_sql.rs

@@ -572,7 +572,7 @@ fn roundtrip_statement_with_dialect() -> Result<()> {
         },
         TestStatementWithDialect {
             sql: "SELECT * FROM UNNEST([1,2,3])",
-            expected: r#"SELECT UNNEST(make_array(Int64(1),Int64(2),Int64(3))) FROM UNNEST([1, 2, 3])"#,
+            expected: r#"SELECT UNNEST(make_array(Int64(1),Int64(2),Int64(3))) FROM UNNEST([1, 2, 3]) AS UNNEST(make_array(Int64(1),Int64(2),Int64(3)))"#,
             parser_dialect: Box::new(GenericDialect {}),
             unparser_dialect: Box::new(CustomDialectBuilder::default().with_unnest_as_table_factor(true).build()),
         },
@@ -590,7 +590,7 @@ fn roundtrip_statement_with_dialect() -> Result<()> {
         },
         TestStatementWithDialect {
             sql: "SELECT * FROM UNNEST([1,2,3]), j1",
-            expected: r#"SELECT UNNEST(make_array(Int64(1),Int64(2),Int64(3))), j1.j1_id, j1.j1_string FROM UNNEST([1, 2, 3]) CROSS JOIN j1"#,
+            expected: r#"SELECT UNNEST(make_array(Int64(1),Int64(2),Int64(3))), j1.j1_id, j1.j1_string FROM UNNEST([1, 2, 3]) AS UNNEST(make_array(Int64(1),Int64(2),Int64(3))) CROSS JOIN j1"#,
             parser_dialect: Box::new(GenericDialect {}),
             unparser_dialect: Box::new(CustomDialectBuilder::default().with_unnest_as_table_factor(true).build()),
         },
@@ -608,7 +608,7 @@ fn roundtrip_statement_with_dialect() -> Result<()> {
         },
         TestStatementWithDialect {
             sql: "SELECT UNNEST([1,2,3])",
-            expected: r#"SELECT * FROM UNNEST([1, 2, 3])"#,
+            expected: r#"SELECT * FROM UNNEST([1, 2, 3]) AS UNNEST(make_array(Int64(1),Int64(2),Int64(3)))"#,
             parser_dialect: Box::new(GenericDialect {}),
             unparser_dialect: Box::new(CustomDialectBuilder::default().with_unnest_as_table_factor(true).build()),
         },
@@ -626,7 +626,7 @@ fn roundtrip_statement_with_dialect() -> Result<()> {
         },
         TestStatementWithDialect {
             sql: "SELECT * FROM unnest_table u, UNNEST(u.array_col)",
-            expected: r#"SELECT u.array_col, u.struct_col, UNNEST(outer_ref(u.array_col)) FROM unnest_table AS u CROSS JOIN UNNEST(u.array_col)"#,
+            expected: r#"SELECT u.array_col, u.struct_col, UNNEST(outer_ref(u.array_col)) FROM unnest_table AS u CROSS JOIN UNNEST(u.array_col) AS UNNEST(outer_ref(u.array_col))"#,
             parser_dialect: Box::new(GenericDialect {}),
             unparser_dialect: Box::new(CustomDialectBuilder::default().with_unnest_as_table_factor(true).build()),
         },
@@ -638,7 +638,7 @@ fn roundtrip_statement_with_dialect() -> Result<()> {
         },
         TestStatementWithDialect {
             sql: "SELECT unnest([1, 2, 3, 4]) from unnest([1, 2, 3]);",
-            expected: r#"SELECT UNNEST([1, 2, 3, 4]) AS UNNEST(make_array(Int64(1),Int64(2),Int64(3),Int64(4))) FROM UNNEST([1, 2, 3])"#,
+            expected: r#"SELECT UNNEST([1, 2, 3, 4]) AS UNNEST(make_array(Int64(1),Int64(2),Int64(3),Int64(4))) FROM UNNEST([1, 2, 3]) AS UNNEST(make_array(Int64(1),Int64(2),Int64(3)))"#,
             parser_dialect: Box::new(GenericDialect {}),
             unparser_dialect: Box::new(CustomDialectBuilder::default().with_unnest_as_table_factor(true).build()),
         },

datafusion/sqllogictest/test_files/count_star_rule.slt

@@ -59,7 +59,7 @@ query TT
 EXPLAIN SELECT t1.a, COUNT() AS cnt FROM t1 GROUP BY t1.a HAVING COUNT() > 0;
 ----
 logical_plan
-01)Projection: t1.a, count(Int64(1)) AS count() AS cnt
+01)Projection: t1.a, count(Int64(1)) AS cnt
 02)--Filter: count(Int64(1)) > Int64(0)
 03)----Aggregate: groupBy=[[t1.a]], aggr=[[count(Int64(1))]]
 04)------TableScan: t1 projection=[a]
@@ -83,7 +83,7 @@ query TT
 EXPLAIN SELECT a, COUNT() OVER (PARTITION BY a) AS count_a FROM t1;
 ----
 logical_plan
-01)Projection: t1.a, count(Int64(1)) PARTITION BY [t1.a] ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING AS count() PARTITION BY [t1.a] ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING AS count_a
+01)Projection: t1.a, count(Int64(1)) PARTITION BY [t1.a] ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING AS count_a
 02)--WindowAggr: windowExpr=[[count(Int64(1)) PARTITION BY [t1.a] ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING]]
 03)----TableScan: t1 projection=[a]
 physical_plan

datafusion/sqllogictest/test_files/group_by.slt

@@ -4188,7 +4188,7 @@ EXPLAIN SELECT SUM(DISTINCT CAST(x AS DOUBLE)), MAX(DISTINCT CAST(x AS DOUBLE))
 logical_plan
 01)Projection: sum(alias1) AS sum(DISTINCT t1.x), max(alias1) AS max(DISTINCT t1.x)
 02)--Aggregate: groupBy=[[t1.y]], aggr=[[sum(alias1), max(alias1)]]
-03)----Aggregate: groupBy=[[t1.y, __common_expr_1 AS t1.x AS alias1]], aggr=[[]]
+03)----Aggregate: groupBy=[[t1.y, __common_expr_1 AS alias1]], aggr=[[]]
 04)------Projection: CAST(t1.x AS Float64) AS __common_expr_1, t1.y
 05)--------TableScan: t1 projection=[x, y]
 physical_plan

datafusion/sqllogictest/test_files/insert.slt

@@ -171,7 +171,7 @@ logical_plan
 01)Dml: op=[Insert Into] table=[table_without_values]
 02)--Projection: a1 AS a1, a2 AS a2
 03)----Sort: aggregate_test_100.c1 ASC NULLS LAST
-04)------Projection: sum(aggregate_test_100.c4) PARTITION BY [aggregate_test_100.c1] ORDER BY [aggregate_test_100.c9 ASC NULLS LAST] ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING AS a1, count(Int64(1)) PARTITION BY [aggregate_test_100.c1] ORDER BY [aggregate_test_100.c9 ASC NULLS LAST] ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING AS count(*) PARTITION BY [aggregate_test_100.c1] ORDER BY [aggregate_test_100.c9 ASC NULLS LAST] ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING AS a2, aggregate_test_100.c1
+04)------Projection: sum(aggregate_test_100.c4) PARTITION BY [aggregate_test_100.c1] ORDER BY [aggregate_test_100.c9 ASC NULLS LAST] ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING AS a1, count(Int64(1)) PARTITION BY [aggregate_test_100.c1] ORDER BY [aggregate_test_100.c9 ASC NULLS LAST] ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING AS a2, aggregate_test_100.c1
 05)--------WindowAggr: windowExpr=[[sum(CAST(aggregate_test_100.c4 AS Int64)) PARTITION BY [aggregate_test_100.c1] ORDER BY [aggregate_test_100.c9 ASC NULLS LAST] ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING, count(Int64(1)) PARTITION BY [aggregate_test_100.c1] ORDER BY [aggregate_test_100.c9 ASC NULLS LAST] ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING]]
 06)----------TableScan: aggregate_test_100 projection=[c1, c4, c9]
 physical_plan

datafusion/sqllogictest/test_files/union.slt

@@ -491,7 +491,7 @@ EXPLAIN
 logical_plan
 01)Limit: skip=0, fetch=3
 02)--Union
-03)----Projection: count(Int64(1)) AS count(*) AS cnt
+03)----Projection: count(Int64(1)) AS cnt
 04)------Limit: skip=0, fetch=3
 05)--------Aggregate: groupBy=[[]], aggr=[[count(Int64(1))]]
 06)----------SubqueryAlias: a
@@ -650,7 +650,7 @@ select x, y from (select 1 as x , max(10) as y) b
 ----
 logical_plan
 01)Union
-02)--Projection: count(Int64(1)) AS count(*) AS count, a.n
+02)--Projection: count(Int64(1)) AS count, a.n
 03)----Aggregate: groupBy=[[a.n]], aggr=[[count(Int64(1))]]
 04)------SubqueryAlias: a
 05)--------Projection: Int64(5) AS n

datafusion/sqllogictest/test_files/unnest.slt

@@ -595,7 +595,7 @@ explain select unnest(unnest(column3)), column3 from recursive_unnest_table;
 ----
 logical_plan
 01)Unnest: lists[] structs[__unnest_placeholder(UNNEST(recursive_unnest_table.column3))]
-02)--Projection: __unnest_placeholder(recursive_unnest_table.column3,depth=1) AS UNNEST(recursive_unnest_table.column3) AS __unnest_placeholder(UNNEST(recursive_unnest_table.column3)), recursive_unnest_table.column3
+02)--Projection: __unnest_placeholder(recursive_unnest_table.column3,depth=1) AS __unnest_placeholder(UNNEST(recursive_unnest_table.column3)), recursive_unnest_table.column3
 03)----Unnest: lists[__unnest_placeholder(recursive_unnest_table.column3)|depth=1] structs[]
 04)------Projection: recursive_unnest_table.column3 AS __unnest_placeholder(recursive_unnest_table.column3), recursive_unnest_table.column3
 05)--------TableScan: recursive_unnest_table projection=[column3]

datafusion/sqllogictest/test_files/window.slt

@@ -1764,7 +1764,7 @@ EXPLAIN SELECT count(*) as global_count FROM
                   ORDER BY c1 ) AS a
 ----
 logical_plan
-01)Projection: count(Int64(1)) AS count(*) AS global_count
+01)Projection: count(Int64(1)) AS global_count
 02)--Aggregate: groupBy=[[]], aggr=[[count(Int64(1))]]
 03)----SubqueryAlias: a
 04)------Projection: