Implement DISTINCT ON from Postgres

datafusion/expr/src/expr.rs

@@ -28,7 +28,7 @@ use crate::Operator;
 use crate::{aggregate_function, ExprSchemable};
 use arrow::datatypes::DataType;
 use datafusion_common::tree_node::{Transformed, TreeNode};
-use datafusion_common::{internal_err, DFSchema};
+use datafusion_common::{internal_err, DFSchema, OwnedTableReference};
 use datafusion_common::{plan_err, Column, DataFusionError, Result, ScalarValue};
 use std::collections::HashSet;
 use std::fmt;
@@ -187,13 +187,20 @@ pub enum Expr {
 #[derive(Clone, PartialEq, Eq, Hash, Debug)]
 pub struct Alias {
     pub expr: Box<Expr>,
+    pub relation: Option<OwnedTableReference>,
     pub name: String,
 }
 
 impl Alias {
-    pub fn new(expr: Expr, name: impl Into<String>) -> Self {
+    /// Create an alias with an optional schema/field qualifier.
+    pub fn new(
+        expr: Expr,
+        relation: Option<impl Into<OwnedTableReference>>,
+        name: impl Into<String>,
+    ) -> Self {
         Self {
             expr: Box::new(expr),
+            relation: relation.map(|r| r.into()),
             name: name.into(),
         }
     }
@@ -844,7 +851,27 @@ impl Expr {
                 asc,
                 nulls_first,
             }) => Expr::Sort(Sort::new(Box::new(expr.alias(name)), asc, nulls_first)),
-            _ => Expr::Alias(Alias::new(self, name.into())),
+            _ => Expr::Alias(Alias::new(self, None::<&str>, name.into())),
+        }
+    }
+
+    /// Return `self AS name` alias expression with a specific qualifier
+    pub fn alias_qualified(
+        self,
+        relation: Option<impl Into<OwnedTableReference>>,
+        name: impl Into<String>,
+    ) -> Expr {
+        match self {
+            Expr::Sort(Sort {
+                expr,
+                asc,
+                nulls_first,
+            }) => Expr::Sort(Sort::new(
+                Box::new(expr.alias_qualified(relation, name)),
+                asc,
+                nulls_first,
+            )),
+            _ => Expr::Alias(Alias::new(self, relation, name.into())),
         }
     }
 

datafusion/expr/src/expr_schema.rs

@@ -305,6 +305,13 @@ impl ExprSchemable for Expr {
                 self.nullable(input_schema)?,
             )
             .with_metadata(self.metadata(input_schema)?)),
+            Expr::Alias(Alias { relation, name, .. }) => Ok(DFField::new(
+                relation.clone(),
+                name,
+                self.get_type(input_schema)?,
+                self.nullable(input_schema)?,
+            )
+            .with_metadata(self.metadata(input_schema)?)),
             _ => Ok(DFField::new_unqualified(
                 &self.display_name()?,
                 self.get_type(input_schema)?,

datafusion/expr/src/logical_plan/builder.rs

@@ -32,8 +32,8 @@ use crate::expr_rewriter::{
     rewrite_sort_cols_by_aggs,
 };
 use crate::logical_plan::{
-    Aggregate, Analyze, CrossJoin, Distinct, EmptyRelation, Explain, Filter, Join,
-    JoinConstraint, JoinType, Limit, LogicalPlan, Partitioning, PlanType, Prepare,
+    Aggregate, Analyze, CrossJoin, Distinct, DistinctOn, EmptyRelation, Explain, Filter,
+    Join, JoinConstraint, JoinType, Limit, LogicalPlan, Partitioning, PlanType, Prepare,
     Projection, Repartition, Sort, SubqueryAlias, TableScan, Union, Unnest, Values,
     Window,
 };
@@ -551,16 +551,29 @@ impl LogicalPlanBuilder {
         let left_plan: LogicalPlan = self.plan;
         let right_plan: LogicalPlan = plan;
 
-        Ok(Self::from(LogicalPlan::Distinct(Distinct {
-            input: Arc::new(union(left_plan, right_plan)?),
-        })))
+        Ok(Self::from(LogicalPlan::Distinct(Distinct::All(Arc::new(
+            union(left_plan, right_plan)?,
+        )))))
     }
 
     /// Apply deduplication: Only distinct (different) values are returned)
     pub fn distinct(self) -> Result<Self> {
-        Ok(Self::from(LogicalPlan::Distinct(Distinct {
-            input: Arc::new(self.plan),
-        })))
+        Ok(Self::from(LogicalPlan::Distinct(Distinct::All(Arc::new(
+            self.plan,
+        )))))
+    }
+
+    /// Project first values of the specified expression list according to the provided
+    /// sorting expressions grouped by the `DISTINCT ON` clause expressions.
+    pub fn distinct_on(
+        self,
+        on_expr: Vec<Expr>,
+        select_expr: Vec<Expr>,
+        sort_expr: Option<Vec<Expr>>,
+    ) -> Result<Self> {
+        Ok(Self::from(LogicalPlan::Distinct(Distinct::On(
+            DistinctOn::try_new(on_expr, select_expr, sort_expr, Arc::new(self.plan))?,
+        ))))
     }
 
     /// Apply a join to `right` using explicitly specified columns and an

datafusion/expr/src/logical_plan/mod.rs

@@ -33,10 +33,10 @@ pub use ddl::{
 };
 pub use dml::{DmlStatement, WriteOp};
 pub use plan::{
-    Aggregate, Analyze, CrossJoin, DescribeTable, Distinct, EmptyRelation, Explain,
-    Extension, Filter, Join, JoinConstraint, JoinType, Limit, LogicalPlan, Partitioning,
-    PlanType, Prepare, Projection, Repartition, Sort, StringifiedPlan, Subquery,
-    SubqueryAlias, TableScan, ToStringifiedPlan, Union, Unnest, Values, Window,
+    Aggregate, Analyze, CrossJoin, DescribeTable, Distinct, DistinctOn, EmptyRelation,
+    Explain, Extension, Filter, Join, JoinConstraint, JoinType, Limit, LogicalPlan,
+    Partitioning, PlanType, Prepare, Projection, Repartition, Sort, StringifiedPlan,
+    Subquery, SubqueryAlias, TableScan, ToStringifiedPlan, Union, Unnest, Values, Window,
 };
 pub use statement::{
     SetVariable, Statement, TransactionAccessMode, TransactionConclusion, TransactionEnd,

datafusion/expr/src/logical_plan/plan.rs

@@ -25,8 +25,8 @@ use std::sync::Arc;
 use super::dml::CopyTo;
 use super::DdlStatement;
 use crate::dml::CopyOptions;
-use crate::expr::{Alias, Exists, InSubquery, Placeholder};
-use crate::expr_rewriter::create_col_from_scalar_expr;
+use crate::expr::{Alias, Exists, InSubquery, Placeholder, Sort as SortExpr};
+use crate::expr_rewriter::{create_col_from_scalar_expr, normalize_cols};
 use crate::logical_plan::display::{GraphvizVisitor, IndentVisitor};
 use crate::logical_plan::extension::UserDefinedLogicalNode;
 use crate::logical_plan::{DmlStatement, Statement};
@@ -163,7 +163,8 @@ impl LogicalPlan {
             }) => projected_schema,
             LogicalPlan::Projection(Projection { schema, .. }) => schema,
             LogicalPlan::Filter(Filter { input, .. }) => input.schema(),
-            LogicalPlan::Distinct(Distinct { input }) => input.schema(),
+            LogicalPlan::Distinct(Distinct::All(input)) => input.schema(),
+            LogicalPlan::Distinct(Distinct::On(DistinctOn { schema, .. })) => schema,
             LogicalPlan::Window(Window { schema, .. }) => schema,
             LogicalPlan::Aggregate(Aggregate { schema, .. }) => schema,
             LogicalPlan::Sort(Sort { input, .. }) => input.schema(),
@@ -367,6 +368,16 @@ impl LogicalPlan {
             LogicalPlan::Unnest(Unnest { column, .. }) => {
                 f(&Expr::Column(column.clone()))
             }
+            LogicalPlan::Distinct(Distinct::On(DistinctOn {
+                on_expr,
+                select_expr,
+                sort_expr,
+                ..
+            })) => on_expr
+                .iter()
+                .chain(select_expr.iter())
+                .chain(sort_expr.clone().unwrap_or(vec![]).iter())
+                .try_for_each(f),
             // plans without expressions
             LogicalPlan::EmptyRelation(_)
             | LogicalPlan::Subquery(_)
@@ -377,7 +388,7 @@ impl LogicalPlan {
             | LogicalPlan::Analyze(_)
             | LogicalPlan::Explain(_)
             | LogicalPlan::Union(_)
-            | LogicalPlan::Distinct(_)
+            | LogicalPlan::Distinct(Distinct::All(_))
             | LogicalPlan::Dml(_)
             | LogicalPlan::Ddl(_)
             | LogicalPlan::Copy(_)
@@ -405,7 +416,9 @@ impl LogicalPlan {
             LogicalPlan::Union(Union { inputs, .. }) => {
                 inputs.iter().map(|arc| arc.as_ref()).collect()
             }
-            LogicalPlan::Distinct(Distinct { input }) => vec![input],
+            LogicalPlan::Distinct(
+                Distinct::All(input) | Distinct::On(DistinctOn { input, .. }),
+            ) => vec![input],
             LogicalPlan::Explain(explain) => vec![&explain.plan],
             LogicalPlan::Analyze(analyze) => vec![&analyze.input],
             LogicalPlan::Dml(write) => vec![&write.input],
@@ -461,8 +474,11 @@ impl LogicalPlan {
                     Ok(Some(agg.group_expr.as_slice()[0].clone()))
                 }
             }
+            LogicalPlan::Distinct(Distinct::On(DistinctOn { select_expr, .. })) => {
+                Ok(Some(select_expr[0].clone()))
+            }
             LogicalPlan::Filter(Filter { input, .. })
-            | LogicalPlan::Distinct(Distinct { input, .. })
+            | LogicalPlan::Distinct(Distinct::All(input))
             | LogicalPlan::Sort(Sort { input, .. })
             | LogicalPlan::Limit(Limit { input, .. })
             | LogicalPlan::Repartition(Repartition { input, .. })
@@ -823,10 +839,29 @@ impl LogicalPlan {
                 inputs: inputs.iter().cloned().map(Arc::new).collect(),
                 schema: schema.clone(),
             })),
-            LogicalPlan::Distinct(Distinct { .. }) => {
-                Ok(LogicalPlan::Distinct(Distinct {
-                    input: Arc::new(inputs[0].clone()),
-                }))
+            LogicalPlan::Distinct(distinct) => {
+                let distinct = match distinct {
+                    Distinct::All(_) => Distinct::All(Arc::new(inputs[0].clone())),
+                    Distinct::On(DistinctOn {
+                        on_expr,
+                        select_expr,
+                        ..
+                    }) => {
+                        let sort_expr = expr.split_off(on_expr.len() + select_expr.len());
+                        let select_expr = expr.split_off(on_expr.len());
+                        Distinct::On(DistinctOn::try_new(
+                            expr,
+                            select_expr,
+                            if !sort_expr.is_empty() {
+                                Some(sort_expr)
+                            } else {
+                                None
+                            },
+                            Arc::new(inputs[0].clone()),
+                        )?)
+                    }
+                };
+                Ok(LogicalPlan::Distinct(distinct))
             }
             LogicalPlan::Analyze(a) => {
                 assert!(expr.is_empty());
@@ -1064,7 +1099,9 @@ impl LogicalPlan {
             LogicalPlan::Subquery(_) => None,
             LogicalPlan::SubqueryAlias(SubqueryAlias { input, .. }) => input.max_rows(),
             LogicalPlan::Limit(Limit { fetch, .. }) => *fetch,
-            LogicalPlan::Distinct(Distinct { input }) => input.max_rows(),
+            LogicalPlan::Distinct(
+                Distinct::All(input) | Distinct::On(DistinctOn { input, .. }),
+            ) => input.max_rows(),
             LogicalPlan::Values(v) => Some(v.values.len()),
             LogicalPlan::Unnest(_) => None,
             LogicalPlan::Ddl(_)
@@ -1667,9 +1704,21 @@ impl LogicalPlan {
                     LogicalPlan::Statement(statement) => {
                         write!(f, "{}", statement.display())
                     }
-                    LogicalPlan::Distinct(Distinct { .. }) => {
-                        write!(f, "Distinct:")
-                    }
+                    LogicalPlan::Distinct(distinct) => match distinct {
+                        Distinct::All(_) => write!(f, "Distinct:"),
+                        Distinct::On(DistinctOn {
+                            on_expr,
+                            select_expr,
+                            sort_expr,
+                            ..
+                        }) => write!(
+                            f,
+                            "DistinctOn: on_expr=[[{}]], select_expr=[[{}]], sort_expr=[[{}]]",
+                            expr_vec_fmt!(on_expr),
+                            expr_vec_fmt!(select_expr),
+                            if let Some(sort_expr) = sort_expr { expr_vec_fmt!(sort_expr) } else { "".to_string() },
+                        ),
+                    },
                     LogicalPlan::Explain { .. } => write!(f, "Explain"),
                     LogicalPlan::Analyze { .. } => write!(f, "Analyze"),
                     LogicalPlan::Union(_) => write!(f, "Union"),
@@ -2132,9 +2181,93 @@ pub struct Limit {
 
 /// Removes duplicate rows from the input
 #[derive(Clone, PartialEq, Eq, Hash)]
-pub struct Distinct {
+pub enum Distinct {
+    /// Plain `DISTINCT` referencing all selection expressions
+    All(Arc<LogicalPlan>),
+    /// The `Postgres` addition, allowing separate control over DISTINCT'd and selected columns
+    On(DistinctOn),
+}
+
+/// Removes duplicate rows from the input
+#[derive(Clone, PartialEq, Eq, Hash)]
+pub struct DistinctOn {
+    /// The `DISTINCT ON` clause expression list
+    pub on_expr: Vec<Expr>,
+    /// The selected projection expression list
+    pub select_expr: Vec<Expr>,
+    /// The `ORDER BY` clause, whose initial expressions must match those of the `ON` clause when
+    /// present. Note that those matching expressions actually wrap the `ON` expressions with
+    /// additional info pertaining to the sorting procedure (i.e. ASC/DESC, and NULLS FIRST/LAST).
+    pub sort_expr: Option<Vec<Expr>>,
     /// The logical plan that is being DISTINCT'd
     pub input: Arc<LogicalPlan>,
+    /// The schema description of the DISTINCT ON output
+    pub schema: DFSchemaRef,
+}
+
+impl DistinctOn {
+    /// Create a new `DistinctOn` struct.
+    pub fn try_new(
+        on_expr: Vec<Expr>,
+        select_expr: Vec<Expr>,
+        sort_expr: Option<Vec<Expr>>,
+        input: Arc<LogicalPlan>,
+    ) -> Result<Self> {
+        if on_expr.is_empty() {
+            return plan_err!("No `ON` expressions provided");
+        }
+
+        let on_expr = normalize_cols(on_expr, input.as_ref())?;
+
+        let schema = DFSchema::new_with_metadata(
+            exprlist_to_fields(&select_expr, &input)?,
+            input.schema().metadata().clone(),
+        )?;
+
+        let mut distinct_on = DistinctOn {
+            on_expr,
+            select_expr,
+            sort_expr: None,
+            input,
+            schema: Arc::new(schema),
+        };
+
+        if let Some(sort_expr) = sort_expr {
+            distinct_on = distinct_on.with_sort_expr(sort_expr)?;
+        }
+
+        Ok(distinct_on)
+    }
+
+    /// Try to update `self` with a new sort expressions.
+    ///
+    /// Validates that the sort expressions are a super-set of the `ON` expressions.
+    pub fn with_sort_expr(mut self, sort_expr: Vec<Expr>) -> Result<Self> {
+        let sort_expr = normalize_cols(sort_expr, self.input.as_ref())?;
+
+        // Check that the left-most sort expressions are the same as the `ON` expressions.
+        let mut matched = true;
+        for (on, sort) in self.on_expr.iter().zip(sort_expr.iter()) {
+            match sort {
+                Expr::Sort(SortExpr { expr, .. }) => {
+                    if on != &**expr {
+                        matched = false;
+                        break;
+                    }
+                }
+                _ => return plan_err!("Not a sort expression: {sort}"),
+            }
+        }
+
+        if self.on_expr.len() > sort_expr.len() || !matched {
+            return plan_err!(
+                "SELECT DISTINCT ON expressions must match initial ORDER BY expressions"
+            );
+        }
+
+        self.sort_expr = Some(sort_expr);
+        Ok(self)
+    }
 }
 
 /// Aggregates its input based on a set of grouping and aggregate

datafusion/expr/src/tree_node/expr.rs

@@ -157,9 +157,11 @@ impl TreeNode for Expr {
         let mut transform = transform;
 
         Ok(match self {
-            Expr::Alias(Alias { expr, name, .. }) => {
-                Expr::Alias(Alias::new(transform(*expr)?, name))
-            }
+            Expr::Alias(Alias {
+                expr,
+                relation,
+                name,
+            }) => Expr::Alias(Alias::new(transform(*expr)?, relation, name)),
             Expr::Column(_) => self,
             Expr::OuterReferenceColumn(_, _) => self,
             Expr::Exists { .. } => self,