feat: Add maintain_order parameter to joins

crates/polars-core/src/chunked_array/ops/sort/options.rs

@@ -149,7 +149,7 @@ impl SortMultipleOptions {
         self
     }
 
-    /// Implement order for all columns. Default `false`.
+    /// Sort order for all columns. Default `false` which is ascending.
     pub fn with_order_descending(mut self, descending: bool) -> Self {
         self.descending = vec![descending];
         self

crates/polars-core/src/frame/mod.rs

@@ -2186,6 +2186,7 @@ impl DataFrame {
 
     /// Return a sorted clone of this [`DataFrame`].
     ///
+    /// In many cases the output chunks will be continuous in memory but this is not guaranteed
     /// # Example
     ///
     /// Sort by a single column with default options:

crates/polars-lazy/src/frame/mod.rs

@@ -34,7 +34,7 @@ use polars_core::prelude::*;
 use polars_expr::{create_physical_expr, ExpressionConversionState};
 use polars_io::RowIndex;
 use polars_mem_engine::{create_physical_plan, Executor};
-use polars_ops::frame::JoinCoalesce;
+use polars_ops::frame::{JoinCoalesce, MaintainOrderJoin};
 #[cfg(feature = "is_between")]
 use polars_ops::prelude::ClosedInterval;
 pub use polars_plan::frame::{AllowedOptimizations, OptFlags};
@@ -2031,8 +2031,9 @@ pub struct JoinBuilder {
     force_parallel: bool,
     suffix: Option<PlSmallStr>,
     validation: JoinValidation,
-    coalesce: JoinCoalesce,
     join_nulls: bool,
+    coalesce: JoinCoalesce,
+    maintain_order: MaintainOrderJoin,
 }
 impl JoinBuilder {
     /// Create the `JoinBuilder` with the provided `LazyFrame` as the left table.
@@ -2045,10 +2046,11 @@ impl JoinBuilder {
             right_on: vec![],
             allow_parallel: true,
             force_parallel: false,
-            join_nulls: false,
             suffix: None,
             validation: Default::default(),
+            join_nulls: false,
             coalesce: Default::default(),
+            maintain_order: Default::default(),
         }
     }
 
@@ -2129,6 +2131,12 @@ impl JoinBuilder {
         self
     }
 
+    /// Whether to preserve the row order.
+    pub fn maintain_order(mut self, maintain_order: MaintainOrderJoin) -> Self {
+        self.maintain_order = maintain_order;
+        self
+    }
+
     /// Finish builder
     pub fn finish(self) -> LazyFrame {
         let mut opt_state = self.lf.opt_state;
@@ -2146,6 +2154,7 @@ impl JoinBuilder {
             slice: None,
             join_nulls: self.join_nulls,
             coalesce: self.coalesce,
+            maintain_order: self.maintain_order,
         };
 
         let lp = self
@@ -2242,6 +2251,7 @@ impl JoinBuilder {
             slice: None,
             join_nulls: self.join_nulls,
             coalesce: self.coalesce,
+            maintain_order: self.maintain_order,
         };
         let options = JoinOptions {
             allow_parallel: self.allow_parallel,

crates/polars-ops/src/frame/join/args.rs

@@ -29,6 +29,7 @@ pub struct JoinArgs {
     pub slice: Option<(i64, usize)>,
     pub join_nulls: bool,
     pub coalesce: JoinCoalesce,
+    pub maintain_order: MaintainOrderJoin,
 }
 
 impl JoinArgs {
@@ -68,6 +69,18 @@ impl JoinCoalesce {
     }
 }
 
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Hash, Default, IntoStaticStr)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[strum(serialize_all = "snake_case")]
+pub enum MaintainOrderJoin {
+    #[default]
+    None,
+    Left,
+    Right,
+    LeftRight,
+    RightLeft,
+}
+
 impl Default for JoinArgs {
     fn default() -> Self {
         Self {
@@ -77,6 +90,7 @@ impl Default for JoinArgs {
             slice: None,
             join_nulls: false,
             coalesce: Default::default(),
+            maintain_order: Default::default(),
         }
     }
 }
@@ -90,6 +104,7 @@ impl JoinArgs {
             slice: None,
             join_nulls: false,
             coalesce: Default::default(),
+            maintain_order: Default::default(),
         }
     }
 

crates/polars-ops/src/frame/join/dispatch_left_right.rs

@@ -67,82 +67,190 @@ pub fn materialize_left_join_from_series(
         s_right = s_right.rechunk();
     }
 
-    let ids = sort_or_hash_left(&s_left, &s_right, verbose, args.validation, args.join_nulls)?;
+    // The current sort_or_hash_left implementation preserves the Left DataFrame order so skip left for now.
+    let requires_ordering = matches!(
+        args.maintain_order,
+        MaintainOrderJoin::Right | MaintainOrderJoin::RightLeft
+    );
+    if requires_ordering {
+        // When ordering we rechunk the series so we don't get ChunkIds as output
+        s_left = s_left.rechunk();
+        s_right = s_right.rechunk();
+    }
+
+    let (left_idx, right_idx) =
+        sort_or_hash_left(&s_left, &s_right, verbose, args.validation, args.join_nulls)?;
+
     let right = if let Some(drop_names) = drop_names {
         right.drop_many(drop_names)
     } else {
         right.drop(s_right.name()).unwrap()
     };
-    Ok(materialize_left_join(&left, &right, ids, args))
+
+    #[cfg(feature = "chunked_ids")]
+    match (left_idx, right_idx) {
+        (ChunkJoinIds::Left(left_idx), ChunkJoinOptIds::Left(right_idx)) => {
+            if requires_ordering {
+                Ok(maintain_order_idx(
+                    &left,
+                    &right,
+                    left_idx.as_slice(),
+                    right_idx.as_slice(),
+                    args,
+                ))
+            } else {
+                Ok(POOL.join(
+                    || materialize_left_join_idx_left(&left, left_idx.as_slice(), args),
+                    || materialize_left_join_idx_right(&right, right_idx.as_slice(), args),
+                ))
+            }
+        },
+        (ChunkJoinIds::Left(left_idx), ChunkJoinOptIds::Right(right_idx)) => Ok(POOL.join(
+            || materialize_left_join_idx_left(&left, left_idx.as_slice(), args),
+            || materialize_left_join_chunked_right(&right, right_idx.as_slice(), args),
+        )),
+        (ChunkJoinIds::Right(left_idx), ChunkJoinOptIds::Right(right_idx)) => Ok(POOL.join(
+            || materialize_left_join_chunked_left(&left, left_idx.as_slice(), args),
+            || materialize_left_join_chunked_right(&right, right_idx.as_slice(), args),
+        )),
+        (ChunkJoinIds::Right(left_idx), ChunkJoinOptIds::Left(right_idx)) => Ok(POOL.join(
+            || materialize_left_join_chunked_left(&left, left_idx.as_slice(), args),
+            || materialize_left_join_idx_right(&right, right_idx.as_slice(), args),
+        )),
+    }
+
+    #[cfg(not(feature = "chunked_ids"))]
+    if requires_ordering {
+        Ok(maintain_order_idx(
+            &left,
+            &right,
+            left_idx.as_slice(),
+            right_idx.as_slice(),
+            args,
+        ))
+    } else {
+        Ok(POOL.join(
+            || materialize_left_join_idx_left(&left, left_idx.as_slice(), args),
+            || materialize_left_join_idx_right(&right, right_idx.as_slice(), args),
+        ))
+    }
 }
 
-#[cfg(feature = "chunked_ids")]
-fn materialize_left_join(
+fn maintain_order_idx(
     left: &DataFrame,
     other: &DataFrame,
-    ids: LeftJoinIds,
+    left_idx: &[IdxSize],
+    right_idx: &[NullableIdxSize],
     args: &JoinArgs,
 ) -> (DataFrame, DataFrame) {
-    let (left_idx, right_idx) = ids;
-    let materialize_left = || match left_idx {
-        ChunkJoinIds::Left(left_idx) => unsafe {
-            let mut left_idx = &*left_idx;
-            if let Some((offset, len)) = args.slice {
-                left_idx = slice_slice(left_idx, offset, len);
-            }
-            left._create_left_df_from_slice(left_idx, true, args.slice.is_some(), true)
-        },
-        ChunkJoinIds::Right(left_idx) => unsafe {
-            let mut left_idx = &*left_idx;
-            if let Some((offset, len)) = args.slice {
-                left_idx = slice_slice(left_idx, offset, len);
-            }
-            left.create_left_df_chunked(left_idx, true, args.slice.is_some())
-        },
+    let mut df = {
+        // SAFETY: left_idx and right_idx are continuous memory that outlive the memory mapped slices
+        let left = unsafe { IdxCa::mmap_slice("a".into(), left_idx) };
+        let right = unsafe { IdxCa::mmap_slice("b".into(), bytemuck::cast_slice(right_idx)) };
+        DataFrame::new(vec![left.into_series().into(), right.into_series().into()]).unwrap()
     };
 
-    let materialize_right = || match right_idx {
-        ChunkJoinOptIds::Left(right_idx) => unsafe {
-            let mut right_idx = &*right_idx;
-            if let Some((offset, len)) = args.slice {
-                right_idx = slice_slice(right_idx, offset, len);
-            }
-            IdxCa::with_nullable_idx(right_idx, |idx| other.take_unchecked(idx))
-        },
-        ChunkJoinOptIds::Right(right_idx) => unsafe {
-            let mut right_idx = &*right_idx;
-            if let Some((offset, len)) = args.slice {
-                right_idx = slice_slice(right_idx, offset, len);
-            }
-            other._take_opt_chunked_unchecked_hor_par(right_idx)
-        },
+    let options = SortMultipleOptions::new()
+        .with_order_descending(false)
+        .with_maintain_order(true);
+
+    let columns = match args.maintain_order {
+        // If the left order is preserved then there are no unsorted right rows
+        // So Left and LeftRight are equal
+        MaintainOrderJoin::Left | MaintainOrderJoin::LeftRight => vec!["a"],
+        MaintainOrderJoin::Right => vec!["b"],
+        MaintainOrderJoin::RightLeft => vec!["b", "a"],
+        _ => unreachable!(),
     };
-    POOL.join(materialize_left, materialize_right)
+
+    df.sort_in_place(columns, options).unwrap();
+    df.rechunk_mut();
+
+    let join_tuples_left = df
+        .column("a")
+        .unwrap()
+        .as_series()
+        .unwrap()
+        .idx()
+        .unwrap()
+        .cont_slice()
+        .unwrap();
+
+    let join_tuples_right = df
+        .column("b")
+        .unwrap()
+        .as_series()
+        .unwrap()
+        .idx()
+        .unwrap()
+        .cont_slice()
+        .unwrap();
+
+    POOL.join(
+        || materialize_left_join_idx_left(left, join_tuples_left, args),
+        || materialize_left_join_idx_right(other, bytemuck::cast_slice(join_tuples_right), args),
+    )
 }
 
-#[cfg(not(feature = "chunked_ids"))]
-fn materialize_left_join(
+fn materialize_left_join_idx_left(
     left: &DataFrame,
-    other: &DataFrame,
-    ids: LeftJoinIds,
+    left_idx: &[IdxSize],
     args: &JoinArgs,
-) -> (DataFrame, DataFrame) {
-    let (left_idx, right_idx) = ids;
-
-    let mut left_idx = &*left_idx;
-    if let Some((offset, len)) = args.slice {
-        left_idx = slice_slice(left_idx, offset, len);
+) -> DataFrame {
+    let left_idx = if let Some((offset, len)) = args.slice {
+        slice_slice(left_idx, offset, len)
+    } else {
+        left_idx
+    };
+    unsafe {
+        left._create_left_df_from_slice(
+            left_idx,
+            true,
+            args.slice.is_some(),
+            matches!(
+                args.maintain_order,
+                MaintainOrderJoin::Left | MaintainOrderJoin::LeftRight
+            ),
+        )
     }
-    let materialize_left =
-        || unsafe { left._create_left_df_from_slice(&left_idx, true, args.slice.is_some(), true) };
+}
 
-    let mut right_idx = &*right_idx;
-    if let Some((offset, len)) = args.slice {
-        right_idx = slice_slice(right_idx, offset, len);
-    }
-    let materialize_right = || {
-        let right_idx = &*right_idx;
-        unsafe { IdxCa::with_nullable_idx(right_idx, |idx| other.take_unchecked(idx)) }
+fn materialize_left_join_idx_right(
+    right: &DataFrame,
+    right_idx: &[NullableIdxSize],
+    args: &JoinArgs,
+) -> DataFrame {
+    let right_idx = if let Some((offset, len)) = args.slice {
+        slice_slice(right_idx, offset, len)
+    } else {
+        right_idx
+    };
+    unsafe { IdxCa::with_nullable_idx(right_idx, |idx| right.take_unchecked(idx)) }
+}
+#[cfg(feature = "chunked_ids")]
+fn materialize_left_join_chunked_left(
+    left: &DataFrame,
+    left_idx: &[ChunkId],
+    args: &JoinArgs,
+) -> DataFrame {
+    let left_idx = if let Some((offset, len)) = args.slice {
+        slice_slice(left_idx, offset, len)
+    } else {
+        left_idx
+    };
+    unsafe { left.create_left_df_chunked(left_idx, true, args.slice.is_some()) }
+}
+
+#[cfg(feature = "chunked_ids")]
+fn materialize_left_join_chunked_right(
+    right: &DataFrame,
+    right_idx: &[ChunkId],
+    args: &JoinArgs,
+) -> DataFrame {
+    let right_idx = if let Some((offset, len)) = args.slice {
+        slice_slice(right_idx, offset, len)
+    } else {
+        right_idx
     };
-    POOL.join(materialize_left, materialize_right)
+    unsafe { right._take_opt_chunked_unchecked_hor_par(right_idx) }
 }

crates/polars-ops/src/frame/join/general.rs

@@ -11,7 +11,8 @@ fn get_suffix(suffix: Option<PlSmallStr>) -> PlSmallStr {
     suffix.unwrap_or_else(|| PlSmallStr::from_static("_right"))
 }
 
-/// Utility method to finish a join.
+/// Renames the columns on the right to not clash with the left using a specified or otherwise default suffix
+/// and then merges the right dataframe into the left
 #[doc(hidden)]
 pub fn _finish_join(
     mut df_left: DataFrame,