SubhamSinghal commented on code in PR #21621: URL: https://github.com/apache/datafusion/pull/21621#discussion_r3115134951
########## datafusion/optimizer/src/push_down_topk_through_join.rs: ########## @@ -0,0 +1,865 @@ +// Licensed to the Apache Software Foundation (ASF) under one +// or more contributor license agreements. See the NOTICE file +// distributed with this work for additional information +// regarding copyright ownership. The ASF licenses this file +// to you under the Apache License, Version 2.0 (the +// "License"); you may not use this file except in compliance +// with the License. You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, +// software distributed under the License is distributed on an +// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY +// KIND, either express or implied. See the License for the +// specific language governing permissions and limitations +// under the License. + +//! [`PushDownTopKThroughJoin`] pushes TopK (Sort with fetch) through outer joins +//! +//! When a `Sort` with a fetch limit sits above an outer join and all sort +//! expressions come from the **preserved** side, this rule inserts a copy +//! of the `Sort(fetch)` on that input to reduce the number of rows +//! entering the join. +//! +//! This is correct because: +//! - A LEFT JOIN preserves every left row (each appears at least once in the +//! output). The final top-N by left-side columns must come from the top-N +//! left rows. +//! - The same reasoning applies symmetrically for RIGHT JOIN and right-side +//! columns. +//! +//! The top-level sort is kept for correctness since a 1-to-many join can +//! produce more than N output rows from N input rows. +//! +//! ## Example +//! +//! Before: +//! ```text +//! Sort: t1.b ASC, fetch=3 +//! Left Join: t1.a = t2.c +//! Scan: t1 ← scans ALL rows +//! Scan: t2 +//! ``` +//! +//! After: +//! ```text +//! Sort: t1.b ASC, fetch=3 +//! Left Join: t1.a = t2.c +//! Sort: t1.b ASC, fetch=3 ← pushed down +//! Scan: t1 +//! Scan: t2 +//! ``` + +use std::sync::Arc; + +use crate::optimizer::ApplyOrder; +use crate::{OptimizerConfig, OptimizerRule}; + +use crate::utils::{has_all_column_refs, schema_columns}; +use datafusion_common::tree_node::{Transformed, TreeNode}; +use datafusion_common::{Column, Result}; +use datafusion_expr::logical_plan::{ + JoinType, LogicalPlan, Projection, Sort as SortPlan, SubqueryAlias, +}; +use datafusion_expr::{Expr, SortExpr}; + +/// Optimization rule that pushes TopK (Sort with fetch) through +/// LEFT / RIGHT outer joins when all sort expressions come from +/// the preserved side. +/// +/// See module-level documentation for details. +#[derive(Default, Debug)] +pub struct PushDownTopKThroughJoin; + +impl PushDownTopKThroughJoin { + #[expect(missing_docs)] + pub fn new() -> Self { + Self {} + } +} + +impl OptimizerRule for PushDownTopKThroughJoin { + fn supports_rewrite(&self) -> bool { + true + } + + fn rewrite( + &self, + plan: LogicalPlan, + _config: &dyn OptimizerConfig, + ) -> Result<Transformed<LogicalPlan>> { + // Match Sort with fetch (TopK) + let LogicalPlan::Sort(sort) = &plan else { + return Ok(Transformed::no(plan)); + }; + let Some(fetch) = sort.fetch else { + return Ok(Transformed::no(plan)); + }; + + // Don't push if any sort expression is non-deterministic (e.g. random()). + // Duplicating such expressions would produce different values at each + // evaluation point, potentially changing the result. + if sort.expr.iter().any(|se| se.expr.is_volatile()) { + return Ok(Transformed::no(plan)); + } + + // Peel through transparent nodes (SubqueryAlias, Projection) to find + // the Join. Track intermediate nodes so we can reconstruct the tree + // and resolve sort expressions through them. + let mut current = sort.input.as_ref(); + let mut intermediates: Vec<&LogicalPlan> = Vec::new(); + let join = loop { + match current { + LogicalPlan::Join(join) => break join, + LogicalPlan::Projection(proj) => { + intermediates.push(current); Review Comment: handled volatile expo in proj.input -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
