perf: Reorder predicates in conjuncts via simple heuristic #22343
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| // 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. | ||
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| //! Reorder conjunctive (`AND`) predicates so that cheap predicates run before | ||
| //! expensive ones. | ||
| //! | ||
| //! DataFusion's `AND` evaluator short-circuits the right-hand side when the | ||
| //! left-hand side keeps few rows, so leading with a cheap predicate shrinks | ||
| //! the batch that expensive ones see. | ||
| //! | ||
| //! The cost of evaluating a predicate is assessed with a simple, conservative | ||
| //! heuristic: we define an allow-list of cheap operations, and consider an | ||
| //! expression to be cheap if it consists ONLY of cheap operations; everything | ||
| //! else is considered expensive. | ||
| //! | ||
| //! The sort is stable, so order within each class is preserved. | ||
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| use datafusion_common::tree_node::TreeNode; | ||
| use datafusion_expr::{BinaryExpr, Expr, Operator}; | ||
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| /// Stable partition of `predicates`: cheap first, then expensive. | ||
| /// | ||
| /// Returns `(predicates, changed)`. When `changed` is `false` the input was | ||
| /// already cheap-first and the caller can skip rebuilding the conjunction. | ||
| pub(crate) fn reorder_predicates(predicates: Vec<Expr>) -> (Vec<Expr>, bool) { | ||
|
Contributor
There was a problem hiding this comment. It could also potentially return |
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| if predicates.len() <= 1 { | ||
| return (predicates, false); | ||
| } | ||
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| // Volatile predicates may have observable side-effects and reordering | ||
| // conjuncts can change how many times they evaluate. Preserve user order | ||
| // if any predicate contains a volatile expression. | ||
| if predicates.iter().any(Expr::is_volatile) { | ||
| return (predicates, false); | ||
| } | ||
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| let classes: Vec<bool> = predicates.iter().map(is_cheap_predicate).collect(); | ||
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| // A reorder is needed iff an expensive predicate precedes a cheap one | ||
| let needs_reorder = classes.windows(2).any(|w| !w[0] && w[1]); | ||
| if !needs_reorder { | ||
| return (predicates, false); | ||
| } | ||
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| let mut cheap = Vec::with_capacity(predicates.len()); | ||
|
Contributor
There was a problem hiding this comment. You could probably save the allocation / sort in place using https://doc.rust-lang.org/std/vec/struct.Vec.html#method.sort_by and a custom comparator |
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| let mut expensive = Vec::new(); | ||
| for (p, is_cheap) in predicates.into_iter().zip(classes) { | ||
| if is_cheap { | ||
| cheap.push(p); | ||
| } else { | ||
| expensive.push(p); | ||
| } | ||
| } | ||
| cheap.extend(expensive); | ||
| (cheap, true) | ||
| } | ||
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| /// Returns true if every node in `expr`'s tree is cheap. | ||
| fn is_cheap_predicate(expr: &Expr) -> bool { | ||
| !expr | ||
| .exists(|node| Ok(!is_cheap_node(node))) | ||
| .expect("is_cheap_node is infallible") | ||
| } | ||
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| /// Returns true if `expr` is itself cheap. | ||
| /// | ||
| /// We use a simple, conservative heuristic to determine if an expression is | ||
| /// cheap to evaluate: we enumerate known-cheap operations (e.g., equality | ||
| /// comparisons, negations, casts), and consider anything outside this list to | ||
| /// be expensive. New/unrecognized expressions therefore default to being | ||
| /// expensive. | ||
| fn is_cheap_node(expr: &Expr) -> bool { | ||
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neilconway marked this conversation as resolved.
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| match expr { | ||
| // Direct reads and literals. | ||
| Expr::Column(_) | ||
| | Expr::Literal(_, _) | ||
| | Expr::ScalarVariable(_, _) | ||
| | Expr::Placeholder(_) | ||
| | Expr::OuterReferenceColumn(_, _) | ||
| | Expr::LambdaVariable(_) | ||
| // Wrappers; children are walked separately by `is_cheap_predicate`. | ||
| | Expr::Alias(_) | ||
| // Single-row unary predicates and arithmetic negation. | ||
| | Expr::Not(_) | ||
| | Expr::Negative(_) | ||
| | Expr::IsNull(_) | ||
| | Expr::IsNotNull(_) | ||
| | Expr::IsTrue(_) | ||
| | Expr::IsFalse(_) | ||
| | Expr::IsUnknown(_) | ||
| | Expr::IsNotTrue(_) | ||
| | Expr::IsNotFalse(_) | ||
| | Expr::IsNotUnknown(_) | ||
| // Composite cheap forms; child expressions are walked separately. | ||
| | Expr::Between(_) | ||
| | Expr::Case(_) | ||
| | Expr::Cast(_) | ||
| | Expr::TryCast(_) | ||
| | Expr::InList(_) => true, | ||
| // BinaryExpr is cheap unless the operator is LIKE or regexp matching. | ||
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neilconway marked this conversation as resolved.
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| Expr::BinaryExpr(BinaryExpr { op, .. }) => !matches!( | ||
| op, | ||
| Operator::LikeMatch | ||
| | Operator::ILikeMatch | ||
| | Operator::NotLikeMatch | ||
| | Operator::NotILikeMatch | ||
| | Operator::RegexMatch | ||
| | Operator::RegexIMatch | ||
| | Operator::RegexNotMatch | ||
| | Operator::RegexNotIMatch | ||
| ), | ||
| _ => false, | ||
| } | ||
| } | ||
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| #[cfg(test)] | ||
| mod tests { | ||
| use super::*; | ||
| use datafusion_expr::{col, lit}; | ||
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| #[test] | ||
| fn like_predicate_moves_after_equality() { | ||
| let cheap = col("a").eq(lit(1)); | ||
| let expensive = col("b").like(lit("%foo%")); | ||
| let (out, changed) = reorder_predicates(vec![expensive.clone(), cheap.clone()]); | ||
| assert_eq!(out, vec![cheap, expensive]); | ||
| assert!(changed); | ||
| } | ||
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| #[test] | ||
| fn order_among_cheap_predicates_is_preserved() { | ||
| let p1 = col("a").eq(lit(1)); | ||
| let p2 = col("b").eq(lit(2)); | ||
| let p3 = col("c").eq(lit(3)); | ||
| let input = vec![p1.clone(), p2.clone(), p3.clone()]; | ||
| let (out, changed) = reorder_predicates(input.clone()); | ||
| assert_eq!(out, input); | ||
| assert!(!changed); | ||
| } | ||
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| #[test] | ||
| fn order_among_expensive_predicates_is_preserved() { | ||
| let p1 = col("a").like(lit("%a%")); | ||
| let p2 = Expr::BinaryExpr(BinaryExpr::new( | ||
| Box::new(col("b")), | ||
| Operator::RegexMatch, | ||
| Box::new(lit("foo")), | ||
| )); | ||
| let p3 = col("c").like(lit("%c%")); | ||
| let input = vec![p1.clone(), p2.clone(), p3.clone()]; | ||
| let (out, changed) = reorder_predicates(input.clone()); | ||
| assert_eq!(out, input); | ||
| assert!(!changed); | ||
| } | ||
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| #[test] | ||
| fn already_cheap_first_reports_no_change() { | ||
| let cheap = col("a").eq(lit(1)); | ||
| let expensive = col("b").like(lit("%a%")); | ||
| let input = vec![cheap.clone(), expensive.clone()]; | ||
| let (out, changed) = reorder_predicates(input.clone()); | ||
| assert_eq!(out, input); | ||
| assert!(!changed); | ||
| } | ||
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| #[test] | ||
| fn nested_expensive_under_not_is_expensive() { | ||
| // The top node is `Not`, which is on the cheap allow-list. The walk | ||
| // must descend into the `Like` to flag this predicate as expensive. | ||
| let cheap = col("a").eq(lit(1)); | ||
| let nested = Expr::Not(Box::new(col("b").like(lit("%foo%")))); | ||
| let (out, changed) = reorder_predicates(vec![nested.clone(), cheap.clone()]); | ||
| assert_eq!(out, vec![cheap, nested]); | ||
| assert!(changed); | ||
| } | ||
| } | ||
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