comphead commented on code in PR #21075: URL: https://github.com/apache/datafusion/pull/21075#discussion_r3046022454
########## datafusion/optimizer/src/unions_to_filter.rs: ########## @@ -0,0 +1,619 @@ +// 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. + +//! Rewrites `UNION DISTINCT` branches that differ only by filter predicates +//! into a single filtered branch plus `DISTINCT`. + +use crate::optimizer::ApplyOrder; +use crate::{OptimizerConfig, OptimizerRule}; +use datafusion_common::Result; +use datafusion_common::tree_node::{Transformed, TreeNode}; +use datafusion_expr::expr_rewriter::coerce_plan_expr_for_schema; +use datafusion_expr::logical_plan::builder::LogicalPlanBuilder; +use datafusion_expr::utils::disjunction; +use datafusion_expr::{ + Distinct, Expr, Filter, LogicalPlan, Projection, SubqueryAlias, Union, +}; +use std::collections::HashMap; +use std::sync::Arc; + +#[derive(Default, Debug)] +pub struct UnionsToFilter; + +impl UnionsToFilter { + #[expect(missing_docs)] + pub fn new() -> Self { + Self + } +} + +impl OptimizerRule for UnionsToFilter { + fn name(&self) -> &str { + "unions_to_filter" + } + + fn apply_order(&self) -> Option<ApplyOrder> { + Some(ApplyOrder::BottomUp) + } + + fn supports_rewrite(&self) -> bool { + true + } + + fn rewrite( + &self, + plan: LogicalPlan, + config: &dyn OptimizerConfig, + ) -> Result<Transformed<LogicalPlan>> { + if !config.options().optimizer.enable_unions_to_filter { + return Ok(Transformed::no(plan)); + } + + match plan { + LogicalPlan::Distinct(Distinct::All(input)) => { + let inner = Arc::unwrap_or_clone(input); + match try_rewrite_distinct_union(inner.clone())? { + Some(rewritten) => Ok(Transformed::yes(rewritten)), + None => Ok(Transformed::no(LogicalPlan::Distinct(Distinct::All( + Arc::new(inner), + )))), + } + } + _ => Ok(Transformed::no(plan)), + } + } +} + +fn try_rewrite_distinct_union(plan: LogicalPlan) -> Result<Option<LogicalPlan>> { + let LogicalPlan::Union(Union { inputs, schema }) = plan else { + return Ok(None); + }; + + if inputs.len() < 2 { + return Ok(None); + } + + let mut grouped: HashMap<GroupKey, Vec<Expr>> = HashMap::new(); + let mut input_order: Vec<GroupKey> = Vec::new(); + let mut transformed = false; + + for input in inputs { + let Some(branch) = extract_branch(Arc::unwrap_or_clone(input))? else { + return Ok(None); + }; + + let key = GroupKey { + source: branch.source, + wrappers: branch.wrappers, + }; + if let Some(conds) = grouped.get_mut(&key) { + conds.push(branch.predicate); + transformed = true; + } else { + input_order.push(key.clone()); + grouped.insert(key, vec![branch.predicate]); + } + } + + if !transformed { + return Ok(None); + } + + let mut builder: Option<LogicalPlanBuilder> = None; + for key in input_order { + let predicates = grouped + .remove(&key) + .expect("grouped predicates should exist for every source"); + let combined = + disjunction(predicates).expect("union branches always provide predicates"); + let branch = LogicalPlanBuilder::from(key.source) + .filter(combined)? + .build()?; + let branch = wrap_branch(branch, &key.wrappers)?; + let branch = coerce_plan_expr_for_schema(branch, &schema)?; + let branch = align_plan_to_schema(branch, Arc::clone(&schema))?; + builder = Some(match builder { + None => LogicalPlanBuilder::from(branch), + Some(builder) => builder.union(branch)?, + }); + } + + let union = builder + .expect("at least one branch after rewrite") + .build()?; + Ok(Some(LogicalPlan::Distinct(Distinct::All(Arc::new(union))))) +} + +struct Branch { + source: LogicalPlan, + predicate: Expr, + wrappers: Vec<Wrapper>, +} + +fn extract_branch(plan: LogicalPlan) -> Result<Option<Branch>> { + let (wrappers, plan) = peel_wrappers(plan); + + // Volatile or subquery expressions in the projection must not be merged: + // they are evaluated once per branch in the original plan but would be + // evaluated once per combined row after the rewrite, which can change the + // output row set. + if !wrapper_projections_are_safe(&wrappers) { + return Ok(None); + } + + match plan { + LogicalPlan::Filter(Filter { + predicate, input, .. + }) => { + if !is_mergeable_predicate(&predicate) { + return Ok(None); + } + Ok(Some(Branch { + source: strip_passthrough_nodes(Arc::unwrap_or_clone(input)), + predicate, + wrappers, + })) + } + // A Limit or Sort node changes the row-set semantics of the branch. + // Merging two such branches into one would silently drop the per-branch + // row restriction (LIMIT) or rely on an order guarantee that UNION does + // not preserve (ORDER BY). Bail out to leave the UNION unchanged. + LogicalPlan::Limit(_) | LogicalPlan::Sort(_) => Ok(None), Review Comment: considering all the introduced guardrails it would probably be useful adding some debug info why the optimizer rule cannot be applied -- 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]
