Dandandan commented on a change in pull request #8961: URL: https://github.com/apache/arrow/pull/8961#discussion_r545960387
########## File path: rust/datafusion/src/optimizer/hash_build_probe_order.rs ########## @@ -0,0 +1,219 @@ +// 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 + +//! Optimizer rule to switch build and probe order of hash join +//! based on statistics of a `TableProvider`. If the number of +//! rows of both sources is known, the order can be switched +//! for a faster hash join. + +use crate::logical_plan::LogicalPlan; +use crate::optimizer::optimizer::OptimizerRule; +use crate::{error::Result, prelude::JoinType}; + +use super::utils; + +/// BuildProbeOrder reorders the build and probe phase of +/// hash joins. This uses the amount of rows that a datasource has. +/// The rule optimizes the order such that the left (build) side of the join +/// is the smallest. +/// If the information is not available, the order stays the same, +/// so that it could be optimized manually in a query. +pub struct HashBuildProbeOrder {} + +// Gets exact number of rows, if known by the statistics of the underlying +fn get_num_rows(logical_plan: &LogicalPlan) -> Option<usize> { Review comment: Indeed I think that could be very beneficial but estimating it before executing can be really hard / impossible I think? Also, if using the left as build side wrong, at this moment, the order could be changed by the user by changing the query itself. I think ideally you should be able to know more about the table size when the query is executing (a la Spark 3 adaptive query execution) so you don't do the wrong thing. BigQuery also has a nice strategy / explanation for this https://cloud.google.com/bigquery/query-plan-explanation This probably requires quite a bit of changes on the execution / planning side, but this would bring much more available statistics to each step during execution to be able to change optimize the plan further. ---------------------------------------------------------------- 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. For queries about this service, please contact Infrastructure at: [email protected]
