andygrove commented on a change in pull request #8961: URL: https://github.com/apache/arrow/pull/8961#discussion_r545974057
########## 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: Some databases use the `STRAIGHT_JOIN` modifier to force joins to happen in the user-specified order. This is from Impala docs: > If statistics are not available for all the tables in the join query, or if Impala chooses a join order that is not the most efficient, you can override the automatic join order optimization by specifying the STRAIGHT_JOIN keyword immediately after the SELECT and any DISTINCT or ALL keywords. In this case, Impala uses the order the tables appear in the query to guide how the joins are processed. I think we can merge this PR as is and continue this discussion. Spark's AQE approach would mean that we have the statistics, but only if we load both sides into memory first (or scan them first for row counts) which would possibly defeat the point of this optimization. It would also mean that the next operator in the query plan wouldn't be able to start streaming until the join has completed? This is a tricky area. ---------------------------------------------------------------- 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]
