xudong963 commented on a change in pull request #1492: URL: https://github.com/apache/arrow-datafusion/pull/1492#discussion_r776359498
########## File path: docs/rfcs/subquery_design.md ########## @@ -0,0 +1,108 @@ +<!--- + 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. +--> + +Feature Name: subquery + +Status: in-progress + +Start Date: 2021-12-27 + +Authors: xudong.w + +RFC PR: #1373 + +Datafusion Issue: #1209 + +--- + +### Background +Datafusion currently only supports subquery in `from`. +Almost all of the tpch queries that datafusion currently fails are due to unsupported subqueries, such as `exist subquery`, `scalar subquery`. +To improve the integrity of Datafusion SQL support, and through all TPCH cases, more complete subquery support is needed. + +--- + +### Goals +1. Support the `Scalar subquery`. +2. Support the `Exist/Not Exist subquery`. +3. Support the `In/Not In subquery`. +4. Support the `Any/Some/All subquery`, currently sqlparser doesn't support Any/AllSubquery expr, I opened an issue to trace. [support Any/AllSubquery exprs](https://github.com/sqlparser-rs/sqlparser-rs/issues/394) +5. Support the non-correlation subquery and correlation subquery + +--- + +### Non-Goals +1. Correlation subquery doesn't use a more complex solution to do decorrelation, for this design, I'll use nest-loop to do decorrelation. +I'll provide some materials about more advanced decorrelation, and we can research them and optimize in subsequent iterations. +2. Some unusual subqueries won't be supported, such as list subqueries. +--- + +### Survey + +#### PostgreSQL +In postgresql, subqueries are classified into sublink and subquery based on where they appear. Sublink occurs in constraints such as WHERE/ON. +Postgres will try to pull up sublink if possible, the relevant code is in `pull_up_sublinks_jointree_recurse` function. If pull up succeeds, sublink will be converted to join. +- [NOT IN] -> [Anti] Semi Join +- [ANY/SOME] -> Semi Join +- [NOT EXISTS] -> [Anti] Semi Join + +**Note that not all sublinks can be pulled up**. Only some simple sublink queries can be pulled up. If contains aggregate functions, sublink won't be pulled up, then there will be a `SubPlan` in `Query Plan` and the upper execution plan and the subquery plan do nested loops to get the final result. + +Subqueries and sublinks have similar processing logic, except they are in different locations of queries + + +#### CockroachDB +The cockroach's subquery design draws on the ideas in this [paper](https://dl.acm.org/doi/10.1145/1247480.1247598), you can see it to learn more details. +That's the [decorrelation rules](https://github.com/cockroachdb/cockroach/blob/master/pkg/sql/opt/norm/rules/decorrelate.opt) of the cockroach. + +#### Materialize +Materialize is a streaming system with incremental backend. This backend does not support nested loops, so it has to be able to handle all decorrelation cases. +Materialize's subquery implementation should be the most complete. This [paper](https://cs.emis.de/LNI/Proceedings/Proceedings241/383.pdf) is relevant to their implementation. The paper is a little indigestible, so I didn't deep into it. + +--- + +### Design +Let's turn our attention to Datafusion. + +There are two overall steps: +1. transfer sqlparser subquery exprs to datafusion exprs. Review comment: `sql_expr_to_logical_expr` function in datafusion/src/sql/planner.rs -- 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: github-unsubscr...@arrow.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org
