LakshSingla opened a new pull request, #14981: URL: https://github.com/apache/druid/pull/14981
### Description MSQ currently doesn't support UNION queries. However, in the query stack, there are two types of UNIONs: 1. UnionDataSource - Very limited. MSQ detects this and throws a `QueryNotSupported` fault, which is the expected behavior 2. Top-level union - Works around the shortcomings of 1. However 2) is executed sequentially by the SQL layer and the results are appended sequentially. For a simple query like ```sql SELECT * FROM foo UNION ALL SELECT * FROM foo2 ``` SQL would execute `SELECT * FROM foo` and `SELECT * FROM foo2` and concat the results together. This works fine for working with engines producing results synchronously like `sql-native` where we return the results, however, for MSQ, which produces results asynchronously, the concatenation logic doesn't work as expected since don't wait for the query to finish, fetch the results and submit the second query. To make matters worse, the SQL layer submits the first query, gets the query ID back as the result, and then executes the second query (that fails). Therefore we only submit the partial query successfully and we might even get the incorrect results back. This PR introduces the engine feature `ALLOW_TOP_LEVEL_UNION_ALL` that dictates whether the planner can plan the query using top-level union alls. MSQ disallows this, so the queries are forced to plan using the union data source, which will return query not supported exception. This flag will also be useful once we start supporting unions in MSQ, which we'd want to exclusively execute using `UnionDataSource`, and the flag would seamlessly tie in with the query paths we'd wanna take when planning unions then. With the change, the following query: <img width="912" alt="Screenshot 2023-09-14 at 2 07 34 AM" src="https://github.com/apache/druid/assets/30999375/dc09b94f-a112-49b6-9c27-7811bc7a92b3";> native tasks plan query as before (top-level union all) <img width="470" alt="Screenshot 2023-09-14 at 2 07 39 AM" src="https://github.com/apache/druid/assets/30999375/f598aff4-2a10-4b16-b227-3cb332660f59";> MSQ tasks plan can't plan query with top-level union all, therefore use the `UnionDataSource` to plan the query, which then ultimately fails with `QueryNotSupported` in MSQ <img width="384" alt="Screenshot 2023-09-14 at 2 07 48 AM" src="https://github.com/apache/druid/assets/30999375/145031d1-4961-41ad-9626-38523fcdacf9";> #### Release note Fixes the bug where top-level unions partially execute with MSQ engine, and can give incorrect results. <hr> <!-- Check the items by putting "x" in the brackets for the done things. Not all of these items apply to every PR. Remove the items which are not done or not relevant to the PR. None of the items from the checklist below are strictly necessary, but it would be very helpful if you at least self-review the PR. --> This PR has: - [x] been self-reviewed. - [ ] using the [concurrency checklist](https://github.com/apache/druid/blob/master/dev/code-review/concurrency.md) (Remove this item if the PR doesn't have any relation to concurrency.) - [x] added documentation for new or modified features or behaviors. - [x] a release note entry in the PR description. - [x] added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links. - [ ] added or updated version, license, or notice information in [licenses.yaml](https://github.com/apache/druid/blob/master/dev/license.md) - [x] added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader. - [x] added unit tests or modified existing tests to cover new code paths, ensuring the threshold for [code coverage](https://github.com/apache/druid/blob/master/dev/code-review/code-coverage.md) is met. - [ ] added integration tests. - [x] been tested in a test Druid cluster. -- 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]
