2010YOUY01 opened a new pull request, #26227: URL: https://github.com/apache/datafusion/pull/26227
## Which issue does this PR close? <!-- We generally require a GitHub issue to be filed for all bug fixes and enhancements and this helps us generate change logs for our releases. You can link an issue to this PR using the GitHub syntax. For example `Closes #123` indicates that this PR will close issue #123. --> Part of - https://github.com/apache/datafusion/issues/26225 ## Rationale for this change <!-- Why are you proposing this change? If this is already explained clearly in the issue then this section is not needed. Explaining clearly why changes are proposed helps reviewers understand your changes and offer better suggestions for fixes. Please explain the problem you are trying to solve in terms of the user-visible behavior, rather than the implementation. For example, "The code in `foo.rs` doesn't handle nulls" is a symptom of the implementation. "COUNT(DISTINCT) returns wrong results when the column contains nulls" is the user-visible problem. --> See [#26225](https://github.com/apache/datafusion/issues/26225) for the rationale (terminology is explained in more detail there). This PR rewrites Hash Join for maintainability. The two major design changes from the existing implementation are: 1. **Deep module:** The join hash table is encapsulated behind a simpler interface. Currently, the state machine directly manipulates low-level hash table components, forcing readers to reason about hash table internals and state transitions simultaneously. 2. **Separate execution path for regular joins:** Regular joins (inner/left/right/full) and existence joins (semi/anti/mark) have sufficiently different requirements to justify separate streams: - **Optimizations:** For example, existence joins can sometimes stop probing after the first match, while regular joins must enumerate all matches. - **Advanced features:** Features such as spilling may require different state transitions. Point 2 justifies a holistic rewrite. This PR incrementally rewrites regular joins, covering the existing functionality while keeping the legacy implementation unchanged. After a similar rewrite for existence joins, the legacy `HashJoinStream` can be removed. Simplified pseudocode: ```rust HashJoinExec::execute() { if config.enable_migration_hash_join && matches!(self.join_type, Inner | Left | Right | Full) { RegularHashJoinStream::new(...) } else { HashJoinStream::new(...) } } ``` ## What changes are included in this PR? <!-- There is no need to duplicate the description in the issue here, but it is sometimes worth providing a summary of the individual changes in this PR. --> 1. add a config `enable_migration_hash_join`, if set to `true`, and join node is regular join, then use the new path; otherwise use the existing implementation as fallback (old implementation not changed). Defaulted to `true`, because it has covered full functionality of existing implementation. 2. Implement a dedicated stream for regular join. To read the implementation, I recommend 1. start with the state machine in `datafusion/physical-plan/src/joins/hash_join/regular_join.rs` 2. The major component for the joining logic is `struct HashJoinTable` in `datafusion/physical-plan/src/joins/hash_join/hash_join_table.rs` -- the struct's comment explains it at high-level. 3. ...and follow along ## What is the testing strategy for this PR? <!-- We typically require tests for all PRs in order to: 1. Prevent the code from being accidentally broken by subsequent changes 4. Serve as another way to document the expected behavior of the code Briefly describe how this PR is tested, and point to the specific tests you added. For example: 'This new feature is covered by the `sqllogictest` cases added in `foo.slt`'. If this PR does not add tests, explain why. For example, if the change is already covered by existing tests, please mention it. You should also check the `codecov` bot reply on this PR to confirm the changed code is exercised. --> Existing tests ## TODO for myself - [ ] check test coverage - [ ] Update comments TODOs to link the Github issue - [ ] Verify benchmark results ## Are there any user-facing changes? No <!-- If there are user-facing changes then we may require documentation to be updated before approving the PR. If there are any breaking changes to public APIs, please add the `api change` label. --> -- 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]
