[GitHub] [kylin] yuanoOo opened a new pull request, #2077: Enhancement: Optimize code logic for pushdown queries

[GitBox <gi...@apache.org>]

yuanoOo opened a new pull request, #2077:
URL: https://github.com/apache/kylin/pull/2077

   ## Proposed changes
   
   It is found that the push-down query in kylin4.0.x is very slow for the simple query eg: `select * from table limit 10`. It should respond in seconds, but it often takes a few minutes, and the larger the query data set, the more time-consuming it takes Long, which is very abnormal.BI tools often execute some simple queries to display detailed data. Abnormal query duration often causes BI tools to time out and return error messages, which is very unfriendly to user experience.
   Through investigation, it is found that there is a **shuffle** process in the query plan of this very simple detailed query, which is outrageous.
   The main logic of Kylin executing push-down query is concentrated in `org.apache.kylin.query.pushdown.SparkSqlClient`.Unnecessary Spark DataFrame type transform in `org.apache.kylin.query.pushdown.SparkSqlClient#DFToList` is the main cause of this problem.
   
   ## Branch to commit
   - [ ] Branch **kylin3** for v2.x to v3.x
   - [x] Branch **kylin4** for v4.x
   - [ ] Branch **kylin5** for v5.x
   
   ## Types of changes
   
   What types of changes does your code introduce to Kylin?
   _Put an `x` in the boxes that apply_
   
   - [x] Bugfix (non-breaking change which fixes an issue)
   - [ ] New feature (non-breaking change which adds functionality)
   - [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
   - [ ] Documentation Update (if none of the other choices apply)
   
   ## Checklist
   
   _Put an `x` in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code._
   
   - [ ] I have created an issue on [Kylin's jira](https://issues.apache.org/jira/browse/KYLIN), and have described the bug/feature there in detail
   - [ ] Commit messages in my PR start with the related jira ID, like "KYLIN-0000 Make Kylin project open-source"
   - [x] Compiling and unit tests pass locally with my changes
   - [ ] I have added tests that prove my fix is effective or that my feature works
   - [ ] I have added necessary documentation (if appropriate)
   - [ ] Any dependent changes have been merged
   
   ## Further comments
   
   If this is a relatively large or complex change, kick off the discussion at user@kylin.apache.org or dev@kylin.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...
   


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