To improve the test coverage, we can port the regression tests from the other popular open source projects to Spark SQL. PostgreSQL is one of the best SQL systems. Below are the links to the test cases and results.
- Regression test cases: https://github.com/postgres/postgres/tree/master/src/test/regress/sql
- Expected results: https://github.com/postgres/postgres/tree/master/src/test/regress/expected
Spark SQL does not support all the feature sets of PostgreSQL. In the current stage, we should first comment out these test cases and create the corresponding JIRAs in SPARK-27764. We can discuss and prioritize which features we should support. Also, these PostgreSQL regression tests could also expose the existing bugs of Spark SQL. We should also create the JIRAs and track them in SPARK-27764.