I believe that ubenchmark MUST remain at source and target 1.6;
That's not a requirement.
Well, it would be nice to be able to build via different JDK versions (e.g. to compare performance of Calcite under different JDKs).
However, I would not mind much if using just 1.8 (i.e. JDK-latest) source and target
It makes sense to update maven-source-plugin to disable default attach-sources goal.
Current configuration (in 816 branch) is bad: https://github.com/julianhyde/calcite/blob/816-sub-query/pom.xml#L327-L337
I believe it causes "double-execution of mvn generate-source" target during a release.
Here's detailed explanation: http://blog.peterlynch.ca/2010/05/maven-how-to-prevent-generate-sources.html
It boils down to "add invalid <phase> for default attach-sources execution id" and "add your own attach-sources-nofork".