Github user concretevitamin commented on the pull request:
https://github.com/apache/spark/pull/1116#issuecomment-46469383
Sorry for introducing the bug in the first place. Just throwing a thought
out there: I have found that there are a lot of arguably hidden assumptions and
contracts in Spark SQL and they turn out to be tricky to spot for newcomers
(e.g. me ;)). As the codebase grows, should we consider using type-level
constraints to enforce some of these constraints? For instance, (again a very
rudimentary thought), we could make the output of planner and
prepareForExecution have different types, and make `execution.ExplainCommand`
only accepts the former and not the latter.
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