Github user jinfengni commented on the pull request:
https://github.com/apache/drill/pull/193#issuecomment-152013964
The original approach (skipping the execution phase for limit 0
completely), actually could potentially have issues in some cases, due to the
difference in Calcite rule and Drill execution rule, in terms of how type is
determined.
For example, sum(int) in calcite is resolved to be int, while in Drill
execution, we changed to bigint. Another case is implicit cast. Currently,
there are some small differences between Calcite and Drill execution. That
means, if we skip the execution for limit 0, then types which are resolved in
Calcite could be different from the type if the query goes through Drill
execution. For BI tool like Tableau, that means the type returned from "limit
0" query and type from a second query w/o "limit 0" could be different.
If we want to avoid the above issues, we have to detect all those cases,
which are painful. That's why Sudheesh and I are now more inclined to this new
approach.
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