Looking at this statement <https://github.com/apache/calcite/blame/master/druid/src/main/java/org/apache/calcite/adapter/druid/DruidQuery.java#L1570>, it suggests that *org.apache.calcite.runtime.Hook.addThread(Function<T, R>)* or *org.apache.calcite.runtime.Hook.add(Function<T, R>)* will be called for QUERY_PLAN somewhere but I couldn't find it.
Is it kept this way just in case someone needs to extends QUERY_PLAN handlers somehow but kept empty by default ? Would someone please explain to me the purpose of this ? Thanks, Gelbana
