As the javadoc says, hooks can be set “by observers”. They are sometimes used 
internally in Calcite but can also be used externally.

QUERY_PLAN is set in CalciteAssert.queryContains.

> On Mar 7, 2018, at 10:27 AM, Muhammad Gelbana <> wrote:
> Looking at this statement
> <>,
> 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
> ​​

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