Canceling the below query that is hung in planning state, leaves the query in ENQUEUED state for ever.
Here is the query that is hung in planning state
Explain plan for that query also just hangs.
The above issues show the following problems:
1. Simple query with reasonable number of concat functions hangs.
In reality query does not hang it just take lots of time to execute. The root cause is that during planning time DrillFuncHolderExpr return type is extensively used to determine matching function, matching type etc. Though this type is retrieved via getter in reality complex logic is executed beaneath it. For example for concat function. Since function return type can not be changes during DrillFuncHolderExpr life time, it is safe to cache it.
2. No mechanism to cancel query during ENQUEUED state.
Currently Drill does not have mechanism to cancel query before STARTING / RUNNING states. Plus ENQUEUED state includes two PLANNING and ENQUEUED.
The following changes were made to address above issues:
a. two new states were added: PREPARING (when foreman is initialized) and PLANNING (includes logical and / or physical planning).
b. ability to cancel query during planning and enqueued states was added.