Function planning_session_context 

pub fn planning_session_context(target_partitions: usize) -> SessionContext

Session context used to plan a query for distributed stage execution.

Round-robin repartitioning is disabled: a RoundRobinBatch exchange left inside a stage subtree would make every task of that stage re-execute all input partitions (RepartitionExec drives all inputs per process), so only hash exchanges — which the builder cuts into shuffle boundaries — are allowed into the plan.