Function assert_single_partition 

pub fn assert_single_partition(
    plan: &Arc<dyn ExecutionPlan>,
    subject: &str,
    insert_range: TimeRange,
    reason: &str,
) -> Result<(), Error>

Bails unless plan is a single-partition physical plan. subject names the query for the error message (e.g. format!("merge query {:?}", query) or format!("extract query for {view}")); reason supplies the full, call-site-specific trailing sentence(s) explaining what a non-single-partition plan means here – executing such a plan would coalesce partitions and silently destroy a declared ordering before it is safe to record or execute. Shared by the query-execution paths that must verify this before executing: QueryMerger::execute_concatenated_merge (only when its ordering is declared), QueryMerger::execute_sorted_merge, and sql_partition_spec::plan_sorted_extract.