Use consistent elapsed_compute time

Author: alamb

datafusion/src/physical_plan/union.rs

@@ -95,14 +95,14 @@ impl ExecutionPlan for UnionExec {
         let baseline_metrics = BaselineMetrics::new(&self.metrics, partition);
         // record the tiny amount of work done in this function so
         // elapsed_compute is reported as non zero
-        let timer = baseline_metrics.elapsed_compute().timer();
+        let elapsed_compute = baseline_metrics.elapsed_compute().clone();
+        let _timer = elapsed_compute.timer(); // record on drop
 
         // find partition to execute
         for input in self.inputs.iter() {
             // Calculate whether partition belongs to the current partition
             if partition < input.output_partitioning().partition_count() {
                 let stream = input.execute(partition).await?;
-                drop(timer);
                 return Ok(Box::pin(ObservedStream::new(stream, baseline_metrics)));
             } else {
                 partition -= input.output_partitioning().partition_count();