refactor to terminate early if tablescan is empty

Author: geruh

core/src/main/java/org/apache/iceberg/rest/CatalogHandlers.java

@@ -678,7 +678,6 @@ public static PlanTableScanResponse planTableScan(
         initial.second() == null
             ? Collections.emptyList()
             : IN_MEMORY_PLANNING_STATE.nextPlanTask(initial.second());
-
     PlanTableScanResponse.Builder builder =
         PlanTableScanResponse.builder()
             .withPlanStatus(PlanStatus.COMPLETED)
@@ -777,20 +776,25 @@ static void clearPlanningState() {
    */
   private static Pair<List<FileScanTask>, String> planFilesFor(
       TableScan tableScan, String planId, int tasksPerPlanTask) {
-    Iterable<List<FileScanTask>> taskGroupings =
-        Iterables.partition(tableScan.planFiles(), tasksPerPlanTask);
+    Iterable<FileScanTask> planTasks = tableScan.planFiles();
+    String planTaskPrefix = planId + "-" + tableScan.table().uuid() + "-";
+
+    // Handle empty table scans
+    if (!planTasks.iterator().hasNext()) {
+      String planTaskKey = planTaskPrefix + "0";
+      // Add empty scan to planning state so async calls know the scan completed
+      IN_MEMORY_PLANNING_STATE.addPlanTask(planTaskKey, Collections.emptyList());
+      return Pair.of(Collections.emptyList(), planTaskKey);
+    }
+
+    Iterable<List<FileScanTask>> taskGroupings = Iterables.partition(planTasks, tasksPerPlanTask);
     int planTaskSequence = 0;
     String previousPlanTask = null;
     String firstPlanTaskKey = null;
     List<FileScanTask> initialFileScanTasks = null;
-    boolean hasPlanTasks = false;
-
     for (List<FileScanTask> taskGrouping : taskGroupings) {
-      hasPlanTasks = true;
-      String planTaskKey =
-          String.format("%s-%s-%s", planId, tableScan.table().uuid(), planTaskSequence++);
+      String planTaskKey = planTaskPrefix + planTaskSequence++;
       IN_MEMORY_PLANNING_STATE.addPlanTask(planTaskKey, taskGrouping);
-
       if (previousPlanTask != null) {
         IN_MEMORY_PLANNING_STATE.addNextPlanTask(previousPlanTask, planTaskKey);
       } else {
@@ -800,11 +804,6 @@ private static Pair<List<FileScanTask>, String> planFilesFor(
 
       previousPlanTask = planTaskKey;
     }
-
-    if (!hasPlanTasks) {
-      return Pair.of(Collections.emptyList(), null);
-    }
-
     return Pair.of(initialFileScanTasks, firstPlanTaskKey);
   }