[Calcite] Make containsNestedAggregator tolerate flat-leaf schemas (opensearch-project#5423)

* [Calcite] Make containsNestedAggregator tolerate flat-leaf schemas

  The check splits each aggregator argument on '.' and looks up the
  leading token as a top-level ARRAY column (the OpenSearch `nested`
  marker). It used relBuilder.field(root), which throws when the column
  doesn't exist.

  That works for the classic path, which always exposes a top-level
  column for object/nested parents. It breaks the analytics-engine
  path, which emits only flat leaves ("city.name", "city.location.latitude")
  — parent placeholders can't round-trip through Substrait. Any
  aggregation against an object-field leaf (e.g. stats max(city.location.latitude))
  crashed with "Field [city] not found".

  Use RelDataType.getField, which returns null on miss. A null lookup
  is the correct "not nested" answer. Behavior unchanged for relations
  that do have a top-level ARRAY column.

Signed-off-by: Marc Handalian <marc.handalian@gmail.com>

* fix spotless

Signed-off-by: Marc Handalian <marc.handalian@gmail.com>

---------

Signed-off-by: Marc Handalian <marc.handalian@gmail.com>

Author: mch2

core/src/main/java/org/opensearch/sql/calcite/CalciteRelNodeVisitor.java

@@ -1538,10 +1538,25 @@ private Pair<List<RexNode>, List<AggCall>> aggregateWithTrimming(
    * count(a.b)] returns true.
    */
   private boolean containsNestedAggregator(RelBuilder relBuilder, List<RexInputRef> aggCallRefs) {
+    // For each aggregator argument, take the part of its column name before the first dot
+    // (e.g. "city" from "city.location.latitude") and check whether that's a top-level
+    // ARRAY column — the marker for an OpenSearch `nested` field.
+    //
+    // The classic path always exposes a top-level column for object/nested parents. The
+    // analytics-engine path emits only the flat leaves ("city.name", "city.location.latitude")
+    // because parent placeholder types (MAP<VARCHAR, ANY>) can't round-trip through Substrait.
+    // RelDataType.getField returns null when the column doesn't exist — for analytics-engine,
+    // that null just means "not nested," which is the right answer.
+    RelDataType rowType = relBuilder.peek().getRowType();
     return aggCallRefs.stream()
-        .map(r -> relBuilder.peek().getRowType().getFieldNames().get(r.getIndex()))
+        .map(r -> rowType.getFieldNames().get(r.getIndex()))
         .map(name -> org.apache.commons.lang3.StringUtils.substringBefore(name, "."))
-        .anyMatch(root -> relBuilder.field(root).getType().getSqlTypeName() == SqlTypeName.ARRAY);
+        .anyMatch(
+            root -> {
+              RelDataTypeField field =
+                  rowType.getField(root, /* caseSensitive= */ true, /* elideRecord= */ false);
+              return field != null && field.getType().getSqlTypeName() == SqlTypeName.ARRAY;
+            });
   }
 
   /**