Improve expand command error message for non-nested fields

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

@@ -3331,6 +3331,17 @@ private void flattenParsedPattern(
 
   private void buildExpandRelNode(
       RexInputRef arrayFieldRex, String arrayFieldName, String alias, CalcitePlanContext context) {
+    // Validate that the field is an array type.
+    // Only nested array fields are supported by expand command.
+    RelDataType fieldType = arrayFieldRex.getType();
+    if (fieldType.getSqlTypeName() != SqlTypeName.ARRAY) {
+      throw new UnsupportedOperationException(
+          String.format(
+              "Expand command only supports nested array fields. "
+                  + "Field '%s' has type '%s' which is not a nested array.",
+              arrayFieldName, fieldType.getSqlTypeName()));
+    }
+
     // 3. Capture the outer row in a CorrelationId
     Holder<RexCorrelVariable> correlVariable = Holder.empty();
     context.relBuilder.variable(correlVariable::set);

integ-test/src/yamlRestTest/resources/rest-api-spec/test/issues/5065.yml

@@ -0,0 +1,45 @@
+setup:
+  - do:
+      indices.create:
+        index: test-idx-5065
+        body:
+          settings:
+            number_of_shards: 1
+          mappings:
+            properties:
+              nums:
+                type: long
+  - do:
+      index:
+        index: test-idx-5065
+        id: 1
+        refresh: true
+        body:
+          nums: [1, 2, 3]
+  - do:
+      query.settings:
+        body:
+          transient:
+            plugins.calcite.enabled: true
+---
+teardown:
+  - do:
+      query.settings:
+        body:
+          transient:
+            plugins.calcite.enabled: false
+  - do:
+      indices.delete:
+        index: test-idx-5065
+        ignore_unavailable: true
+
+---
+"Expand on scalar type should fail with clear error message":
+  - do:
+      catch: bad_request
+      ppl:
+        body:
+          query: 'source=test-idx-5065 | expand nums'
+  - match: { error.type: "UnsupportedOperationException" }
+  - match: { error.reason: "/.*Expand command only works on array types.*/" }
+  - match: { error.details: "/.*nums.*BIGINT.*/" }