Force script fallback for nested IS NULL / IS NOT NULL

ExistsQuery.canSupport() already returns false for nested references, but
the default branch of FilterQueryBuilder.visitFunction() has a second
dispatch: when canSupport() is false, it checks isNestedPredicate() and, if
true, routes to NestedQuery.buildNested(func, query). That helper reads
func.getArguments().get(1) on the assumption of a binary predicate shape,
so unary IS NULL / IS NOT NULL would throw at runtime.

Override isNestedPredicate() to return false in ExistsQuery so the dispatch
falls through to the compiled-script path for nested-field predicates. Add
unit tests covering both IS NULL and IS NOT NULL to lock in the fallback.

Signed-off-by: Eric Wei <mengwei.eric@gmail.com>

Author: mengweieric

opensearch/src/main/java/org/opensearch/sql/opensearch/storage/script/filter/lucene/ExistsQuery.java

@@ -45,6 +45,17 @@ public boolean canSupport(FunctionExpression func) {
         && !isNestedFunction(func.getArguments().get(0));
   }
 
+  /**
+   * Unary IS NULL / IS NOT NULL has no {@code arg[1]}, so we must never route through {@link
+   * org.opensearch.sql.opensearch.storage.script.filter.lucene.NestedQuery#buildNested} — that path
+   * reads {@code func.getArguments().get(1)} and would throw. Returning {@code false} here forces
+   * {@code FilterQueryBuilder} to fall back to the script-query path for nested-field predicates.
+   */
+  @Override
+  public boolean isNestedPredicate(FunctionExpression func) {
+    return false;
+  }
+
   @Override
   public QueryBuilder build(FunctionExpression func) {
     ReferenceExpression ref = (ReferenceExpression) func.getArguments().get(0);

opensearch/src/test/java/org/opensearch/sql/opensearch/storage/script/filter/FilterQueryBuilderTest.java

@@ -205,6 +205,51 @@ void should_build_must_not_exists_query_for_is_null() {
         buildQuery(DSL.is_null(ref("age", INTEGER))));
   }
 
+  @Test
+  void should_fallback_to_script_for_nested_is_not_null() {
+    // Nested IS_NOT_NULL must NOT route through NestedQuery.buildNested(): that path reads
+    // arg[1] and unary IS_NOT_NULL only has arg[0]. ExistsQuery.isNestedPredicate() returns
+    // false precisely to force the script fallback here.
+    mockToStringSerializer();
+    assertJsonEquals(
+        "{\n"
+            + "  \"script\" : {\n"
+            + "    \"script\" : {\n"
+            + "      \"source\" :"
+            + " \"{\\\"langType\\\":\\\"v2\\\",\\\"script\\\":\\\"is"
+            + " not null(FunctionExpression(functionName=nested, arguments=[message.info,"
+            + " message]))\\\"}\",\n"
+            + "      \"lang\" : \"opensearch_compounded_script\"\n"
+            + "    },\n"
+            + "    \"boost\" : 1.0\n"
+            + "  }\n"
+            + "}",
+        buildQuery(
+            DSL.isnotnull(
+                DSL.nested(DSL.ref("message.info", STRING), DSL.ref("message", STRING)))));
+  }
+
+  @Test
+  void should_fallback_to_script_for_nested_is_null() {
+    // Symmetric to the IS_NOT_NULL case: must not crash with an arg[1] lookup via NestedQuery.
+    mockToStringSerializer();
+    assertJsonEquals(
+        "{\n"
+            + "  \"script\" : {\n"
+            + "    \"script\" : {\n"
+            + "      \"source\" :"
+            + " \"{\\\"langType\\\":\\\"v2\\\",\\\"script\\\":\\\"is"
+            + " null(FunctionExpression(functionName=nested, arguments=[message.info,"
+            + " message]))\\\"}\",\n"
+            + "      \"lang\" : \"opensearch_compounded_script\"\n"
+            + "    },\n"
+            + "    \"boost\" : 1.0\n"
+            + "  }\n"
+            + "}",
+        buildQuery(
+            DSL.is_null(DSL.nested(DSL.ref("message.info", STRING), DSL.ref("message", STRING)))));
+  }
+
   @Test
   void should_build_script_query_for_function_expression() {
     mockToStringSerializer();