Handle non-pushdownable WHERE with explicit filter_type

pushDownFilter() did not catch ScriptQueryUnSupportedException, so
non-pushdownable filters (e.g. struct-type fields) would propagate a
raw internal exception instead of a clean SQL-layer error.

With explicit filter_type: throw a clear error explaining the WHERE
clause cannot be pushed down for the requested filter placement.
Without explicit filter_type: return false to fall back to in-memory
filtering, matching the base class behavior.

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

Author: mengweieric

opensearch/src/main/java/org/opensearch/sql/opensearch/storage/scan/VectorSearchQueryBuilder.java

@@ -19,6 +19,7 @@
 import org.opensearch.sql.opensearch.request.OpenSearchRequestBuilder;
 import org.opensearch.sql.opensearch.storage.FilterType;
 import org.opensearch.sql.opensearch.storage.script.filter.FilterQueryBuilder;
+import org.opensearch.sql.opensearch.storage.script.filter.FilterQueryBuilder.ScriptQueryUnSupportedException;
 import org.opensearch.sql.opensearch.storage.serde.DefaultExpressionSerializer;
 import org.opensearch.sql.planner.logical.LogicalFilter;
 import org.opensearch.sql.planner.logical.LogicalLimit;
@@ -73,7 +74,19 @@ public VectorSearchQueryBuilder(
   public boolean pushDownFilter(LogicalFilter filter) {
     FilterQueryBuilder queryBuilder = new FilterQueryBuilder(new DefaultExpressionSerializer());
     Expression queryCondition = filter.getCondition();
-    QueryBuilder whereQuery = queryBuilder.build(queryCondition);
+    QueryBuilder whereQuery;
+    try {
+      whereQuery = queryBuilder.build(queryCondition);
+    } catch (ScriptQueryUnSupportedException e) {
+      if (filterTypeExplicit) {
+        throw new ExpressionEvaluationException(
+            "filter_type requires a pushdownable WHERE clause, but the given condition"
+                + " cannot be pushed down: "
+                + e.getMessage());
+      }
+      // Default mode: fall back to in-memory filtering (matches base class behavior)
+      return false;
+    }
     filterPushed = true;
 
     if (filterType == FilterType.EFFICIENT) {

opensearch/src/test/java/org/opensearch/sql/opensearch/storage/scan/VectorSearchQueryBuilderTest.java

@@ -6,6 +6,7 @@
 package org.opensearch.sql.opensearch.storage.scan;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
@@ -536,6 +537,48 @@ void pushDownSortNonScoreRejectedUnderEfficientMode() {
     assertTrue(ex.getMessage().contains("unsupported sort expression"));
   }
 
+  // ── Non-pushdownable filter handling ──────────────────────────────────
+
+  @Test
+  void pushDownFilterNonPushdownableWithExplicitFilterTypeThrows() {
+    var requestBuilder = createRequestBuilder();
+    var knnQuery = new WrapperQueryBuilder("{\"knn\":{}}");
+    var builder =
+        new VectorSearchQueryBuilder(
+            requestBuilder, knnQuery, Map.of("k", "5"), FilterType.POST, true, null);
+
+    // STRUCT = STRUCT triggers ScriptQueryUnSupportedException in FilterQueryBuilder
+    var condition =
+        DSL.equal(
+            new ReferenceExpression("nested_field", ExprCoreType.STRUCT),
+            new ReferenceExpression("other_field", ExprCoreType.STRUCT));
+    var dummyChild = new LogicalValues(Collections.emptyList());
+    var filter = new LogicalFilter(dummyChild, condition);
+
+    ExpressionEvaluationException ex =
+        assertThrows(ExpressionEvaluationException.class, () -> builder.pushDownFilter(filter));
+    assertTrue(ex.getMessage().contains("filter_type requires a pushdownable WHERE clause"));
+    assertTrue(ex.getMessage().contains("cannot be pushed down"));
+  }
+
+  @Test
+  void pushDownFilterNonPushdownableWithoutExplicitFilterTypeFallsBack() {
+    var requestBuilder = createRequestBuilder();
+    var knnQuery = new WrapperQueryBuilder("{\"knn\":{}}");
+    var builder = new VectorSearchQueryBuilder(requestBuilder, knnQuery, Map.of("k", "5"));
+
+    // STRUCT = STRUCT triggers ScriptQueryUnSupportedException in FilterQueryBuilder
+    var condition =
+        DSL.equal(
+            new ReferenceExpression("nested_field", ExprCoreType.STRUCT),
+            new ReferenceExpression("other_field", ExprCoreType.STRUCT));
+    var dummyChild = new LogicalValues(Collections.emptyList());
+    var filter = new LogicalFilter(dummyChild, condition);
+
+    boolean pushed = builder.pushDownFilter(filter);
+    assertFalse(pushed, "Non-pushdownable filter should return false for in-memory fallback");
+  }
+
   @Test
   void buildSucceedsRadialWithSortEmbeddedLimit() {
     var requestBuilder = createRequestBuilder();