Reject GROUP BY / aggregations on vectorSearch() relations

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

Author: mengweieric

opensearch/src/main/java/org/opensearch/sql/opensearch/storage/VectorSearchIndex.java

@@ -14,7 +14,7 @@
 import org.opensearch.sql.opensearch.client.OpenSearchClient;
 import org.opensearch.sql.opensearch.request.OpenSearchRequestBuilder;
 import org.opensearch.sql.opensearch.storage.scan.OpenSearchIndexScan;
-import org.opensearch.sql.opensearch.storage.scan.OpenSearchIndexScanBuilder;
+import org.opensearch.sql.opensearch.storage.scan.VectorSearchIndexScanBuilder;
 import org.opensearch.sql.opensearch.storage.scan.VectorSearchQueryBuilder;
 import org.opensearch.sql.storage.read.TableScanBuilder;
 
@@ -94,7 +94,7 @@ public TableScanBuilder createScanBuilder() {
                 getClient(),
                 rb.getMaxResponseSize(),
                 rb.build(getIndexName(), cursorKeepAlive, getClient(), getFieldTypes().isEmpty()));
-    return new OpenSearchIndexScanBuilder(queryBuilder, createScanOperator);
+    return new VectorSearchIndexScanBuilder(queryBuilder, createScanOperator);
   }
 
   private QueryBuilder buildKnnQuery() {

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

@@ -0,0 +1,33 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.opensearch.storage.scan;
+
+import java.util.function.Function;
+import org.opensearch.sql.exception.ExpressionEvaluationException;
+import org.opensearch.sql.opensearch.request.OpenSearchRequestBuilder;
+import org.opensearch.sql.planner.logical.LogicalAggregation;
+
+/**
+ * Scan builder for vector search relations. Rejects aggregation pushdown because the semantics of
+ * aggregations over knn-scored documents are ambiguous at the SQL surface (e.g., per-bucket _score
+ * is undefined). Rejecting at push-down time keeps the MVP contract tight; support can be added
+ * later without breaking existing users.
+ */
+public class VectorSearchIndexScanBuilder extends OpenSearchIndexScanBuilder {
+
+  public VectorSearchIndexScanBuilder(
+      PushDownQueryBuilder translator,
+      Function<OpenSearchRequestBuilder, OpenSearchIndexScan> scanFactory) {
+    super(translator, scanFactory);
+  }
+
+  @Override
+  public boolean pushDownAggregation(LogicalAggregation aggregation) {
+    throw new ExpressionEvaluationException(
+        "GROUP BY / aggregations are not supported on vectorSearch() relations. "
+            + "Wrap the vectorSearch() call in a subquery and aggregate over the subquery result.");
+  }
+}

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

@@ -0,0 +1,52 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.sql.opensearch.storage.scan;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.Mockito.mock;
+
+import java.util.Collections;
+import org.junit.jupiter.api.Test;
+import org.opensearch.index.query.WrapperQueryBuilder;
+import org.opensearch.sql.common.setting.Settings;
+import org.opensearch.sql.exception.ExpressionEvaluationException;
+import org.opensearch.sql.opensearch.data.value.OpenSearchExprValueFactory;
+import org.opensearch.sql.opensearch.request.OpenSearchRequestBuilder;
+import org.opensearch.sql.planner.logical.LogicalAggregation;
+import org.opensearch.sql.planner.logical.LogicalValues;
+
+class VectorSearchIndexScanBuilderTest {
+
+  @Test
+  void pushDownAggregationIsRejected() {
+    var requestBuilder =
+        new OpenSearchRequestBuilder(
+            mock(OpenSearchExprValueFactory.class), 10000, mock(Settings.class));
+    var queryBuilder =
+        new VectorSearchQueryBuilder(
+            requestBuilder, new WrapperQueryBuilder("{\"knn\":{}}"), java.util.Map.of("k", "5"));
+    var scanBuilder =
+        new VectorSearchIndexScanBuilder(queryBuilder, rb -> mock(OpenSearchIndexScan.class));
+
+    var agg =
+        new LogicalAggregation(
+            new LogicalValues(Collections.emptyList()),
+            Collections.emptyList(),
+            Collections.emptyList(),
+            false);
+
+    ExpressionEvaluationException ex =
+        assertThrows(
+            ExpressionEvaluationException.class, () -> scanBuilder.pushDownAggregation(agg));
+    assertTrue(
+        ex.getMessage().contains("GROUP BY"),
+        "Error should mention GROUP BY; actual: " + ex.getMessage());
+    assertTrue(
+        ex.getMessage().contains("vectorSearch"),
+        "Error should mention vectorSearch; actual: " + ex.getMessage());
+  }
+}