Reject radial vector search without LIMIT

Radial search (max_distance or min_score) can return unbounded results.
Add build-time validation that rejects radial queries without an explicit
LIMIT clause, with a clear error message guiding the user.

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

Author: mengweieric

integ-test/src/test/java/org/opensearch/sql/sql/VectorSearchIT.java

@@ -203,6 +203,21 @@ public void testMissingRequiredOptionRejects() throws IOException {
     assertThat(ex.getMessage(), containsString("Missing required option"));
   }
 
+  @Test
+  public void testRadialWithoutLimitRejects() throws IOException {
+    ResponseException ex =
+        expectThrows(
+            ResponseException.class,
+            () ->
+                executeQuery(
+                    "SELECT v._id FROM vectorSearch(table='"
+                        + TEST_INDEX
+                        + "', field='embedding', "
+                        + "vector='[1.0, 2.0]', option='max_distance=10.5') AS v"));
+
+    assertThat(ex.getMessage(), containsString("LIMIT is required for radial vector search"));
+  }
+
   // ── Sort restriction validation ─────────────────────────────────────────
 
   @Test

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

@@ -44,6 +44,7 @@ public class VectorSearchQueryBuilder extends OpenSearchIndexScanQueryBuilder {
   private final boolean filterTypeExplicit;
   private final Function<QueryBuilder, QueryBuilder> rebuildKnnWithFilter;
   private boolean filterPushed = false;
+  private boolean limitPushed = false;
 
   /** Full constructor with filter type support. */
   public VectorSearchQueryBuilder(
@@ -95,6 +96,7 @@ public boolean pushDownLimit(LogicalLimit limit) {
             String.format("LIMIT %d exceeds k=%d in top-k vector search", limit.getLimit(), k));
       }
     }
+    limitPushed = true;
     return super.pushDownLimit(limit);
   }
 
@@ -131,6 +133,12 @@ public OpenSearchRequestBuilder build() {
     if (filterTypeExplicit && !filterPushed) {
       throw new ExpressionEvaluationException("filter_type requires a pushdownable WHERE clause");
     }
+    boolean isRadial = !options.containsKey("k");
+    if (isRadial && !limitPushed) {
+      throw new ExpressionEvaluationException(
+          "LIMIT is required for radial vector search (max_distance or min_score)."
+              + " Without LIMIT, the result set size is unbounded.");
+    }
     return super.build();
   }
 }

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

@@ -382,6 +382,56 @@ void buildSucceedsWithFilterTypeAndPushedWhere() {
     assertNotNull(result);
   }
 
+  // ── Radial without LIMIT rejection ─────────────────────────────────
+
+  @Test
+  void buildRejectsRadialMaxDistanceWithoutLimit() {
+    var requestBuilder = createRequestBuilder();
+    var knnQuery = new WrapperQueryBuilder("{\"knn\":{}}");
+    var builder =
+        new VectorSearchQueryBuilder(requestBuilder, knnQuery, Map.of("max_distance", "10.0"));
+
+    ExpressionEvaluationException ex =
+        assertThrows(ExpressionEvaluationException.class, builder::build);
+    assertTrue(ex.getMessage().contains("LIMIT is required for radial vector search"));
+  }
+
+  @Test
+  void buildRejectsRadialMinScoreWithoutLimit() {
+    var requestBuilder = createRequestBuilder();
+    var knnQuery = new WrapperQueryBuilder("{\"knn\":{}}");
+    var builder =
+        new VectorSearchQueryBuilder(requestBuilder, knnQuery, Map.of("min_score", "0.5"));
+
+    ExpressionEvaluationException ex =
+        assertThrows(ExpressionEvaluationException.class, builder::build);
+    assertTrue(ex.getMessage().contains("LIMIT is required for radial vector search"));
+  }
+
+  @Test
+  void buildSucceedsRadialWithLimit() {
+    var requestBuilder = createRequestBuilder();
+    var knnQuery = new WrapperQueryBuilder("{\"knn\":{}}");
+    var builder =
+        new VectorSearchQueryBuilder(requestBuilder, knnQuery, Map.of("max_distance", "10.0"));
+
+    var dummyChild = new LogicalValues(Collections.emptyList());
+    builder.pushDownLimit(new LogicalLimit(dummyChild, 50, 0));
+
+    OpenSearchRequestBuilder result = builder.build();
+    assertNotNull(result);
+  }
+
+  @Test
+  void buildSucceedsTopKWithoutLimit() {
+    var requestBuilder = createRequestBuilder();
+    var knnQuery = new WrapperQueryBuilder("{\"knn\":{}}");
+    var builder = new VectorSearchQueryBuilder(requestBuilder, knnQuery, Map.of("k", "5"));
+
+    OpenSearchRequestBuilder result = builder.build();
+    assertNotNull(result);
+  }
+
   // ── Regression: LIMIT and sort invariants under efficient mode ──────
 
   @Test