K must be set either explicitly or implicitly

Author: carlosdelest

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/vector/Knn.java

@@ -11,6 +11,7 @@
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.index.query.QueryBuilder;
+import org.elasticsearch.xpack.esql.capabilities.PostOptimizationVerificationAware;
 import org.elasticsearch.xpack.esql.common.Failures;
 import org.elasticsearch.xpack.esql.core.InvalidArgumentException;
 import org.elasticsearch.xpack.esql.core.expression.Expression;
@@ -60,7 +61,7 @@
 import static org.elasticsearch.xpack.esql.core.type.DataType.FLOAT;
 import static org.elasticsearch.xpack.esql.core.type.DataType.INTEGER;
 
-public class Knn extends FullTextFunction implements OptionalArgument, VectorFunction {
+public class Knn extends FullTextFunction implements OptionalArgument, VectorFunction, PostOptimizationVerificationAware {
 
     public static final NamedWriteableRegistry.Entry ENTRY = new NamedWriteableRegistry.Entry(Expression.class, "Knn", Knn::readFrom);
 
@@ -288,6 +289,14 @@ protected String notSupportedErroMessage() {
         return "[{}] {} is only supported in WHERE commands";
     }
 
+    @Override
+    public void postOptimizationVerification(Failures failures) {
+        // Checks that k is set, either because of limit or options
+        if (limit == null && knnQueryOptions().get(K_FIELD.getPreferredName()) == null) {
+            failures.add(fail(this, "k must be set either through a LIMIT or using the KNN k option", functionName(), functionType()));
+        }
+    }
+
     @Override
     protected NodeInfo<? extends Expression> info() {
         return NodeInfo.create(this, Knn::new, field(), query(), options());

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/LogicalPlanOptimizerTests.java

@@ -7995,4 +7995,17 @@ public void testSampleNoPushDownChangePoint() {
         var topN = as(changePoint.child(), TopN.class);
         var source = as(topN.child(), EsRelation.class);
     }
+
+    public void testKnnFunctionHasLimitOrKSpecified() {
+        assumeTrue("KNN must be enabled", EsqlCapabilities.Cap.KNN_FUNCTION.isEnabled());
+
+        var query = """
+            FROM test
+            | WHERE KNN(dense_vector, [1.0, 2.0, 3.0])
+            | STATS c = COUNT(*)
+            """;
+
+        VerificationException e = expectThrows(VerificationException.class, () -> planTypes(query));
+        assertThat(e.getMessage(), containsString("k must be set either through a LIMIT or using the KNN k option"));
+    }
 }