Create vectorArgumentsCount() on VectorFunction to determine what params to cast

Author: carlosdelest

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/analysis/Analyzer.java

@@ -1668,8 +1668,11 @@ private static Expression castStringLiteral(Expression from, DataType target) {
         private static Expression processVectorFunction(org.elasticsearch.xpack.esql.core.expression.function.Function vectorFunction) {
             List<Expression> args = vectorFunction.arguments();
             List<Expression> newArgs = new ArrayList<>();
-            for (Expression arg : args) {
-                if (arg.resolved() && arg.dataType().isNumeric()) {
+            // Only the first vector arguments are vectors and considered for casting
+            int vectorArgsCount = ((VectorFunction)vectorFunction).vectorArgumentsCount();
+            for (int i = 0; i < args.size(); i++) {
+                Expression arg = args.get(i);
+                if (i < vectorArgsCount && arg.resolved() && arg.dataType().isNumeric()) {
                     if (arg.foldable()) {
                         Object folded = arg.fold(FoldContext.small() /* TODO remove me */);
                         if (folded instanceof List) {

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

@@ -203,6 +203,11 @@ public DataType dataType() {
         return DataType.BOOLEAN;
     }
 
+    @Override
+    public int vectorArgumentsCount() {
+        return 2;
+    }
+
     @Override
     protected TypeResolution resolveParams() {
         return resolveField().and(resolveQuery()).and(resolveK()).and(Options.resolve(options(), source(), FOURTH, ALLOWED_OPTIONS));

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

@@ -96,6 +96,11 @@ protected TypeResolution resolveType() {
         return isType(field(), dt -> dt == DENSE_VECTOR, sourceText(), TypeResolutions.ParamOrdinal.FIRST, "dense_vector");
     }
 
+    @Override
+    public int vectorArgumentsCount() {
+        return 1;
+    }
+
     /**
      * Functional interface for evaluating the scalar value of the underlying float array.
      */

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

@@ -12,4 +12,11 @@
  * from multi values to dense_vector field types, so parameters are actually
  * processed as dense_vectors in vector functions
  */
-public interface VectorFunction {}
+public interface VectorFunction {
+
+    /**
+     * Number of arguments that should be treated as vectors. The first vectorArgumentsCount() arguments will be implicitly casted as
+     * dense_vector according to the value returned of this method
+     */
+    int vectorArgumentsCount();
+}

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

@@ -77,6 +77,11 @@ public Object fold(FoldContext ctx) {
         return EvaluatorMapper.super.fold(source(), ctx);
     }
 
+    @Override
+    public int vectorArgumentsCount() {
+        return 2;
+    }
+
     @Override
     public final EvalOperator.ExpressionEvaluator.Factory toEvaluator(EvaluatorMapper.ToEvaluator toEvaluator) {
         return new SimilarityEvaluatorFactory(