content: /*
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to you under the Apache License, Version 2.0
   * (the "License"); you may not use this file except in compliance with
   * the License.  You may obtain a copy of the License at
   *
   * http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.calcite.plan;
  
  /**
   * Policy by which operands will be matched by relational expressions with
   * any number of children.
   */
  public enum RelOptRuleOperandChildPolicy {
    /**
     * Signifies that operand can have any number of children.
     */
    ANY,
  
    /**
     * Signifies that operand has no children. Therefore it matches a
     * leaf node, such as a table scan or VALUES operator.
     *
     * <p>{@code RelOptRuleOperand(Foo.class, NONE)} is equivalent to
     * {@code RelOptRuleOperand(Foo.class)} but we prefer the former because
     * it is more explicit.
     */
    LEAF,
  
    /**
     * Signifies that the operand's children must precisely match its
     * child operands, in order.
     */
    SOME,
  
    /**
     * Signifies that the rule matches any one of its parents' children.
     * The parent may have one or more children.
     */
    UNORDERED,
  }
  