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.rel.core;
  
  import org.apache.calcite.linq4j.function.Experimental;
  import org.apache.calcite.plan.RelOptCluster;
  import org.apache.calcite.plan.RelTraitSet;
  import org.apache.calcite.rel.RelNode;
  import org.apache.calcite.rel.RelWriter;
  import org.apache.calcite.rel.SingleRel;
  
  import java.util.List;
  
  import static java.util.Objects.requireNonNull;
  
  /**
   * Relational expression that iterates over its input and, in addition to
   * returning its results, will forward them into other consumers.
   *
   * <p>NOTE: The current API is experimental and subject to change without
   * notice.
   */
  @Experimental
  public abstract class Spool extends SingleRel {
  
    /**
     * Enumeration representing spool read / write type.
     */
    public enum Type {
      EAGER,
      LAZY
    }
  
    /**
     * How the spool consumes elements from its input.
     *
     * <ul>
     * <li>EAGER: the spool consumes the elements from its input at once at the
     *     initial request;
     * <li>LAZY: the spool consumes the elements from its input one by one by
     *     request.
     * </ul>
     */
    public final Type readType;
  
    /**
     * How the spool forwards elements to consumers.
     *
     * <ul>
     * <li>EAGER: the spool forwards each element as soon as it returns it;
     * <li>LAZY: the spool forwards all elements at once when it is done returning
     *     all of them.
     * </ul>
     */
    public final Type writeType;
  
    //~ Constructors -----------------------------------------------------------
  
    /** Creates a Spool. */
    protected Spool(RelOptCluster cluster, RelTraitSet traitSet, RelNode input,
        Type readType, Type writeType) {
      super(cluster, traitSet, input);
      this.readType = requireNonNull(readType, "readType");
      this.writeType = requireNonNull(writeType, "writeType");
    }
  
    @Override public final RelNode copy(RelTraitSet traitSet,
        List<RelNode> inputs) {
      return copy(traitSet, sole(inputs), readType, writeType);
    }
  
    protected abstract Spool copy(RelTraitSet traitSet, RelNode input,
        Type readType, Type writeType);
  
    @Override public RelWriter explainTerms(RelWriter pw) {
      return super.explainTerms(pw)
          .item("readType", readType)
          .item("writeType", writeType);
    }
  }
  