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.metadata;
  
  import org.apache.calcite.rel.RelNode;
  
  /**
   * Metadata about a relational expression.
   *
   * <p>For particular types of metadata, a sub-class defines one of more methods
   * to query that metadata. Then a {@link RelMetadataProvider} can offer those
   * kinds of metadata for particular sub-classes of {@link RelNode}.
   *
   * <p>User code (typically in a planner rule or an implementation of
   * {@link RelNode#computeSelfCost(org.apache.calcite.plan.RelOptPlanner, RelMetadataQuery)})
   * acquires a {@code Metadata} instance by calling {@link RelNode#metadata}.
   *
   * <p>A {@code Metadata} instance already knows which particular {@code RelNode}
   * it is describing, so the methods do not pass in the {@code RelNode}. In fact,
   * quite a few metadata methods have no extra parameters. For instance, you can
   * get the row-count as follows:
   *
   * <blockquote><pre><code>
   * RelNode rel;
   * double rowCount = rel.metadata(RowCount.class).rowCount();
   * </code></pre></blockquote>
   */
  public interface Metadata {
    /** Returns the relational expression that this metadata is about. */
    RelNode rel();
  }
  