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;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  /**
   * Source of metadata about relational expressions.
   *
   * <p>The metadata is typically various kinds of statistics used to estimate
   * costs.
   *
   * <p>Each kind of metadata has an interface that extends {@link Metadata} and
   * has a method. Some examples: {@link BuiltInMetadata.Selectivity},
   * {@link BuiltInMetadata.ColumnUniqueness}.
   */
  public interface MetadataFactory {
    /** Returns a metadata interface to get a particular kind of metadata
     * from a particular relational expression. Returns null if that kind of
     * metadata is not available.
     *
     * @param <M> Metadata type
     *
     * @param rel Relational expression
     * @param mq Metadata query
     * @param metadataClazz Metadata class
     * @return Metadata bound to {@code rel} and {@code query}
     */
    <@Nullable M extends @Nullable Metadata> M query(RelNode rel, RelMetadataQuery mq,
        Class<M> metadataClazz);
  }
  