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.
   */
  
  /**
   * Management of materialized query results.
   *
   * <p>An actor ({@link org.apache.calcite.materialize.MaterializationActor})
   * maintains the state of all
   * materializations in the system and is wrapped in a service
   * ({@link org.apache.calcite.materialize.MaterializationService})
   * for access from other parts of the system.
   *
   * <p>Optimizer rules allow Calcite to rewrite queries using materializations,
   * if they are valid (that is, contain the same result as executing their
   * defining query) and lower cost.
   *
   * <p>In future, the actor may manage the process of updating materializations,
   * instantiating materializations from the intermediate results of queries, and
   * recognize what materializations would be useful based on actual query load.
   */
  @DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.FIELD)
  @DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.PARAMETER)
  @DefaultQualifier(value = NonNull.class, locations = TypeUseLocation.RETURN)
  package org.apache.calcite.materialize;
  
  import org.checkerframework.checker.nullness.qual.NonNull;
  import org.checkerframework.framework.qual.DefaultQualifier;
  import org.checkerframework.framework.qual.TypeUseLocation;
  