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.schema;
  
  import org.apache.calcite.rel.RelCollation;
  import org.apache.calcite.rel.RelDistribution;
  import org.apache.calcite.rel.RelReferentialConstraint;
  import org.apache.calcite.util.ImmutableBitSet;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  import java.util.List;
  
  /**
   * Statistics about a {@link Table}.
   *
   * <p>Each of the methods may return {@code null} meaning "not known".
   *
   * @see Statistics
   */
  public interface Statistic {
    /** Returns the approximate number of rows in the table. */
    default @Nullable Double getRowCount() {
      return null;
    }
  
    /** Returns whether the given set of columns is a unique key, or a superset
     * of a unique key, of the table.
     */
    default boolean isKey(ImmutableBitSet columns) {
      return false;
    }
  
    /** Returns a list of unique keys, or null if no key exist. */
    default @Nullable List<ImmutableBitSet> getKeys() {
      return null;
    }
  
    /** Returns the collection of referential constraints (foreign-keys)
     * for this table. */
    default @Nullable List<RelReferentialConstraint> getReferentialConstraints() {
      return null;
    }
  
    /** Returns the collections of columns on which this table is sorted. */
    default @Nullable List<RelCollation> getCollations() {
      return null;
    }
  
    /** Returns the distribution of the data in this table. */
    default @Nullable RelDistribution getDistribution()  {
      return null;
    }
  }
  