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.linq4j;
  
  import org.apache.calcite.linq4j.tree.Expression;
  
  import java.lang.reflect.Type;
  
  /**
   * Defines methods to create and execute queries that are described by a
   * {@link Queryable} object.
   *
   * <p>Analogous to LINQ's System.Linq.QueryProvider.
   */
  public interface QueryProvider {
    /**
     * Constructs a {@link Queryable} object that can evaluate the query
     * represented by a specified expression tree.
     *
     * <p>NOTE: The {@link org.apache.calcite.linq4j.Queryable#getExpression()}
     * property of the returned {@link Queryable} object is equal to
     * {@code expression}.
     *
     * @param expression Expression
     * @param rowType Row type
     * @param <T> Row type
     *
     * @return Queryable
     */
    <T> Queryable<T> createQuery(Expression expression, Class<T> rowType);
  
    /**
     * Constructs a {@link Queryable} object that can evaluate the query
     * represented by a specified expression tree. The row type may contain
     * generic information.
     *
     * @param expression Expression
     * @param rowType Row type
     * @param <T> Row type
     *
     * @return Queryable
     */
    <T> Queryable<T> createQuery(Expression expression, Type rowType);
  
    /**
     * Executes the query represented by a specified expression tree.
     *
     * <p>This method executes queries that return a single value
     * (instead of an enumerable sequence of values). Expression trees that
     * represent queries that return enumerable results are executed when the
     * {@link Queryable} object that contains the expression tree is
     * enumerated.
     *
     * <p>The Queryable standard query operator methods that return singleton
     * results call {@code execute}. They pass it a
     * {@link org.apache.calcite.linq4j.tree.MethodCallExpression}
     * that represents a linq4j query.
     */
    <T> T execute(Expression expression, Class<T> type);
  
    /**
     * Executes the query represented by a specified expression tree.
     * The row type may contain type parameters.
     */
    <T> T execute(Expression expression, Type type);
  
    /**
     * Executes a queryable, and returns an enumerator over the
     * rows that it yields.
     *
     * @param queryable Queryable
     *
     * @return Enumerator over rows
     */
    <T> Enumerator<T> executeQuery(Queryable<T> queryable);
  }
  