linq4j/src/main/java/org/apache/calcite/linq4j/QueryProvider.java

/*
 * 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.</p>
 */
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}.</p>
   *
   * @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>
   *
   * <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);
}