Mybatis 批量操作-删除、修改和查询

      批量操作的核心就是一次传入多个数据然后进行相关操作，增删改查中掌握其中一个，其它的就可以举一反三，触类旁通。它之所以执行效率高，是因为合并后日志量（MySQL的binlog和InnoDB的事务日志）减少了，降低日志刷盘的数据量和频率，从而提高效率；同时也能减少SQL语句解析的次数，减少网络传输的IO。但是，以下几点需要注意：

1.  SQL语句有长度限制，在进行数据合并在同一SQL中务必不能超过SQL长度限制，通过max_allowed_packet配置可以修改，默认是1M。
2.  事务需要控制大小，事务太大可能会影响执行的效率。MySQL有innodb_log_buffer_size配置项，超过这个值会把InnoDB的数据刷到磁盘中，这时，效率会有所下降。所以比较好的做法是，在数据达到这个值前进行事务提交。

      在《Java 使用线程池分批插入或者更新数据》中介绍了如何在Java端批量切分数据，然后，使用线程池将被切分的数据传入相应DAO层的方法后，即可完整实现批量操作。在《Mybatis批量insert 返回主键值和foreach标签详解》中已经介绍了批量插入操作，而且，详细描述了foreach标签，这里简要概述批量删除、更新和查找。

     首先，mysql需要数据库连接配置&allowMultiQueries=true

jdbc:mysql://127.0.0.1:3306/mybank?useUnicode=true&characterEncoding=utf8&allowMultiQueries=true

批量删除

<delete id= "batchDeleteByIds" parameterType= "list">

       delete from instance where instance_id in

       <foreach collection="list" item= "item" index ="index"

            open= "(" close =")" separator=",">

            #{item}

       </foreach >

</delete >

批量更新

<update id= "updateUpdateTimeByIds" parameterType= "map">

    update instance

    set update_time  = #{ updateTime } where instance_id in

    <foreach collection="idlist" item= "uid" index ="index"

            open= "(" close =")" separator=",">

            #{ uid}

     </foreach >

</update >

      用法和之前的基本相同，但是这里传入的参数是map类型，idlist和updateTime是map的key。

批量查询

<select id="selectByIds" resultType="list" parameterType="map"> 

 SELECT infos, create_time, update_time FROM instance WHERE instance_id in

     <foreach collection="ids" item="id" index="index" open="(" close=")" separator=","> 

           #{id}

     </foreach>

</select>

    这里提供一下DAO层：

List<Instance> selectByIds (Map<String, Object> map);

void batchDeleteByIds (List<Long> list);

void updateUpdateTimeByIds(Map<String, Object> map);

      乍看上去这个foreach没有问题，但是经过项目实践发现，当表的列数较多（20+），以及一次性插入的行数较多（5000+）时，整个插入的耗时十分漫长，达到了14分钟，这是不能忍的。在资料中也提到了一句话：

 

   Of course don't combine ALL of them, if the amount is HUGE. Say you have 1000 rows you need to insert, then don't do it one at a time. You shouldn't equally try to have all 1000 rows in a single query. Instead break it into smaller sizes.

      它强调，当插入数量很多时，不能把所有的鸡蛋放在同一个篮子里，即一次性全放在一条语句里。可是为什么不能放在同一条语句里呢？这条语句为什么会耗时这么久呢？我查阅了资料发现： 

   Insert inside MyBatis foreach is not batch, this is a single (could become giant) SQL statement and that brings drawbacks:

• some database such as Oracle here does not support.
• in relevant cases: there will be a large number of records to insert and the database configured limit (by default around 2000 parameters per statement) will be hit, and eventually possibly DB stack error if the statement itself become too large.

   Iteration over the collection must not be done in the mybatis XML. Just execute a simple Insertstatement in a Java Foreach loop. The most important thing is the session Executor type.

SqlSession session = sessionFactory.openSession(ExecutorType.BATCH);
for (Model model : list) {
    session.insert("insertStatement", model);
}
session.flushStatements();

   Unlike default ExecutorType.SIMPLE, the statement will be prepared once and executed for each record to insert.

       虽然MyBatis官网推荐使用ExecutorType.BATCH 的插入方式，因为，其性能更好；但是，其SQL写在了Java里，如果SQL比较复杂，则不易于维护。因此，本文只详细介绍了常见的使用foreach标签的方式。下面从MyBatis官网借用一个Batch Insert

示例。 

      A batch insert is a collection of statements that can be used to execute a JDBC batch. A batch is the preferred method of doing bulk inserts with JDBC. The basic idea is that you configure the connection for a batch insert, then execute the same statement multiple times, with different values for each inserted record. MyBatis has a nice abstraction of JDBC batches that works well with statements generated from this library. A batch insert looks like this:

...
    SqlSession session = sqlSessionFactory.openSession(ExecutorType.BATCH);
    try {
        SimpleTableMapper mapper = session.getMapper(SimpleTableMapper.class);
        List<SimpleTableRecord> records = getRecordsToInsert(); // not shown

        BatchInsert<SimpleTableRecord> batchInsert = insert(records)
                .into(simpleTable)
                .map(id).toProperty("id")
                .map(firstName).toProperty("firstName")
                .map(lastName).toProperty("lastName")
                .map(birthDate).toProperty("birthDate")
                .map(employed).toProperty("employed")
                .map(occupation).toProperty("occupation")
                .build()
                .render(RenderingStrategy.MYBATIS3);

        batchInsert.insertStatements().stream().forEach(mapper::insert);

        session.commit();
    } finally {
        session.close();
    }
...

Reference 

  http://www.pianshen.com/article/215691520/

  http://www.mybatis.org/mybatis-dynamic-sql/docs/insert.html