statement/mybatis操作mysql普通查询；游标查询；流式查询

概述

我当前的jdbc驱动版本如下
mysql-connector-java-8.0.30.jar

在实际业务中一般用普通查询就可以覆盖90%的场景了。但是在一些环境需要去处理大量数据，而这些数据可能会超过JVM的堆内存上限。所以此时需要改变一下查询方式；改为一批一批查询数据然后进行业务处理，有点分治法的味道。

那怎么看当前查询是哪种查询呢，其实只要看获取数据时是实现类就很清晰了。

• com.Mysql.cj.protocol.a.result.ResultsetRowsStatic：普通查询，将结果集一次性全部拉取到内存
• com.mysql.cj.protocol.a.result.ResultsetRowsCursor：游标查询，将结果集分批拉取到内存，会占用当前连接直到连接关闭。在mysql那边会建立一个临时表写入磁盘（查询结束后由mysql回收处理），会导致mySQL Server磁盘io飙升。
• com.mysql.cj.protocol.a.result.ResultsetRowsStreaming:流式查询，将结果集一条一条的拉取进内存，占用当前mysql连接。

普通查询

这个没啥好说的就是普通的sql查询；

游标查询

我当前使用的mysql是8.0的需要在jdbc链接参数里声明开启游标查询
useCursorFetch=true

原生查询如下

DataSource dataSource = null;Connection connection = null;Statement statement = null;ResultSet resultSet = null;AtomicLong count = new AtomicLong(0);try {    dataSource = CycleDependenciesUtil.getBean(DataSource.class);    connection = dataSource.getConnection();    statement = connection.createStatement(ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY);    String sql = "select * from xxx";    statement.setFetchSize(1000);    resultSet = statement.executeQuery(sql);    while (resultSet.next()){        long countResult = count.incrementAndGet();//处理业务    }    System.out.println("总共获取行数：" + count.get());} catch (SQLException e) {    throw new RuntimeException(e);}finally {    if(resultSet!=null){        try {resultSet.close();} catch (SQLException e) { e.printStackTrace();}    }    if(statement!=null){        try {statement.close();} catch (SQLException e) {e.printStackTrace();}    }    if(connection!=null){        try {connection.close();} catch (SQLException e) {e.printStackTrace();}    }}

mybatis游标查询
mysql8.0必需在jdbc驱动链接处加上useCursorFetch=true开启游标查询

//mapper接口Cursor<Map<String, Object>> cursorQuery();

xml语句

<select id="cursorQuery" resultType="java.util.Map" resultMap="streaMQuery" resultSetType="FORWARD_ONLY" fetchSize="1000">    select * from `suppliers-system`.as_fact_asselect>

//使用demo代码AtomicLong count = new AtomicLong(0);Cursor<Map<String, Object>> cursor = null;SqlSession sqlSession = null;try {    SqlSessionFactory sqlSessionFactory = CycleDependenciesUtil.getBean(SqlSessionFactory.class);    sqlSession = sqlSessionFactory.openSession();    ProductAccessoriesSkuMapper mapper = sqlSession.getMapper(ProductAccessoriesSkuMapper.class);    cursor = mapper.cursorQuery();    Iterator<Map<String, Object>> iterator = cursor.iterator();    while (iterator.hasNext()){        count.incrementAndGet();        //mybatis相对于原生jdbc操作映射了结果集，方便操作        Map<String, Object> rowData = iterator.next();//todo 业务操作    }    System.out.println("总共获取行数：" + count.get());} finally {    if(Objects.nonNull(cursor)){        try {            cursor.close();        } catch (IOException e) {            throw new RuntimeException(e);        }    }    if(Objects.nonNull(sqlSession)){        sqlSession.close();    }}

流式查询

原生statement查询

DataSource dataSource = null;Connection connection = null;Statement statement = null;ResultSet resultSet = null;AtomicLong count = new AtomicLong(0);String sql = "select * from `suppliers-system`.as_fact_as";try {    dataSource = CycleDependenciesUtil.getBean(DataSource.class);    //流式查询会一直占用这个tcp连接，直到close    connection = dataSource.getConnection();    //ResultSet.TYPE_FORWARD_ONLY 设置游标只能向下    //ResultSet.CONCUR_READ_ONLY 设置结果集只读不可修改    statement = connection.createStatement(ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY);    //这里必需设置为Integer.MIN_VALUE，源码里做了强制校验    statement.setFetchSize(Integer.MIN_VALUE);    resultSet = statement.executeQuery(sql);    while (resultSet.next()){        long countResult = count.incrementAndGet();        //处理业务        //String code = resultSet.getString("serv_info_id");    }    System.out.println("总共获取行数：" + count.get());} catch (SQLException e) {    throw new RuntimeException(e);}finally {    if(resultSet!=null){        try {resultSet.close();} catch (SQLException e) { e.printStackTrace();}    }    if(statement!=null){        try {statement.close();} catch (SQLException e) {e.printStackTrace();}    }    if(connection!=null){        try {connection.close();} catch (SQLException e) {e.printStackTrace();}    }}

mybatis流式查询

//mapper接口void streamQuery(ResultHandler<Map<String,Object>> resultHandler);

<resultMap id="streamQuery" type="java.util.HashMap">    <result column="reg_date" property="reg_date"/>resultMap><select id="streamQuery" resultSetType="FORWARD_ONLY" fetchSize="-2147483648" resultMap="streamQuery">    select * from `suppliers-system`.as_fact_asselect>

//使用demo代码AtomicLong count = new AtomicLong(0);SqlSession sqlSession = null;try {    SqlSessionFactory sqlSessionFactory = CycleDependenciesUtil.getBean(SqlSessionFactory.class);    sqlSession = sqlSessionFactory.openSession();    ProductAccessoriesSkuMapper mapper = sqlSession.getMapper(ProductAccessoriesSkuMapper.class);    //流式查询每次只获取一条    //当前sqlSession只能处理当前流式查询，不能再查询其他    mapper.streamQuery(new ResultHandler<Map<String, Object>>() {        @Override        public void handleResult(ResultContext<? extends Map<String, Object>> resultContext) {            long countResult = count.incrementAndGet();            Map<String, Object> rowData = resultContext.getResultObject();            //处理业务        }    });    System.out.println("总共获取行数：" + count.get());} finally {    if(sqlSession!=null){       sqlSession.close();    }}

最后概括 流式查询、游标查询可以避免 OOM，数据量大可以考虑此方案。
但是这两种方式会占用数据库连接，使用中不会释放，所以线上针对大数据量业务用到游标和流式操作，一定要进行并发控制 另外针对 JDBC
原生流式查询，Mybatis 中也进行了封装，虽然会慢一些，但是 功能以及代码的整洁程度会好上不少

来源地址：https://blog.csdn.net/weixin_43944305/article/details/128897778