浅谈PageHelper插件分页实现原理及大数据量下SQL查询效率问题解决

前因：项目一直使用的是PageHelper实现分页功能，项目前期数据量较少一直没有什么问题。随着业务扩增，数据库扩增PageHelper出现了明显的性能问题。几十万甚至上百万的单表数据查询性能缓慢，需要几秒乃至十几秒的查询时间。故此特地研究了一下PageHelper源码，查找PageHelper分页的实现方式。

一段较为简单的查询，跟随debug开始源码探寻之旅。

public ResultContent select(Integer id) {





        Page<Test> blogPage = PageHelper.startPage(1,3).doSelectPage( () -> testDao.select(id));





        List<Test> test = (List<Test>)blogPage.getResult();





        return new ResultContent(0, "success", test);





    }

主要保存由前端传入的pageNum(页数)、pageSize(每页显示数量)和count(是否进行count(0)查询)信息。

这里是简单的创建page并保存当前线程的变量副本心里，不做深究。

public static <E> Page<E> startPage(int pageNum, int pageSize) {





        return startPage(pageNum, pageSize, DEFAULT_COUNT);





    }





 





    public static <E> Page<E> startPage(int pageNum, int pageSize, boolean count) {





        return startPage(pageNum, pageSize, count, (Boolean)null, (Boolean)null);





    }





 





    public static <E> Page<E> startPage(int pageNum, int pageSize, String orderBy) {





        Page<E> page = startPage(pageNum, pageSize);





        page.setOrderBy(orderBy);





        return page;





    }





 





    public static <E> Page<E> startPage(int pageNum, int pageSize, boolean count, Boolean reasonable, Boolean pageSizeZero) {





        Page<E> page = new Page(pageNum, pageSize, count);





        page.setReasonable(reasonable);





        page.setPageSizeZero(pageSizeZero);





        Page<E> oldPage = getLocalPage();





        if(oldPage != null && oldPage.isOrderByOnly()) {





            page.setOrderBy(oldPage.getOrderBy());





        }





 





        setLocalPage(page);





        return page;





    }

开始执行真正的select语句

public <E> Page<E> doSelectPage(ISelect select) {





        select.doSelect();





        return this;





    }

进入MapperProxy类执行invoke方法获取到方法名称及参数值

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {





    if (Object.class.equals(method.getDeclaringClass())) {





      try {





        return method.invoke(this, args);





      } catch (Throwable t) {





        throw ExceptionUtil.unwrapThrowable(t);





      }





    }





    final MapperMethod mapperMethod = cachedMapperMethod(method);





    return mapperMethod.execute(sqlSession, args);





  }

接着是MapperMethod方法执行execute语句，判断是增、删、改、查。判断返回值是多个，进入executeForMany方法

public Object execute(SqlSession sqlSession, Object[] args) {





    Object result;





    if (SqlCommandType.INSERT == command.getType()) {





      Object param = method.convertArgsToSqlCommandParam(args);





      result = rowCountResult(sqlSession.insert(command.getName(), param));





    } else if (SqlCommandType.UPDATE == command.getType()) {





      Object param = method.convertArgsToSqlCommandParam(args);





      result = rowCountResult(sqlSession.update(command.getName(), param));





    } else if (SqlCommandType.DELETE == command.getType()) {





      Object param = method.convertArgsToSqlCommandParam(args);





      result = rowCountResult(sqlSession.delete(command.getName(), param));





    } else if (SqlCommandType.SELECT == command.getType()) {





      if (method.returnsVoid() && method.hasResultHandler()) {





        executeWithResultHandler(sqlSession, args);





        result = null;





      } else if (method.returnsMany()) {





        result = executeForMany(sqlSession, args);





      } else if (method.returnsMap()) {





        result = executeForMap(sqlSession, args);





      } else {





        Object param = method.convertArgsToSqlCommandParam(args);





        result = sqlSession.selectOne(command.getName(), param);





      }





    } else if (SqlCommandType.FLUSH == command.getType()) {





        result = sqlSession.flushStatements();





    } else {





      throw new BindingException("Unknown execution method for: " + command.getName());





    }





    if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {





      throw new BindingException("Mapper method '" + command.getName() 





          + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");





    }





    return result;





  }

这个方法开始调用SqlSessionTemplate、DefaultSqlSession等类获取到Mapper.xml文件的SQL语句

private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {





    List<E> result;





    Object param = method.convertArgsToSqlCommandParam(args);





    if (method.hasRowBounds()) {





      RowBounds rowBounds = method.extractRowBounds(args);





      result = sqlSession.<E>selectList(command.getName(), param, rowBounds);





    } else {





      result = sqlSession.<E>selectList(command.getName(), param);





    }





    // issue #510 Collections & arrays support





    if (!method.getReturnType().isAssignableFrom(result.getClass())) {





      if (method.getReturnType().isArray()) {





        return convertToArray(result);





      } else {





        return convertToDeclaredCollection(sqlSession.getConfiguration(), result);





      }





    }





    return result;





  }

开始进入PageHelper的真正实现，Plugin通过实现InvocationHandler进行动态代理获取到相关信息

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {





    try {





      Set<Method> methods = signatureMap.get(method.getDeclaringClass());





      if (methods != null && methods.contains(method)) {





        return interceptor.intercept(new Invocation(target, method, args));





      }





      return method.invoke(target, args);





    } catch (Exception e) {





      throw ExceptionUtil.unwrapThrowable(e);





    }





  }

PageInterceptor 实现Mybatis的Interceptor 接口，进行拦截

public Object intercept(Invocation invocation) throws Throwable {





        try {





            Object[] args = invocation.getArgs();





            MappedStatement ms = (MappedStatement)args[0];





            Object parameter = args[1];





            RowBounds rowBounds = (RowBounds)args[2];





            ResultHandler resultHandler = (ResultHandler)args[3];





            Executor executor = (Executor)invocation.getTarget();





            CacheKey cacheKey;





            BoundSql boundSql;





            if(args.length == 4) {





                boundSql = ms.getBoundSql(parameter);





                cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql);





            } else {





                cacheKey = (CacheKey)args[4];





                boundSql = (BoundSql)args[5];





            }





 





            this.checkDialectExists();





            List resultList;





            if(!this.dialect.skip(ms, parameter, rowBounds)) {





                if(this.dialect.beforeCount(ms, parameter, rowBounds)) {





                    Long count = this.count(executor, ms, parameter, rowBounds, resultHandler, boundSql);





                    if(!this.dialect.afterCount(count.longValue(), parameter, rowBounds)) {





                        Object var12 = this.dialect.afterPage(new ArrayList(), parameter, rowBounds);





                        return var12;





                    }





                }





 





                resultList = ExecutorUtil.pageQuery(this.dialect, executor, ms, parameter, rowBounds, resultHandler, boundSql, cacheKey);





            } else {





                resultList = executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql);





            }





 





            Object var16 = this.dialect.afterPage(resultList, parameter, rowBounds);





            return var16;





        } finally {





            this.dialect.afterAll();





        }





    }

转到ExecutorUtil抽象类的pageQuery方法

public static <E> List<E> pageQuery(Dialect dialect, Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql, CacheKey cacheKey) throws SQLException {





        if(!dialect.beforePage(ms, parameter, rowBounds)) {





            return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql);





        } else {





            parameter = dialect.processParameterObject(ms, parameter, boundSql, cacheKey);





            String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, cacheKey);





            BoundSql pageBoundSql = new BoundSql(ms.getConfiguration(), pageSql, boundSql.getParameterMappings(), parameter);





            Map<String, Object> additionalParameters = getAdditionalParameter(boundSql);





            Iterator var12 = additionalParameters.keySet().iterator();





 





            while(var12.hasNext()) {





                String key = (String)var12.next();





                pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key));





            }





 





            return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, pageBoundSql);





        }





    }

在抽象类AbstractHelperDialect的getPageSql获取到对应的Page对象

public String getPageSql(MappedStatement ms, BoundSql boundSql, Object parameterObject, RowBounds rowBounds, CacheKey pageKey) {





        String sql = boundSql.getSql();





        Page page = this.getLocalPage();





        String orderBy = page.getOrderBy();





        if(StringUtil.isNotEmpty(orderBy)) {





            pageKey.update(orderBy);





            sql = OrderByParser.converToOrderBySql(sql, orderBy);





        }





 





        return page.isOrderByOnly()?sql:this.getPageSql(sql, page, pageKey);





    }

进入到MySqlDialect类的getPageSql方法进行SQL封装，根据page对象信息增加Limit。分页的信息就是这么拼装起来的

public String getPageSql(String sql, Page page, CacheKey pageKey) {





        StringBuilder sqlBuilder = new StringBuilder(sql.length() + 14);





        sqlBuilder.append(sql);





        if(page.getStartRow() == 0) {





            sqlBuilder.append(" LIMIT ? ");





        } else {





            sqlBuilder.append(" LIMIT ?, ? ");





        }





 





        return sqlBuilder.toString();





    }

将最后拼装好的SQL返回给DefaultSqlSession执行查询并返回

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {





    try {





      MappedStatement ms = configuration.getMappedStatement(statement);





      return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);





    } catch (Exception e) {





      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);





    } finally {





      ErrorContext.instance().reset();





    }

至此整个查询过程完成，原来PageHelper的分页功能是通过Limit拼接SQL实现的。查询效率低的问题也找出来了，那么应该如何解决。

首先分析SQL语句，limit在数据量少或者页数比较靠前的时候查询效率是比较高的。(单表数据量百万进行测试)

select * from user where age = 10 limit 1,10;结果显示0.43s

当where条件后的结果集较大并且页数达到一个量级整个SQL的查询效率就十分低下(哪怕where的条件加上了索引也不行)。

select * from user where age = 10 limit 100000,10;结果显示4.73s

那有什么解决方案呢？mysql就不能单表数据量超百万乃至千万嘛？答案是NO，显然是可以的。

SELECT a.* FROM USER a

INNER JOIN 

    (SELECT id FROM USER WHERE age = 10 LIMIT 100000,10) b 

ON a.id = b.id;

结果0.53s

完美解决了查询效率问题！！！其中需要对where条件增加索引，id因为是主键自带索引。select返回减少回表可以提升查询性能,所以采用查询主键字段后进行关联大幅度提升了查询效率。

PageHelper想要优化需要在拦截器的拼接SQL部分进行重构，由于博主能力有限暂未实现。能力较强的读者可以自己进行重构

附上PageHelper的git地址：https://github.com/pagehelper/Mybatis-PageHelper/