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

本文深入探讨了PageHelper分页插件的实现原理，分析了其在大数据量下查询效率低的原因，并提出了一种通过内连接优化SQL查询的高效分页方案。

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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
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作者：岁月安然
来源：CSDN
原文：https://blog.csdn.net/baidu_38083619/article/details/82463058
版权声明：本文为博主原创文章，转载请附上博文链接！