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。
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public ResultContent select(Integer id) {
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Page<Test> blogPage = PageHelper.startPage(
1,
3).doSelectPage( () -> testDao.select(id));
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List<Test> test = (
List<Test>)blogPage.getResult();
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return
new ResultContent(
0,
"success", test);
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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。
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public
static <E>
Page<E> startPage(int pageNum, int pageSize) {
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return startPage(pageNum, pageSize, DEFAULT_COUNT);
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-
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public
static <E>
Page<E> startPage(int pageNum, int pageSize, boolean count) {
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return startPage(pageNum, pageSize, count, (Boolean)
null, (Boolean)
null);
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-
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public
static <E>
Page<E> startPage(int pageNum, int pageSize, String orderBy) {
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Page<E> page = startPage(pageNum, pageSize);
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page.setOrderBy(orderBy);
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-
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public
static <E>
Page<E> startPage(int pageNum, int pageSize, boolean count, Boolean reasonable, Boolean pageSizeZero) {
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Page<E> page =
new Page(pageNum, pageSize, count);
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page.setReasonable(reasonable);
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page.setPageSizeZero(pageSizeZero);
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Page<E> oldPage = getLocalPage();
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if(oldPage !=
null && oldPage.isOrderByOnly()) {
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page.setOrderBy(oldPage.getOrderBy());
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-
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开始执行真正的select语句
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public <E>
Page<E> doSelectPage(ISelect select) {
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-
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进入MapperProxy类执行invoke方法获取到方法名称及参数值
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public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
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if (Object.
class.equals(method.getDeclaringClass())) {
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return method.invoke(
this, args);
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throw ExceptionUtil.unwrapThrowable(t);
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-
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final MapperMethod mapperMethod = cachedMapperMethod(method);
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return mapperMethod.execute(sqlSession, args);
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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
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public
Object execute(SqlSession sqlSession,
Object[] args) {
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if (SqlCommandType.INSERT == command.getType()) {
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Object param = method.convertArgsToSqlCommandParam(args);
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result = rowCountResult(sqlSession.insert(command.getName(), param));
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}
else
if (SqlCommandType.UPDATE == command.getType()) {
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Object param = method.convertArgsToSqlCommandParam(args);
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result = rowCountResult(sqlSession.update(command.getName(), param));
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}
else
if (SqlCommandType.DELETE == command.getType()) {
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Object param = method.convertArgsToSqlCommandParam(args);
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result = rowCountResult(sqlSession.delete(command.getName(), param));
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}
else
if (SqlCommandType.SELECT == command.getType()) {
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if (method.returnsVoid() && method.hasResultHandler()) {
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executeWithResultHandler(sqlSession, args);
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}
else
if (method.returnsMany()) {
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result = executeForMany(sqlSession, args);
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}
else
if (method.returnsMap()) {
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result = executeForMap(sqlSession, args);
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Object param = method.convertArgsToSqlCommandParam(args);
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result = sqlSession.selectOne(command.getName(), param);
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}
else
if (SqlCommandType.FLUSH == command.getType()) {
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result = sqlSession.flushStatements();
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throw
new BindingException(
"Unknown execution method for: " + command.getName());
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if (result ==
null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
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throw
new BindingException(
"Mapper method '" + command.getName()
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+
" attempted to return null from a method with a primitive return type (" + method.getReturnType() +
").");
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这个方法开始调用SqlSessionTemplate、DefaultSqlSession等类获取到Mapper.xml文件的SQL语句
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private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
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Object param = method.convertArgsToSqlCommandParam(args);
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if (method.hasRowBounds()) {
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RowBounds rowBounds = method.extractRowBounds(args);
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result = sqlSession.<E>selectList(command.getName(), param, rowBounds);
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result = sqlSession.<E>selectList(command.getName(), param);
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if (!method.getReturnType().isAssignableFrom(result.getClass())) {
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if (method.getReturnType().isArray()) {
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return convertToArray(result);
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return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
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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
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public
Object invoke(
Object proxy, Method method,
Object[] args) throws Throwable {
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Set<Method> methods = signatureMap.get(method.getDeclaringClass());
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if (methods !=
null && methods.contains(method)) {
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return interceptor.intercept(
new Invocation(target, method, args));
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return method.invoke(target, args);
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throw ExceptionUtil.unwrapThrowable(e);
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PageInterceptor 实现Mybatis的Interceptor 接口，进行拦截
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public Object intercept(Invocation invocation) throws Throwable {
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Object[] args = invocation.getArgs();
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MappedStatement ms = (MappedStatement)args[
0];
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Object parameter = args[
1];
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RowBounds rowBounds = (RowBounds)args[
2];
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ResultHandler resultHandler = (ResultHandler)args[
3];
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Executor executor = (Executor)invocation.getTarget();
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boundSql = ms.getBoundSql(parameter);
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cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql);
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cacheKey = (CacheKey)args[
4];
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boundSql = (BoundSql)args[
5];
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this.checkDialectExists();
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if(!
this.dialect.skip(ms, parameter, rowBounds)) {
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if(
this.dialect.beforeCount(ms, parameter, rowBounds)) {
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Long count =
this.count(executor, ms, parameter, rowBounds, resultHandler, boundSql);
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if(!
this.dialect.afterCount(count.longValue(), parameter, rowBounds)) {
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Object var12 =
this.dialect.afterPage(new ArrayList(), parameter, rowBounds);
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resultList = ExecutorUtil.pageQuery(
this.dialect, executor, ms, parameter, rowBounds, resultHandler, boundSql, cacheKey);
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resultList = executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql);
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Object var16 =
this.dialect.afterPage(resultList, parameter, rowBounds);
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转到ExecutorUtil抽象类的pageQuery方法
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public
static <E> List<E> pageQuery(Dialect dialect, Executor executor, MappedStatement ms,
Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql, CacheKey cacheKey) throws SQLException {
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if(!dialect.beforePage(ms, parameter, rowBounds)) {
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return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql);
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parameter = dialect.processParameterObject(ms, parameter, boundSql, cacheKey);
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String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, cacheKey);
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BoundSql pageBoundSql =
new BoundSql(ms.getConfiguration(), pageSql, boundSql.getParameterMappings(), parameter);
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Map<
String,
Object> additionalParameters = getAdditionalParameter(boundSql);
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Iterator var12 = additionalParameters.keySet().iterator();
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String key = (
String)var12.next();
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pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key));
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return executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, pageBoundSql);
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在抽象类AbstractHelperDialect的getPageSql获取到对应的Page对象
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public
String getPageSql(MappedStatement ms, BoundSql boundSql,
Object parameterObject, RowBounds rowBounds, CacheKey pageKey) {
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String sql = boundSql.getSql();
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Page page =
this.getLocalPage();
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String orderBy = page.getOrderBy();
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if(StringUtil.isNotEmpty(orderBy)) {
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sql = OrderByParser.converToOrderBySql(sql, orderBy);
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return page.isOrderByOnly()?sql:
this.getPageSql(sql, page, pageKey);
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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
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public String getPageSql(String sql, Page page, CacheKey pageKey) {
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StringBuilder sqlBuilder =
new StringBuilder(sql.length() +
14);
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if(page.getStartRow() ==
0) {
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sqlBuilder.
append(
" LIMIT ? ");
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sqlBuilder.
append(
" LIMIT ?, ? ");
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return sqlBuilder.toString();
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将最后拼装好的SQL返回给DefaultSqlSession执行查询并返回
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public <E>
List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
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MappedStatement ms = configuration.getMappedStatement(statement);
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return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
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throw ExceptionFactory.wrapException(
"Error querying database. Cause: " + e, e);
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ErrorContext.instance().reset();
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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/
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发表于 2021-4-21 13:38:38