hibernate的面向对象查询的效率有多低？

前言

　　老平台的查询速度很慢，需要进行优化(...说白了就是优化sql语句)，老平台用的strus2+hibernate框架，查询基本都是使用的HSQL。

关于hsql

　　HQL是Hibernate Query Language即Hibernate查询语言

　　优点：

　　　　不需要再编写繁复的SQL 语句，针对实体类及其属性进行查询

　　　查询结果是直接存放在List 中的对象，不需要再次封装

　　独立于数据库(针对不同数据库进行查询，跨数据库,Mysql->Oracle)，

　　 对不同的数据库根据Hibernate dialect 属性的配置自动生成不同的SQL 语句执行

　　缺点：

　　　　涉及到复杂的sql或者数据量大的情况，不好优化

　　

例子

　　我的项目里就是一张单表的分页查询，一共2百多万条数据。(oracle数据库)

下面是sql分页查询时打印出来的原生sql是这样的。。

     select
         *
        from
         ( select
             row_.*,
             rownum rownum_
         from
             ( select
                 tblmerinfo0_.MER_CODE as MER1_19_,
                 tblmerinfo0_.MER_NAME as MER2_19_,
                 tblmerinfo0_.MER_SHORT_NAME as MER3_19_,
                 tblmerinfo0_.MER_CITY as MER4_19_,
                 tblmerinfo0_.MER_NATION as MER5_19_,
                 tblmerinfo0_.MER_STATUS as MER6_19_,
                 tblmerinfo0_.MER_ACQ_CODE as MER7_19_,
                 tblmerinfo0_.MER_TYPE as MER8_19_,
                 tblmerinfo0_.MER_WARNING_AMT as MER9_19_,
                 tblmerinfo0_.MER_GROUP_ID as MER10_19_,
                 tblmerinfo0_.MER_COMMISION_TYPE as MER11_19_,
                 tblmerinfo0_.MER_COMMISION_VALUE as MER12_19_,
                 tblmerinfo0_.MER_TRANS_CURR as MER13_19_,
                 tblmerinfo0_.MER_SETT_CURR as MER14_19_,
                 tblmerinfo0_.MER_SETT_ACCT_NAME as MER15_19_,
                 tblmerinfo0_.MER_SETT_ACCT as MER16_19_,
                 tblmerinfo0_.MER_CB_CODE as MER17_19_,
                 tblmerinfo0_.MER_CB_NAME as MER18_19_,
                 tblmerinfo0_.MER_CORP as MER19_19_,
                 tblmerinfo0_.MER_MASTER as MER20_19_,
                 tblmerinfo0_.MER_REL_MAN as MER21_19_,
                 tblmerinfo0_.MER_REL_TEL as MER22_19_,
                 tblmerinfo0_.MER_FAX as MER23_19_,
                 tblmerinfo0_.MER_EMAIL as MER24_19_,
                 tblmerinfo0_.MER_ADDR as MER25_19_,
                 tblmerinfo0_.MER_POSTCODE as MER26_19_,
                 tblmerinfo0_.MER_PASSWORD as MER27_19_,
                 tblmerinfo0_.PASSWORD_INITIAL_VAL as PASSWOR28_19_,
                 tblmerinfo0_.PASSWORD_EXPIRED_DATE as PASSWOR29_19_,
                 tblmerinfo0_.PASSWORD_EXPIRED as PASSWOR30_19_,
                 tblmerinfo0_.BATCH_NO as BATCH31_19_,
                 tblmerinfo0_.REC_CREATE_TM as REC32_19_,
                 tblmerinfo0_.REC_UPDATE_TM as REC33_19_,
                 tblmerinfo0_.MER_OPEN_TM as MER34_19_,
                 tblmerinfo0_.MER_CLOSE_TM as MER35_19_,
                 tblmerinfo0_.MER_DAY_LIMIT as MER36_19_,
                 tblmerinfo0_.MER_SINGLE_LIMIT as MER37_19_,
                 tblmerinfo0_.SUPPORT_BRAND_FLAG as SUPPORT38_19_,
                 tblmerinfo0_.ROUTE_SCHEME as ROUTE39_19_,
                 tblmerinfo0_.BATCH_SETT_POINT as BATCH40_19_,
                 tblmerinfo0_.DELAY_SETT_DAYS as DELAY41_19_,
                 tblmerinfo0_.DCC_FLAG as DCC42_19_,
                 tblmerinfo0_.SUPPORT_FUNC_FLAG as SUPPORT43_19_,
                 tblmerinfo0_.CITY_CODE as CITY44_19_,
                 tblmerinfo0_.SUPPORT_DCC_FLAG as SUPPORT45_19_
             from
                 SWTONLINE.TBL_MER_INFO tblmerinfo0_
             order by
                 tblmerinfo0_.MER_CODE ) row_ )
         where
             rownum_ <= 24
             and rownum_ > 12

执行时间：63s

个人分析了一下慢的原因：里层的查询像是查询了所有，然后外层的查询在分页。仅个人观点。如果有大神看到这里有不同的看法欢迎指出

下面是我个人的原生sql:

 select * from
           (
             select a.*, rownum r from
             （
               select * from tbl_mer_info where 1=1 order by mer_code
             )  a  where rownum <= 24
           ) b where b.r > 12

执行时间：0.2s

个人分析：与上面的sql不同就是我的rownum先查询的<=24,然后外层在>12，这样会大大提交效率

写到这里，不禁就开始比较hibernate与mybatis的各自优点，个人觉得还是mybatis好，够灵活。虽然hibernate针对各个数据库设定了"方言"，号称兼容性好，可是谁会在开发的过程中替换数据库呢？