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。。
1 2 select 3 * 4 from 5 ( select 6 row_.*, 7 rownum rownum_ 8 from 9 ( select 10 tblmerinfo0_.MER_CODE as MER1_19_, 11 tblmerinfo0_.MER_NAME as MER2_19_, 12 tblmerinfo0_.MER_SHORT_NAME as MER3_19_, 13 tblmerinfo0_.MER_CITY as MER4_19_, 14 tblmerinfo0_.MER_NATION as MER5_19_, 15 tblmerinfo0_.MER_STATUS as MER6_19_, 16 tblmerinfo0_.MER_ACQ_CODE as MER7_19_, 17 tblmerinfo0_.MER_TYPE as MER8_19_, 18 tblmerinfo0_.MER_WARNING_AMT as MER9_19_, 19 tblmerinfo0_.MER_GROUP_ID as MER10_19_, 20 tblmerinfo0_.MER_COMMISION_TYPE as MER11_19_, 21 tblmerinfo0_.MER_COMMISION_VALUE as MER12_19_, 22 tblmerinfo0_.MER_TRANS_CURR as MER13_19_, 23 tblmerinfo0_.MER_SETT_CURR as MER14_19_, 24 tblmerinfo0_.MER_SETT_ACCT_NAME as MER15_19_, 25 tblmerinfo0_.MER_SETT_ACCT as MER16_19_, 26 tblmerinfo0_.MER_CB_CODE as MER17_19_, 27 tblmerinfo0_.MER_CB_NAME as MER18_19_, 28 tblmerinfo0_.MER_CORP as MER19_19_, 29 tblmerinfo0_.MER_MASTER as MER20_19_, 30 tblmerinfo0_.MER_REL_MAN as MER21_19_, 31 tblmerinfo0_.MER_REL_TEL as MER22_19_, 32 tblmerinfo0_.MER_FAX as MER23_19_, 33 tblmerinfo0_.MER_EMAIL as MER24_19_, 34 tblmerinfo0_.MER_ADDR as MER25_19_, 35 tblmerinfo0_.MER_POSTCODE as MER26_19_, 36 tblmerinfo0_.MER_PASSWORD as MER27_19_, 37 tblmerinfo0_.PASSWORD_INITIAL_VAL as PASSWOR28_19_, 38 tblmerinfo0_.PASSWORD_EXPIRED_DATE as PASSWOR29_19_, 39 tblmerinfo0_.PASSWORD_EXPIRED as PASSWOR30_19_, 40 tblmerinfo0_.BATCH_NO as BATCH31_19_, 41 tblmerinfo0_.REC_CREATE_TM as REC32_19_, 42 tblmerinfo0_.REC_UPDATE_TM as REC33_19_, 43 tblmerinfo0_.MER_OPEN_TM as MER34_19_, 44 tblmerinfo0_.MER_CLOSE_TM as MER35_19_, 45 tblmerinfo0_.MER_DAY_LIMIT as MER36_19_, 46 tblmerinfo0_.MER_SINGLE_LIMIT as MER37_19_, 47 tblmerinfo0_.SUPPORT_BRAND_FLAG as SUPPORT38_19_, 48 tblmerinfo0_.ROUTE_SCHEME as ROUTE39_19_, 49 tblmerinfo0_.BATCH_SETT_POINT as BATCH40_19_, 50 tblmerinfo0_.DELAY_SETT_DAYS as DELAY41_19_, 51 tblmerinfo0_.DCC_FLAG as DCC42_19_, 52 tblmerinfo0_.SUPPORT_FUNC_FLAG as SUPPORT43_19_, 53 tblmerinfo0_.CITY_CODE as CITY44_19_, 54 tblmerinfo0_.SUPPORT_DCC_FLAG as SUPPORT45_19_ 55 from 56 SWTONLINE.TBL_MER_INFO tblmerinfo0_ 57 order by 58 tblmerinfo0_.MER_CODE ) row_ ) 59 where 60 rownum_ <= 24 61 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:
1 select * from 2 ( 3 select a.*, rownum r from 4 （ 5 select * from tbl_mer_info where 1=1 order by mer_code 6 ) a where rownum <= 24 7 ) 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？