目前创新互联已为上1000家的企业提供了网站建设、域名、网页空间、网站托管运营、企业网站设计、鄠邑网站维护等服务，公司将坚持客户导向、应用为本的策略，正道将秉承"和谐、参与、激情"的文化，与客户和合作伙伴齐心协力一起成长，共同发展。

Oracle case when 改写 SQL

--- 说明：案例来自《 收获，不止SQL 优化 》

创建测试数据：

SQL >   drop   table  t1 purge ;

SQL >   drop   table  t2 purge ;

SQL >   create   table  t1 as   select   *   from  dba_objects ;

SQL >   create   table  t2 as   select   *   from  dba_objects ;

SQL >   update  t2 set  status = 'INVALID'   WHERE   ROWNUM <= 10000 ;

SQL >   update  t2 set   generated = 'Y'   WHERE   ROWNUM <= 10000 ;

SQL >   update  t2 set   temporary = 'Y'   WHERE   ROWNUM <= 10000 ;

SQL >   update  t2 set   temporary = 'M'   WHERE   temporary <> 'Y' ;

SQL >   update  t2 set   temporary = 'Q'   WHERE   temporary <> 'Y'   or   temporary <> 'M' ;

SQL >   COMMIT ;

SQL >   set  autotrace traceonly

SQL >   set  linesize 1000    

原 SQL ：

SQL>           
select t1.object_name,
       t1.object_id,
       (select count(*)
          from t2
         where temporary = 'Y'
           and t2.object_id = t1.object_id) CNT_TEMPORARY_Y,
       (select count(*)
          from t2
         where created >= sysdate - 365
           and t2.object_id = t1.object_id) CNT_CREATED_NEW,
       (select sum(object_id)
          from t2
         where status <> 'VALUD'
           and t2.object_id = t1.object_id) SUM_OBJID_STATUS_V,
       (select sum(object_id)
          from t2
         where generated = 'Y'
           and t2.object_id = t1.object_id) SUM_OBJID_GENERATED_Y,
       (select sum(object_id)
          from t2
         where generated = 'M'
           and t2.object_id = t1.object_id) SUM_OBJID_GENERATED_M,
       (select sum(object_id)
          from t2
         where generated = 'Q'
           and t2.object_id = t1.object_id) SUM_OBJID_GENERATED_Q
  from t1
 where t1.object_id <= 50;

case when改造 后的 SQL ：

with w_t2 as
(select
t2.object_id,
count(case when t2.temporary='Y' then 1 end ) CNT_TEMPORARY_Y,
count(case when created >=sysdate-365  then 1 end ) CNT_CREATED_NEW,
sum(case when t2.status<>'VALID' then t2.object_id end ) SUM_OBJID_STATUS_V,
sum(case when t2.generated = 'Y' then t2.object_id end ) SUM_OBJID_GENERATED_Y,
sum(case when t2.generated = 'M' then t2.object_id end ) SUM_OBJID_GENERATED_M,
sum(case when t2.generated = 'Q' then t2.object_id end ) SUM_OBJID_GENERATED_Q
from  t2
group by t2.object_id)
select t1.object_name,t1.object_id,w_t2.* from t1,w_t2
where t1.object_id=w_t2.object_id
and t1.object_id<=50;

结论： SQL 改写后 T2 表访问次数由 6 次降到 1 次，逻辑读 consistent gets 由 320100 降到 2580 ，性能有所提升。

欢迎关注我的微信公众号"IT小Chen"，共同学习，共同成长！！！

本文名称：Oraclecasewhen改写SQL
转载来源：http://azwzsj.com/article/jdegjg.html