Oracle case when改写SQL

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(selectt2.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_Qfrom  t2group by t2.object_id)select t1.object_name,t1.object_id,w_t2.* from t1,w_t2where t1.object_id=w_t2.object_idand 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。

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