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 (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!!!