​_optimizer_unnest_scalar_sq

​​_optimizer_unnest_scalar_sq​​参数是​12.1引入的隐含参数，目的是支持标量子查询展开，通常能展开的子查询被改写为hash外连接的形式，能够看到HASH GROUP BY关键字，可能导致临时表空间激增或升级前sql异常缓慢。

默认该参数启用：

set linesize 132
column name format a30
column value format a25
select
  x.ksppinm  name,
  y.ksppstvl  value,
  y.ksppstdf  isdefault,
  decode(bitand(y.ksppstvf,7),1,'MODIFIED',4,'SYSTEM_MOD','FALSE')  ismod,
  decode(bitand(y.ksppstvf,2),2,'TRUE','FALSE')  isadj
from
  sys.x$ksppi x,
  sys.x$ksppcv y
where
  x.inst_id = userenv('Instance') and
  y.inst_id = userenv('Instance') and
  x.indx = y.indx and
  x.ksppinm like '%_&par%'
order by
  translate(x.ksppinm, ' _', ' ')
/

Enter value for par: optimizer_unnest_scalar_sq
old  14:   x.ksppinm like '%_&par%'
new  14:   x.ksppinm like '%_optimizer_unnest_scalar_sq%'

NAME                           VALUE                     ISDEFAULT ISMOD      ISADJ
------------------------------ ------------------------- --------- ---------- -----
_optimizer_unnest_scalar_sq    TRUE                      TRUE      FALSE      FALSE

禁用：

sql中加提示或直接修改为false
/*+ OPTIMIZER_FEATURES_ENABLE('11.2.0.3') */
/*+ OPT_PARAM('OPTIMIZER_FEATURES_ENABLE','11.2.0.3') */ 
alter session set "_optimizer_unnest_scalar_sq"=false;