诡异的CBO参数_optimizer_use_histograms

_optimizer_use_histograms是一个CBO隐藏参数，目前所知该参数负责让CBO optimizer启用/禁用Histogram(This parameter enables/disables the usage of histograms by the optimizer.)   不过诡异的是这个参数在10gR2、12cR1中均存在，但是在11gR2中消失了。。。   11gR2: SQL> select * from v$version; BANNER -------------------------------------------------------------------------------- Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production PL/SQL Release 11.2.0.3.0 - Production CORE 11.2.0.3.0 Production TNS for Linux: Version 11.2.0.3.0 - Production NLSRTL Version 11.2.0.3.0 - Production SQL> alter session set "_optimizer_use_histograms"=false; alter session set "_optimizer_use_histograms"=false * ERROR at line 1: ORA-02248: invalid option for ALTER SESSION   12cR1: SQL> select * from v$version; BANNER -------------------------------------------------------------------------------- CON_ID ---------- Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production 0 PL/SQL Release 12.1.0.1.0 - Production 0 CORE 12.1.0.1.0 Production 0 BANNER -------------------------------------------------------------------------------- CON_ID ---------- TNS for Linux: Version 12.1.0.1.0 - Production 0 NLSRTL Version 12.1.0.1.0 - Production 0 SQL> alter session set "_optimizer_use_histograms"=false; Session altered.       10gR2:   SQL> select * from v$version; BANNER ---------------------------------------------------------------- Oracle Database 10g Enterprise Edition Release 10.2.0.5.0 - 64bi PL/SQL Release 10.2.0.5.0 - Production CORE 10.2.0.5.0 Production TNS for Linux: Version 10.2.0.5.0 - Production NLSRTL Version 10.2.0.5.0 - Production SQL> alter session set "_optimizer_use_histograms"=false; Session altered.