根据18c官方描述cancel sql功能是在18c中引起,但是实测发现在oracle 12.2中已经有了cancel sql功能，可以实现终止掉某个sql的当前sql正在执行的sql语句,而不是传统的直接kill某个会话.ALTER SYSTEM CANCEL SQL语句有四个参数分别为：

--会话1
SQL> set lines 150
SQL> select * from v$version;
BANNER                                                                               CON_ID
-------------------------------------------------------------------------------- ----------
Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production              0
PL/SQL Release 12.2.0.1.0 - Production                                                    0
CORE    12.2.0.1.0      Production                                                        0
TNS for Linux: Version 12.2.0.1.0 - Production                                            0
NLSRTL Version 12.2.0.1.0 - Production                                                    0
SQL> select sid, serial# from v$session where sid in
  2  (select  sid from v$mystat where rownum=1);
       SID    SERIAL#
---------- ----------
       278       4019
SQL> create table t_xifenfei tablespace users as select * from dba_source;
Table created.
SQL> insert into t_xifenfei select * from t_xifenfei;
274132 rows created.                    <<===没有提交
SQL> select count(*)from t_xifenfei;
  COUNT(*)
----------
    548264
SQL> insert into t_xifenfei select * from t_xifenfei;
548264 rows created.     <<===没有提交
SQL> select count(*)from t_xifenfei;
  COUNT(*)
----------
   1096528
SQL> insert into t_xifenfei select * from t_xifenfei;
--会话2
SQL> select count(*)from t_xifenfei;
  COUNT(*)
----------
    274132
SQL> alter system cancel sql '278,4019';
System altered.
SQL> select count(*)from t_xifenfei;
  COUNT(*)
----------
    274132
--会话1
SQL> insert into t_xifenfei select * from t_xifenfei;
insert into t_xifenfei select * from t_xifenfei
*
ERROR at line 1:
ORA-01013: user requested cancel of current operation
SQL> select count(*)from t_xifenfei;
  COUNT(*)
----------
   1096528

这里可以看到会话1的最后一个insert被cancel，但是前面两个没有提交的insert没有被回滚/提交，看到了cancel sql的功能的实现.