问题起源于客户的一个12c的数据库，需要启动到非real time apply的模式，但是发现执行：
alter database recover managed standby database cancel;
alter database recover managed standby database disconnect from session;
之后，数据库还是一直工作在real time apply的模式。
去alertlog中找了一下，发现了答案：
Thu Jun 09 12:16:03 2016
Errors in file /cust/mydb/rdbms/oracle/diag/rdbms/rmydb/mydb/trace/mydb_pr00_24168.trc:
ORA-16037: user requested cancel of managed recovery operation
Thu Jun 09 12:16:03 2016
MRP0: Background Media Recovery process shutdown (mydb)
Thu Jun 09 12:16:04 2016
Managed Standby Recovery Canceled (mydb)
Completed: alter database recover managed standby database cancel
alter database recover managed standby database disconnect from session <==我们平时的发起语句
Thu Jun 09 12:16:13 2016
Attempt to start background Managed Standby Recovery process (mydb)
Starting background process MRP0
Thu Jun 09 12:16:13 2016
MRP0 started with pid=27, OS id=17971
Thu Jun 09 12:16:13 2016
MRP0: Background Managed Standby Recovery process started (mydb)
Thu Jun 09 12:16:19 2016
Started logmerger process
Thu Jun 09 12:16:19 2016
Managed Standby Recovery starting Real Time Apply <==使用了real time apply，而上述语句在11g中的效果是使用real time apply，在12c中行为发生了变化。
Thu Jun 09 12:17:06 2016
Only allocated 127 recovery slaves (requested 128)
Thu Jun 09 12:17:06 2016
Parallel Media Recovery started with 127 slaves
Thu Jun 09 12:17:12 2016
Waiting for all non-current ORLs to be archived...
Thu Jun 09 12:17:12 2016
Wed Apr 27 14:56:52 2016
MRP0: Background Media Recovery process shutdown (mydb)
Wed Apr 27 14:56:53 2016
Managed Standby Recovery Canceled (mydb)
Completed: alter database recover managed standby database cancel
alter database recover managed standby database parallel 16 USING ARCHIVED LOGFILE disconnect <== 使用using archived log
Wed Apr 27 14:57:29 2016
Attempt to start background Managed Standby Recovery process (mydb)
Starting background process MRP0
Wed Apr 27 14:57:29 2016
MRP0 started with pid=27, OS id=23908
Wed Apr 27 14:57:29 2016
MRP0: Background Managed Standby Recovery process started (mydb)
Started logmerger process
Wed Apr 27 14:57:35 2016
Managed Standby Recovery not using Real Time Apply <==可以看到，不使用real time apply了！
Wed Apr 27 14:57:38 2016
Parallel Media Recovery started with 16 slaves
Wed Apr 27 14:57:38 2016
Waiting for all non-current ORLs to be archived...
Wed Apr 27 14:57:38 2016
All non-current ORLs have been archived.
Wed Apr 27 14:57:39 2016
Media Recovery Waiting for thread 1 sequence 2287 (in transit)
Completed: alter database recover managed standby database parallel 16 USING ARCHIVED LOGFILE disconnect
同时，在在线文档也发现了相关说明：
即using current logfile 已经过期，如果要启用real time apply，不再需要加这个语句。（所以我们无论加了using current logfile，还是不加，都是使用real time apply的。）
要使用非real time apply，就需要使用using archived log了。
综上：
在11g中，如要使用real time apply，需要加using current logfile，
在12c中，如果要不使用real time apply，需要加using archived log，using current logfile已经过期作废。
不带using语句，在11g中，默认是不使用real time apply，而在12c中是默认使用real time apply。