在Exadata系统上Oracle11g 数据库中执行某个调度procedure Job_SP_DWR_AR_COLLECTION_WEEKLY_P 时，发现卡在某个SQL : 5jctyad1z1s6a 执行不下去，同时出现大量的 enq: KO - fast object checkpoint 等待事件 。

1. 查询文档 enq: KO - fast object checkpoint

You see this event because object level checkpoint which happens when you run direct path reads on a segment (like with serial direct read full table scan or parallel execution full segment scans).

KO enqueue - fast object checkpoint means that your session has sent CKPT process a message with instruction to do object level checkpoint and is waiting for a reply.

CKPT in turn asks DBWR process to perform the checkpoint and may wait for response. 

So you should check what the DBWR and CKPT processes are doing - whether they're stuck waiting for something (such controlfile enqueue) or doing IO or completely idle. 
This you can do using v$session_wait or v$session_event... 

You can run this: 
select sid,program from v$session where program like '%CKPT%' or program like '%DBW%'; 
and then run 
select sid,event,state,p1,p2,p3,seconds_in_wait from v$session_wait where sid in 
 (select sid from v$session where program like '%CKPT%' or program like '%DBW%'); 

---- 
fast object checkpoint can be disabled by setting 
"_db_fast_obj_ckpt"=false". 
What is the impact to parallel scans with this setting? Any other adverse impact from setting this parameter?