Plan_handle in sys.dm_exec_requests and sys.dm_exec_cached_plans

Question

We are a SQL 2008 R2 Ent shop and recently I am troubling a performance issue which one SQL server is performing much faster than another SQL server for the exact same query.  Each SQL server produces quite a different execution plan than
the other.  This is another topic on its own which I won't touch here.   While using DMV to trouble shoot, I come across a plan handle question regarding these two DMV's. I use sys.dm_exec_requests to check what are currently running
on a SQL server and sys.dm_exec_cached_plans to check what plans are being cached.    When I run the 'long running' query and check the plan handle in dm_exec_requests and dm_exec_cached_plans, the plan handle number in dm_exec_requests doesn't
show up in sys.dm_exec_cached_plans.  In fact, the plan handle in dm_exec_cached_plans is different for the same 'long running' query.   Can someone explain why? 
It is important for me to know because when I am trouble shooting a frequently long running query, I would first try to kill its execution plan because it may become inefficient due to a parameter sniffing issue.   I normally
use the plan handle from sys.dm_exec_requests to get a 'cached' plan handle number and use DBCC FREEPROCCACHE (xxxxxxxxxxx) to kill it.  However, I would expect the same plan handle number from sys.dm_exec_cached_plans for that query but
it is not.  So this leads me to think that I may not be removing the plan I really want.
Besides answering my 'why they are different' question, I also like to know if my way of removing a cached plan is correct or not.  Please advice as well.
Thanks in advance,
OD
Ocean Deep

Answer 1

Erland,
Thanks for the reply.  I think I find the answer to my question.  The correct fact is that dm_exec_requests, dm_exec_cached_plans and even your suggested dm_exec_query_stats all find the same plan handler If I search by the plan hander number.  
If I search by the query content (st.text) , it either doesn't return any record or return records with a different plan handler number.   Let me show you what I use and may be you can correct it. 
SELECT plan_handle, st.text, cacheobjtype, objtype
FROM sys.dm_exec_cached_plans
CROSS APPLY sys.dm_exec_sql_text(plan_handle) AS st
WHERE st.text LIKE N'  select lastName, FirstName %'
If I search the actual plan handler number, it would find that cached plan.
SELECT plan_handle, st.text, cacheobjtype, objtype
FROM sys.dm_exec_cached_plans
CROSS APPLY sys.dm_exec_sql_text(plan_handle) AS st
where plan_handle =0x06001B00693E700240418F25030000000000000000000000
Couple points about the query using the text name for searching.  First the query was copied from one of the SQL expert blog so I assume it is working.   Secondly, the actual query text I use is straight out from a query result using
the following query so I am not guessing what the actual query format it may be.
select r.session_id, r.status,
r.start_time, r.command, s.text, r.plan_handle
from sys.dm_exec_requests r
cross apply sys.dm_exec_sql_text(r.sql_handle) s
Let me know if there is a proper way to search for a cached plan by the query content if there is one.  This is important because sometimes I may not have a plan handle number to work with and yet I need to kill a cached plan for a particular sql query
so the only thing I have is the query itself.
By the way, my long running query is not a stored procedure.  It is a query sending from a web application.
Thanks in advance,
OD
Ocean Deep