Glitch using dm_exec_query_plan

Question

I was experimenting with sys.dm_exec_query_plan the other day and ran into a strange glitch. The select from dm_exec_query_plan does not seem to want to be restricted to specific databases. I know this because one of the databases on my server was set to
single-user mode, and when I ran the following query I was getting an error because of that:
SELECT
cp.refcounts
,cp.usecounts
,cp.objtype
,cp.[dbid]
,cp.objectid
,cp.[text]
,qp.query_plan
FROM sys.dm_exec_cached_plans AS cp
CROSS APPLY sys.dm_exec_sql_text(cp.plan_handle)
AS st
CROSS APPLY sys.dm_exec_query_plan(cp.plan_handle)
AS qp
JOIN sys.databases db ON db.database_id = qp.dbid
AND db.name = 'mydb'
I would have thought that restricting the results to a particular database (one that isn't in single-user mode) would have caused the error not to occur, but it did not. I fiddled around with a couple of other query formulas and finally found one that worked:
WITH cteCachedPlans AS
( SELECT *
FROM sys.dm_exec_cached_plans AS cp
CROSS APPLY sys.dm_exec_sql_text(cp.plan_handle)
AS st
JOIN sys.databases AS db
ON db.database_id = st.dbid
AND db.name = 'STAX'
SELECT
cp.refcounts
,cp.usecounts
,cp.objtype
,cp.[dbid]
,cp.objectid
,cp.[text]
,qp.query_plan
FROM cteCachedPlans AS cp
OUTER APPLY sys.dm_exec_query_plan(cp.plan_handle)
AS qp
WHERE qp.query_plan IS NULL
Notice that I am using an "outer apply" here. If I use an "inner apply" the error still occurs, even though the results from the CTE are restricted to a single database that is not in single-user mode. This seems like strange behavior
to me, has anyone else run into it?
Thanks,
Ron Rice
Ron Rice

Answer 1

I'm not sure what error message you are getting. I guess that I could test myself, but I'm about to withdraw for the night.
In any case, the filter is necessarily not applied before the operator that results in an error because of the single-user database. Changing CROSS to OUTER APPLY may change the query plan, but that may be pure coincidence.
Erland Sommarskog, SQL Server MVP, [email protected]