Query against sys.schemas and sys.synonyms runs very slow for one user Scenario: SQL Server 2014 (v12.0.4100.1)

.NET Service runs this query:

SELECT name, base_object_name 
FROM sys.synonyms 
WHERE schema_id IN (SELECT schema_id 
                    FROM sys.schemas 
                    WHERE name = N'XXXX')
ORDER BY name


...which returns about 6500 rows but it often times out after 3+ minutes. The XXXX above is not 'dbo'.

If I run this query in SSMS as UserA, the query returns in less than a second.

When run as UserB (which is how the .NET service connects), the query takes 3-6 minutes, and has the CPU% at 25% (of a 4 cores) the entire time.

UserA is a Domain Login in the sysadmin role.

UserB is a SQL Login with:

EXEC sp_addrolemember N'db_datareader', N'UserB'
EXEC sp_addrolemember N'db_datawriter', N'UserB'
EXEC sp_addrolemember N'db_ddladmin', N'UserB'
GRANT EXECUTE TO [UserB]
GRANT CREATE SCHEMA TO [UserB]
GRANT VIEW DEFINITION TO [UserB]


I can duplicate this in SSMS by wrapping the above SQL in an Execute as...Revert block, so the .NET code is out of the picture.

The execution plan looks the same. I diff'ed the XML and there are only minor differences (CompileTime, CompileCPU, CompileMemory).

IO Stats all show no physical reads:


Table 'sysobjvalues'. Scan count 0, logical reads 19970, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Workfile'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'sysschobjs'. Scan count 1, logical reads 9122, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'sysclsobjs'. Scan count 0, logical reads 2, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.


XEvent waits status (for a ~3min query) are:


+---------------------+------------+----------------------+------------------------------+-----------------------------+
|      Wait Type      | Wait Count | Total Wait Time (ms) | Total Resource Wait Time (ms) | Total Signal Wait Time (ms) |
+---------------------+------------+----------------------+-------------------------------+-----------------------------+
| SOS_SCHEDULER_YIELD |      37300 |                  427 |                            20 |                         407 |
| NETWORK_IO          |          5 |                   26 |                            26 |                           0 |
| IO_COMPLETION       |          3 |                    1 |                             1 |                           0 |
+---------------------+------------+----------------------+-------------------------------+-----------------------------+


If I rewrite the query (in SSMS, I don't have access to the App Code) to

declare @id int 
SELECT @id=schema_id FROM sys.schemas WHERE name = N'XXXX'
SELECT a.name, base_object_name FROM sys.synonyms a
WHERE schema_id = @id
ORDER BY name


then UserB runs at the same (fast) speed as UserA.

If I add db_owner to UserB, then, again, the query runs &lt; 1 sec.

Schema created via this template:

DECLARE @TranName VARCHAR(20)
SELECT @TranName = 'MyTransaction'

BEGIN TRANSACTION @TranName
GO

IF NOT EXISTS (SELECT SCHEMA_NAME FROM INFORMATION_SCHEMA.SCHEMATA
        WHERE SCHEMA_NAME = '{1}')
BEGIN
    EXEC('CREATE SCHEMA [{1}]')
    EXEC sp_addextendedproperty @name='User', @value='{0}', @level0type=N'Schema', @level0name=N'{1}'
END
GO

{2}

COMMIT TRANSACTION MyTransaction;
GO


And {2} is, I believe, a list of Synonyms created in that schema.

Query Profile at two points into the query:





I've opened a ticket with Microsoft.

Also, we tried adding UserB to db_owner, and then DENYing all of the privileges we know that are associated with db_owner.  The result is a fast query.  Either we missed something (entirely possible), or there is a special check for the db_owner role.
You may want to re-write your query as follows (I'm using dbo rather than XXXX so that I do find some synonyms on my testing database). This is similar to the re-write you found to be more efficient, but avoids the need to declare a variable and use two queries.

SELECT name, base_object_name 
FROM sys.synonyms 
WHERE schema_id = SCHEMA_ID(N'dbo')
ORDER BY name


This yields a plan like the following:



One very interesting thing about the Filter operator in this plan is that it has a predicate that performs an internal has_access() check. This filter removes any objects that the current account does not have sufficient permissions to see. However, this check is short-circuited (i.e., completes much more quickly) if you are a member of the db_owner role, which may explain the performance differences you are seeing.



Here is the query plan for your original query. Notice that all synonyms on the database (1,126 in my case, but likely many more in your case) pass through the very expensive has_access() filter, even though only 2 synonyms match the schema. By using the simplified query above, we can ensure that has_access() is only invoked for the synonyms that match your query rather than for all synonyms in the database.




Using sys.dm_exec_query_profiles to explore further

As Martin suggests, we can confirm that the has_access() check is a significant bottleneck by using sys.dm_exec_query_profiles on SQL Server 2014+. If I run the following query using a db_owner account on a database with ~700K objects, the query takes ~500ms:

SELECT COUNT(*)
FROM sys.objects


When run with an account that is not a db_owner, this same query takes about eight minutes! Running with actual plan on and using a p_queryProgress procedure that I wrote to help parse sys.dm_exec_query_profiles output more easily, we can see that almost all of the processing time is spent on the Filter operator that is performing the has_access() check: