How can I tell if a database table is being access

问题:

I have a very large database with hundreds of tables, and after many, many product upgrades, I'm sure half of them aren't being used anymore. How can I tell if a table is is actively being selected from? I can't just use Profiler - not only do I want to watch for more than a few days, but there are thousands of stored procedures as well, and profiler won't translate the SP calls into table access calls.

The only thing I can think of is to create a clustered index on the tables of interest, and then monitor the sys.dm_db_index_usage_stats to see if there are any seeks or scans on the clustered index, meaning that data from the table was loaded. However, adding a clustered index on every table is a bad idea (for any number of reasons), as isn't really feasible.

Are there other options I have? I've always wanted a feature like a "SELECT trigger", but there are probably other reasons why SQL Server doesn't have that feature either.

SOLUTION:

Thanks, Remus, for pointing me in the right direction. Using those columns, I've created the following SELECT, which does exactly what I want.

  WITH LastActivity (ObjectID, LastAction) AS 
  (
       SELECT object_id AS TableName,
              last_user_seek as LastAction
         FROM sys.dm_db_index_usage_stats u
        WHERE database_id = db_id(db_name())
        UNION 
       SELECT object_id AS TableName,
              last_user_scan as LastAction
         FROM sys.dm_db_index_usage_stats u
        WHERE database_id = db_id(db_name())
        UNION
       SELECT object_id AS TableName,
              last_user_lookup as LastAction
         FROM sys.dm_db_index_usage_stats u
        WHERE database_id = db_id(db_name())
  )
  SELECT OBJECT_NAME(so.object_id) AS TableName,
         MAX(la.LastAction) as LastSelect
    FROM sys.objects so
    LEFT
    JOIN LastActivity la
      on so.object_id = la.ObjectID
   WHERE so.type = 'U'
     AND so.object_id > 100
GROUP BY OBJECT_NAME(so.object_id)
ORDER BY OBJECT_NAME(so.object_id)

回答1:

Look in sys.dm_db_index_usage_stats. The columns last_user_xxx will contain the last time the table was accessed from user requests. This table resets its tracking after a server restart, so you must leave it running for a while before relying on its data.

回答2:

Re: Profiler, if you monitor for SP:StmtCompleted, that will capture all statements executing within a stored procedure, so that will catch table accesses within a sproc. If not everything goes through stored procedures, you may also need the SQL:StmtCompleted event.

There will be a large number of events so it's probably still not practical to trace over a long time due to the size of trace. However, you could apply a filter - e.g. where TextData contains the name of your table you want to check for. You could give a list of table names to filter on at any one time and work through them gradually. So you should not get any trace events if none of those tables have been accessed.

Even if you feel it's not a suitable/viable approach for you, I thought it was worth expanding on.

Another solution would be to do a global search of your source code to find references to the tables. You can query the stored procedure definitions to check for matches for a given table, or just generate a complete database script and do a Find on that for table names.

回答3:

For SQL Server 2008 you should take a look at SQL Auditing. This allows you to audit many things including selects on a table and reports to a file or Events Log.

回答4:

A side-note : if your intent is to drop those tables, you might have to consider legal obligations that impose it on you to keep the data involved anyway for x years.

回答5:

I had in mind to play with user permissions for different tables, but then I remembered you can turn on trace with an ON LOGON trigger you might benefit from this:

CREATE OR REPLACE TRIGGER SYS.ON_LOGON_ALL

AFTER LOGON ON DATABASE
WHEN (

USER 'MAX'

)
BEGIN

EXECUTE IMMEDIATE 'ALTER SESSION SET SQL_TRACE TRUE';

--EXECUTE IMMEDIATE 'alter session set events ''10046 trace name context forever level 12''';

EXCEPTION

WHEN OTHERS THEN

NULL;

END;

/

Then you can check your trace files.

回答6:

This solution works better for me then the solution above. But, is still limted that the server was not re-started as well, but still gives you a good idea of tables not used.

SELECT [name]
      ,[object_id]
      ,[principal_id]
      ,[schema_id]
      ,[parent_object_id]
      ,[type]
      ,[type_desc]
      ,[create_date]
      ,[modify_date]
      ,[is_ms_shipped]
      ,[is_published]
      ,[is_schema_published]
  FROM [COMTrans].[sys].[all_objects]
  where object_id not in (
select object_id from sys.dm_db_index_usage_stats

)
and type='U'
order by name