SQL Server doesn't have the statistics for the table variable?

Question

I found the MSDN says that \"table variables don\'t have statistics\"

https://msdn.microsoft.com/en-us/library/dd535534(v=sql.100).aspx

Also the following ar

Answer 1

No SQL Server doesn't maintain statistics for table variables.

There are three indexes on the table variable in your question and SQL Server inserts a corresponding three rows to sysidxstats

Contains a row for each index or statistics for tables and indexed views

These rows do have the 2 flag set in the status column indicating they are for statistics and so are returned by sys.stats but there is no corresponding statistics object.

You can see this using the query here (requires connecting via the DAC)

SELECT name,
       imageval
FROM   tempdb.sys.stats AS s
       INNER JOIN tempdb.sys.sysobjvalues AS o
               ON s.object_id = o.objid
                  AND s.stats_id = o.subobjid
WHERE  s.object_id = @table_variable_id; 

Which returns

+--------------------------------+----------+
|              name              | imageval |
+--------------------------------+----------+
| PK__#AD773B9__FFEE74513158C9C9 | NULL     |
| IX_Indate                      | NULL     |
| UQ__#AD773B9__DD5A978A62C2C478 | NULL     |
+--------------------------------+----------+

The imageval which would contain the actual statistics is NULL.

This is nothing new. You also see the same in 2008 (though only for the PK and UQ as the inline index definition cannot be used there).

The maintenance of rowcounts is done separately and does not need a full blown statistics object with histogram. And again this is not new.

Answer 2

I found the following phrase in this article by Itzik Ben-Gan Improvements in Table Variables and Temporary Tables in SQL Server 2014:

Cardinality Estimates with TF 2453

The last improvement that I’ll discuss in this article concerns table variables and is available in SQL Server 2014 RTM CU3 and in SQL Server 2012 SP2. You can find the support entry describing it here.

It’s a well-known fact that SQL Server doesn’t maintain statistics like histograms and density vectors for table variables. However, SQL Server does maintain a count of rows in the table, which in some cases can go a long way in helping the optimizer make optimal choices.

You are using Enterprise Edition. I think this explains it. The article by Kendra Little does say that Enterprise Edition is smarter.

I'd try to check with DBCC SHOW_STATISTICS the details of the statistics objects that you see and confirm whether they have all the data the normal statistics objects have. Statistics objects created for table variables should have less data, for example, no histogram. I don't have Enterprise Edition at hand to test.