Table Valued Parameters with Estimated Number of Rows 1

问题

I have been searching the internet for hours trying to figure out how to improve the performance of my query using table-valued parameters (TVP).

After hours of searching, I finally determined what I believe is the root of the problem. Upon examining the Estimated Execution plan of my query, I discovered that the estimated number of rows for my query is 1 anytime I use a TVP. If I exchange the TVP for a query that selects the data I am interested in, then the estimated number of rows is much more accurate at around 7400. This significantly increases the performance.

However, in the real scenario, I cannot use a query, I must use a TVP. Is there any way to have SQL Server more accurately predict the number of rows when using a TVP so that a more appropriate plan will be used?

回答1:

TVPs are Table Variables which don't maintain statistics and hence report only have 1 row. There are two ways to improve statistics on TVPs:

1. If you have no need to modify any of the values in the TVP or add columns to it to track operational data, then you can do a simple, statement-level OPTION (RECOMPILE) on any query that uses a table variable (TVP or locally created) and is doing more with that table variable than a simple SELECT (i.e. doing INSERT INTO RealTable (columns) SELECT (columns) FROM @TVP; does not need the statement-level recompile). Do the following test in SSMS to see this behavior in action:

   DECLARE @TableVariable TABLE (Col1 INT NOT NULL);
   
   INSERT INTO @TableVariable (Col1)
     SELECT so.[object_id]
     FROM   [master].[sys].[objects] so;
   
   -- Control-M to turn on "Include Actual Execution Plan"
   
   SELECT * FROM @TableVariable; -- Estimated Number of Rows = 1 (incorrect)
   
   SELECT * FROM @TableVariable
   OPTION (RECOMPILE); -- Estimated Number of Rows = 91 (correct)
   
   SELECT * FROM @TableVariable; -- Estimated Number of Rows = 1 (back to incorrect)
   

2. Create a local temporary table (single #) and copy the TVP data to that. While this does duplicate the data in tempdb, the benefits are:

   • better statistics for a temp table as opposed to table variable (i.e. no need for statement-level recompiles)
   • ability to add columns
   • ability to modify values

来源：https://stackoverflow.com/questions/23120360/table-valued-parameters-with-estimated-number-of-rows-1