Question

I have c# web application that submits a query to sql server. The query has three parameters, two are ints and one is nchar. I’m getting a different query plan for each length of the nchar parameter. Is there a way to only get one plan?

Some more details:

The query is fairly large and complex, so I won’t inlcude it all here. However, there WHERE part is

WHERE gp.Control=@Control AND r.RegionType=@RegionType

r.RegionType is a column in a table defined as nchar(10). My c# code, sets the parameter with the following:

cmd.Parameters.Add(parmRegionType, System.Data.SqlDbType.NChar).Value = rgn.Key;

where rgn.key may have one of the following four values: “All”, “County”, “DGP”, or “State”.

When I dump the query plan with

SELECT usecounts, cacheobjtype, objtype, text, query_plan
FROM sys.dm_exec_cached_plans 
CROSS APPLY sys.dm_exec_sql_text(plan_handle) 
CROSS APPLY sys.dm_exec_query_plan(plan_handle) 
WHERE text LIKE '%(@RegionType nchar(%),@CacheCnt int,@Control int)%'
AND text NOT LIKE '%this query%'

I get the following results (edited format and truncated text)

usecounts cacheobjtype  objtype  text                                                                                                                                                                                                                                                             query_plan
--------- ------------- -------- -----------------------------------------------
1         Compiled Plan Prepared (@RegionType nchar(5),@CacheCnt int,@Control...
1         Compiled Plan Prepared (@RegionType nchar(6),@CacheCnt int,@Control...
2         Compiled Plan Prepared (@RegionType nchar(3),@CacheCnt int,@Control...

(3 row(s) affected)

So you can see that there is a query plan for when @RegionType is three characters long (“All” and “DGP”), five characters long (“State”) and six characters long (“County”).

Is there a way to code this so that only one query plan is prepared independent of the length of the @RegionType parameter?

Answer 1

Use NVarChar and set the length to the maximum expected possible value (e.g. whatever the max length of the RegionType column is).

Looks like 6 in your case as that is the length of County

cmd.Parameters.Add(parmRegionType, System.Data.SqlDbType.NVarChar, 6).Value = rgn.Key;