What causes DMV to show a NULL query plan?

问题

When I execute this simple SP and then check for the query plan from the DMV query that follows, the query plan comes back as NULL. But if I comment out the else block in the SP (or change it to not reference temp table), the DMV query does show a query plan. Any idea what is causing this difference? And does a NULL query plan mean that nothing was cached? I'm using SQL Server 2008 R2.

CREATE PROCEDURE dbo.SampleSp
(
    @Option TINYINT
)
AS
    DECLARE @RowCount INT;

    -- Get ID into local temp table

    CREATE TABLE #P (ID INT PRIMARY KEY);
    INSERT INTO #P
    VALUES (1), (2)

    IF @Option = 1
    BEGIN
        SELECT ID FROM #P
    END
    ELSE
    BEGIN
        SELECT ID FROM #P
    END
GO

    --Call SP
    EXEC dbo.SampleSp 1
GO
    --DMV Query to find query_plan
    SELECT  
            CP.objtype, 
            CP.usecounts AS Ct, 
            ST.last_execution_time,
            SUBSTRING(Text, (statement_start_offset / 2) + 1, (CASE statement_end_offset WHEN -1 THEN DATALENGTH(Text) ELSE statement_end_offset END  - ((statement_start_offset / 2) + 1))) AS query,
            T.text,
            PL.query_plan
    FROM sys.dm_exec_cached_plans AS CP WITH(NOLOCK)
    CROSS APPLY sys.dm_exec_sql_text(CP.plan_handle) AS T
    OUTER APPLY sys.dm_exec_query_plan(CP.plan_handle) AS PL
    LEFT OUTER JOIN sys.dm_exec_query_stats AS ST WITH(NOLOCK) ON ST.plan_handle = CP.plan_handle
    WHERE 1=1
    AND ST.last_execution_time > DATEADD(MINUTE, -5, GETDATE())
    AND T.text LIKE '%SampleSp%'
    AND T.text NOT LIKE '%dm_exec_cached_plans%'
    ORDER BY ST.last_execution_time DESC

回答1:

I believe I figured it out. It seems that the plan will show up as NULL until every branch in the SP has been executed. (You can see this by making another call to the SP with a parameter value of 2, and then the plan does show up.) If only part of the SP has been executed, SQL Server does cache than plan internally (you can see this by looking at usecounts, plus some other more extensive testing I did to prove this), but it will not show up in the DMV query I have used. Which raises the question: How do you view the plan of just the part of the SP that has been cached?

来源：https://stackoverflow.com/questions/14326146/what-causes-dmv-to-show-a-null-query-plan

标签

sql-server

sql-server-2008-r2