Question

I wrote a stored procedure with optional parameters.

 CREATE PROCEDURE dbo.GetActiveEmployee
   @startTime DATETIME=NULL,
   @endTime   DATETIME=NULL
 AS
   SET NOCOUNT ON

   SELECT columns
   FROM table
   WHERE (@startTime is NULL or table.StartTime >= @startTime) AND
         (@endTIme is NULL or table.EndTime <= @endTime)

I’m wondering whether indexes on StartTime and EndTime will be used?

Answer 1

Yes they will be used (well probably, check the execution plan – but I do know that the optional-ness of your parameters shouldn’t make any difference)

If you are having performance problems with your query then it might be a result of parameter sniffing. Try the following variation of your stored procedure and see if it makes any difference:

CREATE PROCEDURE dbo.GetActiveEmployee
    @startTime DATETIME=NULL,
    @endTime   DATETIME=NULL
AS
    SET NOCOUNT ON

    DECLARE @startTimeCopy DATETIME
    DECLARE @endTimeCopy DATETIME
    set @startTimeCopy = @startTime
    set @endTimeCopy = @endTime

    SELECT columns
    FROM table
    WHERE (@startTimeCopy is NULL or table.StartTime >= @startTimeCopy) AND
         (@endTimeCopy is NULL or table.EndTime <= @endTimeCopy)

This disables parameter sniffing (SQL server using the actual values passed to the SP to optimise it) – In the past I’ve fixed some weird performance issues doing this – I still can’t satisfactorily explain why however.

Another thing that you might want to try is splitting your query into several different statements depending on the NULL-ness of your parameters:

IF @startTime is NULL
BEGIN
    IF @endTime IS NULL
        SELECT columns FROM table
    ELSE
        SELECT columns FROM table WHERE table.EndTime <= @endTime
END    
ELSE
    IF @endTime IS NULL
        SELECT columns FROM table WHERE table.StartTime >= @startTime
    ELSE
        SELECT columns FROM table WHERE table.StartTime >= @startTime AND table.EndTime <= @endTime
BEGIN

This is messy, but might be worth a try if you are having problems – the reason it helps is because SQL server can only have a single execution plan per sql statement, however your statement can potentially return vastly different result sets.

For example, if you pass in NULL and NULL you will return the entire table and the most optimal execution plan, however if you pass in a small range of dates it is more likely that a row lookup will be the most optimal execution plan.

With this query as a single statement SQL server is forced to choose between these two options, and so the query plan is likely to be sub-optimal in certain situations. By splitting the query into several statements however SQL server can have a different execution plan in each case.

(You could also use the exec function / dynamic SQL to achieve the same thing if you preferred)