T-SQL Daylight Saving lookup table - poorly performing table-valued function I've created a "Daylight Savings" lookup calendar table for the GMT region. The function I'm using to query the table to return the local datetime from a UTC datetime is performing poorly.

Any help to improve this, including changing the way the TVF is coded, would be appreciated.

The function will be used in queries that can return 1m+ rows on a frequent basis. The function is used when querying warehouse tables containing trip data.

The start and end datetimes of the trips are stored in UTC and the function above is used to convert them into local time. A developer, long since gone from the company, wrote a scalar function that converts UTC time to local time. I was tasked to rewrite that function using a calendar table and a TVF as TVF are supposed to better performing than scalar functions

Without the function:

SQL Server Execution Times:    CPU time = 4633 ms,  elapsed time = 4909 ms.


execution plan without function

With the function:

SQL Server Execution Times:    CPU time = 20795 ms,  elapsed time = 21176 ms.


execution plan with function

Here's a sample output from the table

CREATE TABLE dbo.DSTLookup 
(
     [Id] int, 
     [Tzid] int, 
     [DT_WhenSwitch] datetime, 
     [DSTOffSetSeconds] int, 
     [GMTOffSetSeconds] int 
)

INSERT INTO dbo.DSTLookup
VALUES (29, 2, N'2014-03-30T01:00:00', 3600, 0), 
       (30, 2, N'2014-10-26T02:00:00', 0, 0), 
       (31, 2, N'2015-03-29T01:00:00', 3600, 0), 
       (32, 2, N'2015-10-25T02:00:00', 0, 0), 
       (33, 2, N'2016-03-27T01:00:00', 3600, 0), 
       (34, 2, N'2016-10-30T02:00:00', 0, 0), 
       (35, 2, N'2017-03-26T01:00:00', 3600, 0), 
       (36, 2, N'2017-10-29T02:00:00', 0, 0), 
       (37, 2, N'2018-03-25T01:00:00', 3600, 0), 
       (38, 2, N'2018-10-28T02:00:00', 0, 0)


This is the TVF:

CREATE FUNCTION dbo.FN_GetLocalTime_FromUTC_BasedOnTZId 
     (@StartDateTime DATETIME, @EndDateTime DATETIME, @Tzid INT)
/*=========================================================================
*   2017-03-27
*   Returns local time from UTC time based on timeZoneId
*
==========================================================================*/
RETURNS TABLE 
AS
    RETURN
        (
         WITH cteStartDate AS
         (
            SELECT
                RN = ROW_NUMBER() OVER (ORDER BY D.Id DESC),
                D.DSTOffSetSeconds 's_DST_OffSet',
                D.GMTOffSetSeconds 's_GMT_OffSet'
            FROM
                dbo.DSTLookup D
            WHERE
                D.DT_WhenSwitch &lt;= @StartDateTime
                AND D.Tzid = @Tzid
         ),
         cteEndDate AS
         (
             SELECT
                 RN = ROW_NUMBER() OVER (ORDER BY D.Id DESC),
                 D.DSTOffSetSeconds 'e_DST_OffSet',
                 D.GMTOffSetSeconds 'e_GMT_OffSet'
             FROM
                 dbo.DSTLookup D
             WHERE
                 D.DT_WhenSwitch &lt;= @EndDateTime
                 AND D.Tzid = @Tzid
         ),
         cteConvertStartDate AS
         (
              SELECT
                  DATEADD(SECOND, (COALESCE(S.s_DST_OffSet, 0) + COALESCE(S.s_GMT_OffSet, 0)), @StartDateTime) 'LocalStartDateTime'
              FROM
                  cteStartDate S
              WHERE
                  S.RN = 1
         ),
         cteConvertEndDate AS
         (
              SELECT
                  DATEADD(SECOND, (COALESCE(E.e_DST_OffSet, 0) + COALESCE(E.e_GMT_OffSet, 0)), @EndDateTime)    'LocalEndDateTime'
              FROM
                  cteEndDate E
              WHERE
                  E.RN = 1
         )
         SELECT
             S.LocalStartDateTime, E.LocalEndDateTime
         FROM
             cteConvertStartDate S, cteConvertEndDate E
);
GO


To query the TVF:

SELECT * 
FROM dbo.FN_GetLocalTime_FromUTC_BasedOnTzId
    ('2017-03-27 10:00:30', '2017-03-27 10:15:54', 2);


Execution plan following Max's recommendations to include the primary key.
If Tzid and DT_WhenSwitch define a unique row I recommend clustering the dbo.DSTLookup table by those two columns. You can make those columns the primary key if you want or you can just make them be the clustered index.
CREATE TABLE dbo.DSTLookup 
(
     [Id] int, 
     [Tzid] int, 
     [DT_WhenSwitch] datetime, 
     [DSTOffSetSeconds] int, 
     [GMTOffSetSeconds] int 
);

CREATE CLUSTERED INDEX CI_DSTLookup ON dbo.DSTLookup ([Tzid], [DT_WhenSwitch]); -- new

INSERT INTO dbo.DSTLookup
VALUES (29, 2, N'2014-03-30T01:00:00', 3600, 0), 
       (30, 2, N'2014-10-26T02:00:00', 0, 0), 
       (31, 2, N'2015-03-29T01:00:00', 3600, 0), 
       (32, 2, N'2015-10-25T02:00:00', 0, 0), 
       (33, 2, N'2016-03-27T01:00:00', 3600, 0), 
       (34, 2, N'2016-10-30T02:00:00', 0, 0), 
       (35, 2, N'2017-03-26T01:00:00', 3600, 0), 
       (36, 2, N'2017-10-29T02:00:00', 0, 0), 
       (37, 2, N'2018-03-25T01:00:00', 3600, 0), 
       (38, 2, N'2018-10-28T02:00:00', 0, 0);

The reason for doing that is it will allow very fast individual row lookups. For both queries against the table you want to filter on [Tzid] and to find the first [DT_WhenSwitch] value in descending order. With the right clustered index getting that row can be a single clustered index seek.
To get that the plan I want I'm going to simplify the TVF a bit with the APPLY and TOP operators. I also want to make it very obvious to the optimizer that I get back just one row every time. Here's one implementation:
CREATE FUNCTION dbo.FN_GetLocalTime_FromUTC_BasedOnTZId 
     (@StartDateTime DATETIME, @EndDateTime DATETIME, @Tzid INT)
/*=========================================================================
*   2017-03-27
*   Returns local time from UTC time based on timeZoneId
*
==========================================================================*/
RETURNS TABLE 
WITH SCHEMABINDING
AS
RETURN
(
        SELECT
               DATEADD(SECOND, (COALESCE(S.s_DST_OffSet, 0) + COALESCE(S.s_GMT_OffSet, 0)), @StartDateTime) 'LocalStartDateTime'
             , DATEADD(SECOND, (COALESCE(E.e_DST_OffSet, 0) + COALESCE(E.e_GMT_OffSet, 0)), @EndDateTime)   'LocalEndDateTime'
        FROM (SELECT 1 t) t
        OUTER APPLY (
            SELECT TOP 1 
                D.DSTOffSetSeconds 's_DST_OffSet',
                D.GMTOffSetSeconds 's_GMT_OffSet'
            FROM dbo.DSTLookup D
            WHERE D.DT_WhenSwitch &lt;= @StartDateTime AND D.Tzid = @Tzid
            ORDER BY D.DT_WhenSwitch DESC
        ) s
        OUTER APPLY (
            SELECT TOP 1  
                 D.DSTOffSetSeconds 'e_DST_OffSet',
                 D.GMTOffSetSeconds 'e_GMT_OffSet'
             FROM dbo.DSTLookup D
             WHERE D.DT_WhenSwitch &lt;= @EndDateTime AND D.Tzid = @Tzid
             ORDER BY D.DT_WhenSwitch DESC
        ) e
);

Here is the query plan for your example query in the question:

As expected we only do two seeks against the clustered index:

(1 row(s) affected)
Table 'DSTLookup'. Scan count 2, logical reads 4, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SQL Server Execution Times:
CPU time = 0 ms,  elapsed time = 1 ms.

I wasn't able to test against SQL Server 2008, but I think that syntax will work on that platform. db fiddle for SQL Server 2014.