CREATE TABLE [dbo].[PenData](
    [IDUkazatel] [smallint] NOT NULL,
    [Cas] [datetime2](0) NOT NULL,
    [Hodnota] [real] NULL,
    [HodnotaMax] [real] NULL,
    [HodnotaMin] [real] NULL,
 CONSTRAINT [PK_Data] PRIMARY KEY CLUSTERED
(
    [IDUkazatel] ASC,
    [Cas] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

ALTER TABLE [dbo].[PenData]  WITH NOCHECK ADD  CONSTRAINT [FK_Data_Ukazatel] FOREIGN KEY([IDUkazatel])
REFERENCES [dbo].[Ukazatel] ([IDUkazatel])

ALTER TABLE [dbo].[PenData] CHECK CONSTRAINT [FK_Data_Ukazatel]

DECLARE @t1 DATETIME;
DECLARE @t2 DATETIME;

SET @t1 = GETDATE();
SELECT min(cas) from PenData p WHERE IDUkazatel=24
SELECT min(cas) from PenData p WHERE IDUkazatel=25
SET @t2 = GETDATE();
SELECT DATEDIFF(millisecond,@t1,@t2) AS elapsed_ms;


SET @t1 = GETDATE();
SELECT min(cas) from PenData p WHERE IDUkazatel=24 OR IDUkazatel=25
SET @t2 = GETDATE();
SELECT DATEDIFF(millisecond,@t1,@t2) AS elapsed_ms;

SELECT MIN(cas) from PenData p WHERE IDUkazatel IN (SELECT IDUkazatel FROM  ...)

SELECT MIN(cas)
FROM   (
        SELECT cas=MIN(cas) FROM PenData p WHERE p.IDUkazatel = 24
        UNION ALL
        SELECT cas=MIN(cas) FROM PenData p WHERE p.IDUkazatel = 25
    ) AS minimums

SELECT
    MinCas = MIN(CA.PartialMinimum)
FROM @T AS T
CROSS APPLY
(
    SELECT
        PartialMinimum = MIN(PD.Cas)
    FROM dbo.PenData AS PD
    WHERE
        PD.IDUkazatel = T.IDUkazatel
) AS CA;

SELECT MIN(cas) from PenData p WHERE IDUkazatel IN (...)

DECLARE @IDZapisovac int = 1

DECLARE @MinCas DateTime2(2) = NULL
DECLARE @IDUkazatel smallint
DECLARE @Cas DateTime2(2) = NULL
DECLARE helper CURSOR FOR
   SELECT IDUkazatel FROM ...
OPEN helper
FETCH NEXT FROM helper INTO @IDUkazatel
WHILE @@FETCH_STATUS = 0
BEGIN
    SELECT @Cas = MIN(Cas) FROM PenData WHERE PenData.IDUkazatel=@IDUkazatel
    IF (@cas IS NOT NULL AND @cas<ISNULL(@MaxCas, '21000101'))
        SET @MinCas = @cas
    FETCH NEXT FROM helper INTO @IDUkazatel
END
CLOSE helper;
DEALLOCATE helper;
SELECT @MinCas