I have a question about indexer and execution plans in T-SQL.

My database is SQL Server 2008.

I have a simple three-table db schema:

[![database schema][1]][1]

The `InverterData` table is very large and partitioned (28,880,436 rows).

The `Date` column is calculated like this:

    [Date] AS (ISNULL(CONVERT([date], [TimeStamp]), CONVERT([DATE], '19000101', (112)))) PERSISTED NOT NULL,

There is also a index for this column:

    CREATE NONCLUSTERED INDEX [NonClusteredIndex] 
    ON [InverterData] ([Date] DESC, [InverterID] ASC)

**Select query #1**:

I now want to make a simple select that include all three tables and the 'Date' column in a where clause:

    SELECT 
        [TimeStamp], [ACPower], [DCPower]
    FROM 
        [InverterData]
    JOIN 
        [Inverter] ON [InverterData].[InverterID] = [Inverter].[ID]
    JOIN 
        [DataLogger] ON [Inverter].[DataLoggerID] = [DataLogger].[ID]
    WHERE 
   	    [InverterData].[Date] = '05.01.2016'
	    AND [DataLogger].[ProjectID] = 20686

It took round about 19 seconds on me current database (result ~80 rows).

This is the execution plan:

[![slow query by date][2]][2]

**Select query #2**:

In the first select I detected that there is a long duration index seek for the 'Date' column. So I run a second select that only include the primary key column 'TimeStamp'.

This is the second select:

    SELECT 
        [TimeStamp], [ACPower], [DCPower]
    FROM 
        [InverterData]
    JOIN 
        [Inverter] ON [InverterData].[InverterID] = [Inverter].[ID]
    JOIN 
        [DataLogger] ON [Inverter].[DataLoggerID] = [DataLogger].[ID]
    WHERE 
  	    [TimeStamp] >= '05.01.2016' AND [TimeStamp] < '06.01.2016'
	    AND [DataLogger].[ProjectID] = 20686

It took only about 2 seconds on me current database.

This is the execution plan:

[![fast query by timestamp][3]][3]

**Question:**

Why are there two index seeks? I included all used columns from select 1 in one index. Why did it took so much longer?

[![details][4]][4]

**Update 1**:

If someone needed the complete schema I will add it to a SQL fiddle and post it.

**Update 2**:

Tooltip of index seek

[![details2][5]][5]


  [1]: https://i.sstatic.net/VtDmE.png
  [2]: https://i.sstatic.net/5UdMH.png
  [3]: https://i.sstatic.net/kgNcx.png
  [4]: https://i.sstatic.net/NvwHQ.png
  [5]: https://i.sstatic.net/uuMg0.png