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:

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:

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:

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?

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