Force Sql server index usage in query

Sometimes sql server surprises me, I have a really stupid table with seven columns and one of these columns contains great amount of text data. I need to select the minimum Id based on a date filter, and so I issued a really simple query like:

Select min(TN.Id)
from TableName TN
where TN.timestamp >= ‘20110201’

This is a really simple query, but since the table is about 15 GB due to the large amount of text stored in it, it got executed in 140 secs, so I decided to create a simple index based on timestamp and id columns, but with my great surprise, even with the index, previous query still resort to a full table scan and needs 140 secs to be executed. The index is newly created, it is not fragmented, statistics are updated, so I really do not understand where is the problem.

Then I decided to force usage index with Query hints changing the query to

Select min(TN.Id)
from TableName TN with (INDEX(IX_TIMESTAMP)) where TN.timestamp >= ‘20110201’

And now the query executes in 2 seconds……..that behavior really surprise me, but thanks to query hint I can at least force the execution plan if sql server did not choose the right one.