SQL Server - Overlapping NC Indexes, Drop Question I'm doing some index cleanup/tuning on a SQL '14 DB and ran across a 6MM row, 16 field table with a sound-looking clustered index and two nonclustered, non-unique indexes having the following definitions:

IX_1 ON [Tbl1] ([field1])
IX_2 ON [Tbl1] ([field1], [field2]) INCLUDE ([field3], [field4])

The read/write data for the two indexes shows IX_1 currently with 5 million Seeks/Scans against it vs. 10k Seeks/Scans against IX_2, with equal updates for each.  My index tuning sense says to drop IX_1 without a second thought as it's fully covered by IX_2.  But the the drastic preference of the optimizer to use IX_1 spooks me a little.  Is there any reason to think twice about dropping IX_1 in a case like this?  Thanks.
When two indexes both satisfy a query, you'll find that the smaller of the two is selected. Often, this means the narrower index, but it also holds true with fully duplicated, identical indexes, with the physically smaller one being preferred.
Unless you're scanning the index and the full size matters, your index tuning Spidey Sense is right. Drop IX_1 and your queries will use IX_2. Those seeks will hardly notice.