Why isn't SQL Server using my index?

Question

In our database we have this table with 200.000 rows

CREATE TABLE dbo.UserTask (
    UserTask_ID int NOT NULL IDENTITY (1, 1),
    UserTask_SequenceNumber in         


        

Answer 1

I faced with situation when the same query makes different plan on different databases. On the one DB it use non-clustered index and on the other - table scan.

Also this index doesn't have all field in INCLUDE, and the best solution here would be to add all necessary selected fields to index INCLUDE. In my case drop free cache helps though.

DBCC freeproccache

Sometimes query plan builder ignores index if it has fragmentation more 50%, because it spend more time to find row in index than to scan the entire table.