I have to regularly import records into a table in the database that also contains records that were created internally. Those that are imported have an ImportRecordID column that I use to ensure that I don't import a record twice. In the import code, I query the table on the ImportRecordID column, and if I get no results, I go ahead and add the new record. Very straightforward .
The problem is that I noticed that when I removed this "existing record" check (SELECT query), the import sped up by nearly 10x. Why should a simple SELECT take longer than 3 or 4 INSERTs? I asked myself.
So I added an Index on the ImportRecordID. It had to be non-Unique because there are many records in the table for which this value is NULL. I looked at the execution plan in Management Studio both before and after adding the index, and it assured me that I would get massive performance gains. Great!
Except that I didn't get massive performance gains when I actually ran the import. The performance on that SELECT was exactly the same. Why? How can I tell that the new Index is actually working?
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