Could this be a case of the Halloween problem?
Read: The Halloween Problem – Part 2 by Paul White (there are 4 parts linked in at the bottom).
Look for an "Eager Table Spool" in your INSERT's execution plan.
If it is there, divide and conquer might help.
It could also help to break the 1.5m row INSERT into six that INSERT 250K each.
My thought question: Have you ever been told that your query runs too fast?
My advice:
INDEXing a poor-performing query is like putting sugar on cat food. Yeah, it probably tastes better but are you sure you want to eat it?
The path of least resistance can be a slippery slope. Take care that fixing your fixes of fixes doesn't snowball and end up costing you more than fixing the root cause would have in the first place.
Need to UNPIVOT? Why not CROSS APPLY VALUES instead?[/url]
Since random numbers are too important to be left to chance, let's generate some![/url]
Learn to understand recursive CTEs by example.[/url]
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