Erik Darling looks at how available indexes may contribute to parameter sniffing problems:
When you’re troubleshooting parameter sniffing, the plans might not be totally different.
Sometimes a subtle change of index usage can really throw gas on things.
It’s also a good example of how Key Lookups aren’t always a huge problem.
Both plans had them, just in different places.
The plans had a small change, but that made a big difference.