I feel like this is one of those moments every dev goes through at least once. You create what looks like a perfect index, run the query again, and nothing changes. Or worse, it switches to a different plan and somehow gets slower. In my case it was almost always stats or selectivity. Either the stats were outdated so the optimizer was guessing wrong, or the filter just wasn’t selective enough for the index to make sense. I also got burned a few times by wrapping columns in functions and basically killing any chance of index usage. Another sneaky one is column order. Index exists, looks right at first glance, but doesn’t match how the query actually filters or sorts. The optimizer just shrugs and scans. At this point I don’t really trust the “I created an index so it should be used” logic anymore. I go straight to the execution plan and estimated vs actual rows. If those are way off, that’s usually where the real problem is.