This is a strange situation, but I'm hoping someone has an answer.
During some performance troubleshooting, we added a NONCLUSTERED INDEX to a table, as was requested by
sp_BlitzIndex. We checked its usage the next day, and it showed 0 reads (0 scans/seeks, 0 singleton lookups), so we disabled it.
The very next minute, we get a complaint of the same app-slowness (performance trouble) that we were trying to check out & resolve in the first place when we added the INDEX.
Now, I know in theory, this sounds purely coincidental. The INDEX was provably, measurably, NOT USED. Disabling it SHOULD NOT have caused query performance degradation. But it's almost TOO coincidental.
So my question, simply enough, is thus:
Is it at all possible, that a NONCLUSTERED INDEX, whose usage-stats (from the DMVs /
sp_BlitzIndex) show NO usage, still have been helping query performance somehow on the affected table?