Setup & the O(N) wall·2 min
Why not just brute force?
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You just built brute force. It returns perfect results — so why does every vector DB on earth ship something more complicated?
Knowledge check
A corpus has 1,000,000 vectors of dimension 128. At roughly 10M L2 comparisons/sec single-threaded, about how long does ONE brute-force query take, and why is that the problem?
Knowledge check
What single number, beyond recall, will the later metric steps use to PROVE your index isn't secretly brute-forcing?