What you're looking for doesn't exist. Even if you have a single 256-bit public key in your table, if you want it to have an instant lookup you need a contiguous array of 2**256 items, where everything is zeroed out except for the location of your public key. Each location needs to have an exact number of bits for the value, in order to be able to compute the lookup location. You can do the math.  You might say that it's absurd, and you can simply compare any key with your stored key, but this doesn't scale. Once you add a second public key, you're already looking at having to choose between a O(1) (unfeasible), O(2) (linear comparisons) and a mixed O(~1) + O(log2) lookup algorithm. And so on.

The best you can get is already explained several times. You can get pretty close to instant by using binning (or a bloom filter), followed by a somewhat log-N lookup (or a small maximum amount of suffix check steps). Most of the items won't pass the filter / binning check anyway, which is already an "instant" operation. If you can find something faster than this, you've broken classical computing!