[HALtheWise]

In practice, for any memory system with caches and limited by the speed of light, random (unpredictable) access to an array of size n takes much closer to O(sqrt(n)), not O(log(n)). There's an excellent article discussing this that you can search for, and it holds both in emperical tests on modern hardware and in the theoretical physical limit.

[jltsiren]

That depends on your perspective. If multiple levels of memory hierarchy are relevant (such as when scaling from 1 MiB to 1 GiB), you will see something resembling O(sqrt(n)). If you remain within the same level (e.g. from 1 GiB to 1 TiB), the scaling resembles O(log n) more closely. Or, in other words, it depends on whether you assume that cache size grows with n or is independent of it.