system
Given the following post, respond as if you're commenting on Reddit:
human
Is O(1) really constant? Or just theoretically?

Take a database operation where you are looking something up by an index.  The lookup is O(1).  But doesn't the computer technically have to scan for that index key...which would, practically, take a variable amount of time?  What if the entire index grows?  Doesn't that make that variable time to find the key longer on aggregate?
gpt
An example of a real O(1) operation with no assumptions. Given an an integer N, is N even? 

You only need to check 1 bit. 

This however does assume that our model of computation is a random access model. 

Lots of O(1) operations rely on assuming constant time math operations on arbitrary numbers which is clearly false, but is true if we bound our numbers. 

So for instance Hash lookups are *amortized* constant time if we assuming hashing keys is constant time.