There are some options. For example, if the indexes are known to be within a certain range, a plain array will do better. For tables below a certain size, an array of key/value tuples might also perform better. An implementation could even switch between the underlying approaches depending on the data at hand, choosing between hash tables and something else depending on what should perform better.
Yes, the "hash" part is an implementation detail. The performance is also an implementation detail by the way. Often, tables will optimize for a different criterion such as memory efficiency. Lookup will be less efficient then. And, as another person noted, you cannot really expect O(1) from a hash table anyway.
Yes, the "hash" part is an implementation detail. The performance is also an implementation detail by the way. Often, tables will optimize for a different criterion such as memory efficiency. Lookup will be less efficient then. And, as another person noted, you cannot really expect O(1) from a hash table anyway.