B-Tree vs Hash Table

Question

In MySQL, an index type is a b-tree, and access an element in a b-tree is in logarithmic amortized time O(log(n)).

On the other hand, accessing an eleme

Answer 1

The time complexity of hashtables is constant only for sufficiently sized hashtables (there need to be enough buckets to hold the data). The size of a database table is not known in advance so the table must be rehashed now and then to get optimal performance out of an hashtable. The rehashing is also expensive.