What is optimal binary search tree
Answers
1 An optimal binary search tree is a binary search tree for which the nodes are arranged on levels such that the tree cost is minimum. For the purpose of a better presentation of optimal binary search trees, we will consider “extended binary search trees”, which have the keys stored at their internal nodes.
if you want to compare AVL tree with simple binary search tree (BST) without balancing, then AVL will consume more memory (each node has to remember its balance factor) and each operation can be slower (because you need to maintain the balance factor and sometimes perform rotations).
As you said, BST without balancing has a very bad (linear) worst case. But if you know that this worst case won't happen to you, or if you're okay if the operation is slow in rare cases, BST without balancing might be better than AVL.