If ci[x] has only t - 1 keys but has an immediate sibling with at least t keys, give ci[x] an extra key by moving a key from x down into ci[x], moving a key from ci[x]'s immediate left or right sibling up into x, and moving the appropriate child pointer from the sibling into ci[x].
If ci[x] and both of ci[x]'s immediate siblings have t - 1 keys, merge ci[x] with one sibling, which involves moving a key from x down into the new merged node to become the median key for that node.
Since most of the keys in a B-tree are in the leaves, we may expect that in practice, deletion operations are most often used to delete keys from leaves. The B-TREE-DELETE procedure then acts in one downward pass through the tree, without having to back up. When deleting a key in an internal node, however, the procedure makes a downward pass through the tree but may have to return to the node from which the key was deleted to replace the key with its predecessor or successor (cases 2a and 2b).
Although this procedure seems complicated, it involves only O(h) disk operations for a B-tree of height h, since only O(1) calls to DISK-READ and DISK-WRITE are made between recursive invocations of the procedure. The CPU time required is O(th) = O(t logt n).
