Hierarchial Queries in SQL

Recently there is a requirement in my project to display a tree structure from a database. I couldn't  explain the exact scenarios in the blog as it has some confidential information.  Hence, I am using the scenario which is being used across google.

Consider a scenario where program managers have a list of project managers reporting under them and the project manager have a list of employees under them.

EId  | Name     | Manager ID
-----------------------------------------
1     | Arun       | 2
2     | Balu       | 3
3     | Chandan  | NULL
------------------------------------------

If we need to display the list of employees under Chandan. The following query will do.

SELECT eid,name,manager_id from employees start with eid-1 CONNECT BY PRIOR eid=manager_id

Consider that there are more than 2 project managers for a same Program manager and each have an Eng. So,we need to know which engineer reports to which manager. In That scenario, SYS_CONNECT_BY_PATH clause would help.

SELECT eid, name, manager_id, SYS_CONNECT_BY_PATH(name, '/') PATH, LEVEL FROM employees start with eid=1 CONNECT BY PRIOR eid = manager id; 

This would print the path for Arun as Chandan/Balu/Arun and for Balu it would print as chandan/balu.

LEVEL clause would help in finding the position of the Node in the tree. In the above scenario, Chandan would get 2, Balu - 1 and Arun would get it as 0. These types of Queries are very useful if you need to build in a Hierarchy in your application.

Hashmap in Java 8

As most of you are aware, when 2 keys hash to a same index in HashMap, JAVA internally creates a Linked List internally to store the values in JAVA 7. 

The performance of the HashMap suffers a lot when the Hash Algorithm is poor or most of the data hash to the same index. If the bucket has got 100 values and the value we are trying to retrieve is in the 100th location or say greater than 50 th index, the performance of the hashmap suffers.

In-order to overcome this performance issue, JAVA 8 replaces Linked List with Balanced Tree(BST). As you are aware, in a BST , the elements that are present in the left of the root are smaller than the root and the elements that are on the right of the root are larger than the root. The time complexity to find the element in a BST is O(log n). Hence, the performance of the HashMap under worst case is increased in JAVA 8.