Clustered index vs Non clustered index in easy words
Think of a clustered index like the index at the back of a book that lists page numbers for different topics. In a clustered index, the data in the table is actually organized in the order of the index. Imagine you have a book about animals, and the book is organized by the animals’ names from A to Z. If you want to find information about zebras, you’d go to the “Z” section of the book and find everything about zebras in that order.
Now, a non-clustered index is like a separate list that tells you where to find information without changing the actual order of the book. Going back to the animal book, let’s say you have a list at the end of the book that tells you on which pages you can find information about specific topics. So, there might be a list that says: “Zebras — Pages 45, 89, 112.” This list doesn’t change the way the book is organized, but it helps you quickly find pages about specific animals.
In a database, a clustered index physically reorders the rows in the table to match the order of the index. This can speed up certain types of queries but might slow down others. A non-clustered index, on the other hand, creates a separate list that points to the actual rows, helping to speed up searches without changing the original order of the data.
So, in simple terms:
- Clustered index: Data in the table is organized like the index itself.
- Non-clustered index: A separate list that points to where the data is located in the table without changing the original order.
Both types of indexes help databases quickly find and retrieve information, similar to how indexes and page numbers help you find specific topics in a book.