This is a basic topic on SQL performance tuning:
Simply put, an index is a pointer to data in a table. An index in a database is similar to an index in the back of a book. For example, if you want to reference all pages in a book that discuss a certain topic, you first refer to the
index, which lists all topics alphabetically, and it refers you to one or more specific page numbers. An index in a database works the same way in that a query is pointed to the exact physical location of data in a table.
You are actually being directed to the data’s location in an underlying file of the database, but as far as you are concerned, you are referring to a table. Which would be faster, looking through a book page by page for some
information or searching the book’s index and getting a page number? Of course, using the book’s index is the most efficient method. It can save a lot of time, especially if the book is large.
If you have a book of just a few pages, however, it might be faster to check the pages for the information
than to flip back and forth between the index and pages of the book. When a database does not use an index, it is performing what is typically.