Table scan in SQL

Question

Is there any impact of Table scan on performance of function.

Javeed M Shaikh · Answer

Hi Lucy Imagine if I give a really fat book with thousand pages and tell you that at page number 325 there is an interesting article about SQL so what would you do? go page by page till you reach page 325 or just open the book in the middle and then go back or forth of the pages depending upon what page number the middle of the book is opened too. So this is the same way query works. A Full Table Scan is the equivalent of having a book that doesn't have any page numbers but being told that somewhere in the book is some piece of information that you need to know. So the only way to access this information is to look at every page one at a time searching for the information you need its not fun right? In a 10 page Children's book this might be fine but a thousand page book not so much. So having an index the SQL will use the Index for searching and not use Full Table Scan by which the results can dramatically improve the performance of accessing the data. Please do not forget to mark Accepted Answer.