Let's take an example table called "sample" with only two rows - "number" and "character". If you run a simple query such as:
SELECT * FROM sample WHERE number = 4;
MySQL will check all records and will return only the one that has its number value set to 4. If you have 1 million entries for example, this will be a slow query. In this case we have a unique field - "number". Therefore, we can create an index for it. Indexing will create an internal register that is saved in by the MySQL service. It can be done with the following query:
ALERT TABLE sample ADD INDEX(number);
Once this index is set, next time you want to get the information for employee number 4, the service will go directly to it using the index and will return the information much faster.
This is just a very basic example. For bigger databases, the difference in the loading time can be significant. Indexing your database can drastically decrease the loading time of your web applications.
There is another query that you can use to increase the loading speed of your database:
OPTIMIZE TABLE sample
After the optimization, the MySQL service will take much less time to search in your database which will result in better performance of your scripts.