Question

I read somewhere that one should not create many indexes on the columns of a table for it reduces the performance of operations.

But when I create a table with UNIQUE NOT NULL fields, the MySQL is creating indexes on all the fields by itself. Doesn’t that reduce the performance?? If yes, what flags I need to change the default behaviour? If not, then where I am wrong?

My Table:

CREATE TABLE Users(
Id_usr INT UNSIGNED NOT NULL AUTO_INCREMENT,
PRIMARY KEY(Id_usr),
Email_id varchar(45) UNIQUE NOT NULL,
username varchar(30) UNIQUE NOT NULL,
userpass varchar(30) NOT NULL
);

When I see the table on phpmyadmin:

Indexes: Documentation
Action  Keyname Type    Unique  Packed  Column  Cardinality Collation   Null    Comment
Edit Edit   Drop Drop   PRIMARY BTREE   Yes No  Id_usr  0   A       
Edit Edit   Drop Drop   Email_id    BTREE   Yes No  Email_id    0   A       
Edit Edit   Drop Drop   username    BTREE   Yes No  username    0   A       

Answer 1

But when I create a table with UNIQUE NOT NULL fields, the MySQL is creating indexes on all the fields by itself.

No, Mysql is creating index on only those fields that you told it to. When you create a column with UNIQUE at the end it means you are telling mysql to add UNIQUE index on that column.

So for two UNIQUE columns there are two indexes. And for the primary key there is one index. In total 3 indexes. userpass column has no index. Because you didn’t tell it to.

If you need index and you know why you need UNIQUE index then just add it. Dont think about the performance now. After deploying if you experience any performance problem then think about optimization.