Testing of improper indexing in MySQL is slower than without an index

The following figure shows the data inserted into the tuangou table:

Id web city type
1. Beijing Food & Beverage
2. Shanghai leisure and entertainment
3% group Tianjin Food & Beverage
4. Online Shopping in Shenzhen
Discount coupons for Shijiazhuang
6% group Handan beauty and health care
..
Chongqing Travel Hotel 4999
5000 discount coupons in Xi'an

Run the MySQL statement:
$ SQL = "select from tuangou where web = 'handler' and city = 'shanghai '";

(1) If no index is added, the execution time is: 0.0041 seconds.

(2) If you only add an index to the Web, the execution time is 0.0064 seconds.

(3) If both the web and city are indexed, the execution time is 0.0007 seconds.

Although the index is added in (2), the query time is longer than the time when no limit is referenced. Therefore, I suggest you add indexes to fields frequently queried.