Different Count and Mysql_num_rows in MYSQL

Use COUNT, internally the server will process the request differently.

When doing COUNT, the server will only allocate memory to store the result of the count.

When using mysql_num_rows, the server will process the entire result set, allocate memory for all those results, and put the server in fetching mode, which involves a lot of different details, such as locking.

Think of it like the following pseudo scenario:

1) Hey Bob, how many people are in the class room? (count)

2) Hey Bob, get me a list of all the people in the classroom, ... I'll calculate the number of people myself (mysql_num_rows)