最佳答案：
这句话应该是摘自 《PHP and MySQL Web development》 这本书
楼主之把其中一句单独取出来，让人很难理解。上下文是这样的
PRIMARY KEY after a column name specifies that this cloumn is the primary key for the table. Entries in this column have to be unique. MySQL will automatically index this column. Notice that where we've used it above with customerid in the customers table we've used it with AUTO_INCREMENT. The automatic index on the primary key takes care of the index required by AUTO_INCREMENT.
customers 的表结构是这样的
create table customers
(
customerid int unsigned not null auto_increment primary key,
name char(30) not null,
address char(40) not null,
city char(20) not null
);
从整个上下文的意思来看
这句话应该理解为 主键的自动索引可以支持自增长类型的索引。或者直接从字面翻译就是
主键的自动索引已经考虑到（照顾到）了对自增长类型的索引的支持
