Why can't fields with high duplicate values be indexed (such as gender fields)

Conclusion （ With innodb For example ）

a、 Non clustered indexes store references to primary keys , If select Field is not in a non clustered index , You need to jump to the primary key index （ In the above figure, jump from the index tree on the right to the index tree on the left ）, Get more select field value

b、 If the non clustered index value has a high repetition rate , Then, when querying, there will be a large number of jumps from the right to the left in the above figure , The whole process is very slow

c、 If where Fields with high value repetition rate ,select It was used limit, Check only a few data , That is, when the number of jumps is very small , You can still build an index （ Later on, there was no need ,limit Limited quantity , Full table scanning is also fast , Unless the field values are sorted , All previous values must be scanned ）

d、 without 3 This premise , It is not recommended to build an index on a field with a high value repetition rate , Because the query efficiency is low , You also need to maintain the index