What does "seq_in_index" in MySQL database indexing means?

Question

When an index is created, there is one column I have doubts about, in concern with the physical linking of the data nodes. When 2 "seq_in_index" are same, how are they placed in the node? How is the structure of the index binary tree in that case?

Answer 1

It is the position of the column in the index.

Example:

INDEX some_idx(column_a, column_b)

seq_in_index for column_a is 1 and for column_b is 2

In your case the compound index is

INDEX email(email, status)