SQL Server MYSQL foreign key indexing is necessary

Background:

You know that when you define a foreign key, you are given an on delete ... on update ...;

What is specified here is how the columns of the table should be changed to cater for the changes in the columns of the main table. The value of indexing from a foreign key in a table is here,

When the primary key of the main table changes, it is necessary to propagate the changes from the table, first to find the rows from the table. Think about it. If you don't have an index, you need to

Scan, if you have an index, you can make index seek on the table, which can greatly improve the speed.

SQL Server MYSQL foreign key indexing is necessary