Question: Shall adding foreign key constraint in access_tokens table for user_id column be an additional overhead

[SamarRizvi]

If foreign key constraint for user_id is added in access_tokens table, shall it be an additional overhead when there are large number of users. I am asking this because, when I add foreign key constraint to user_id field, the index on it is removed, and indexing is for sure, good when there are large number of users. Or is it that adding a foreign key constraint to a column is automatically indexed.

So what to use, foreign key constraint or index for user_id?

[kot13]

Is there a search in access_token table by user_id?
It seems no. Therefore, I think that the index is not needed.

But foreign key constraint for user_id also increases overhead. Maybe it is not necessary?

For maximum performance, you should use key-value storage (Redis for example).