Re: InnoDB engine doesn't honor unique key constraints, under a heavy load

[Rick James]

SELECT  ACCESS_TOKEN, AUTHZ_USER, TOKEN_SCOPE, TOKEN_STATE
    FROM  IDN_OAUTH2_ACCESS_TOKEN
    WHERE  CONSUMER_KEY = 'a7ede2ff2ccf4946aa4176ff9577448d'
      AND  REFRESH_TOKEN = '0f972d6fea304c5ea15e325941a97666'
    ORDER BY  TIME_CREATED DESC
    LIMIT  1

begs for
INDEX(CONSUMER_KEY, REFRESH_TOKEN, TIME_CREATED)
In that order. Or you could swap the first two columns.

The FOREIGN KEY constraints requires that the column be UNIQUE in the _other_ table.