Know what tables each user is accessing
This would have a considerable impact on your system, but you could enable the MySQL General Query Log by running the following:

SET GLOBAL general_log = 'ON';


The default setting for the logs will output to a file that can be identified by running this:

SHOW GLOBAL VARIABLES WHERE Variable_name = 'general_log_file';


Otherwise, you can change the setting to log in a table with the line below, and then you would be able to run queries on the table mysql.general_log

SET GLOBAL log_output = 'TABLE';