We have in every site (a dozen) one big database with lots of tables and record, using innodb as default and with innodb_file_per_table. from time to time we have space issue on some site and I am able to reclaim space from big table with a
ALTER TABLE mydb.mytable ENGINE=InnoDB.
Hopefully that works, but I don't know table will return useful unused space a with table will not. I mean some big table have sometimes NO useless space, and some have a lot, depending how much DELETE have been done (I guess).
Is there a way to know in advance how much space is allowed for deleted record so I can run
ALTER TABLE mydb.mytable ENGINE=InnoDB only on table with lot of useless space.