guessing unused junk space in table. (innodb mysql/mariadb ) 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.
After digging the question, I went with a SELECT like this one:

SELECT table_name from information_schema.tables WHERE table_schema = 'mydb' AND DATA_FREE &gt; 0 ORDER by DATA_FREE DESC;


So I have a list of table to process.

a scripted form would be :

#! /bin/bash
[ -z "$1" ] &amp;&amp; echo usage "$0 [database_name]"
[ -z "$1" ] &amp;&amp; exit 1
for T in $(mysql -ss -e "SELECT table_name from information_schema.tables WHERE table_schema = \"$1\" AND DATA_FREE &gt; 0 ORDER by DATA_FREE DESC;");do echo \"$T\";time mysql -e "SET sql_log_bin=0;alter table $1.$T ENGINE=InnoDB";done 2&gt;&amp;1