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Calculating the InnoDB free space

Recently someone asked my if it's possible to find the total free space within InnoDB. I thought this would be very easy as the INFORMATION_SCHEMA.TABLES table has a DATA_FREE column. So we could just use SELECT SUM(DATA_FREE) FROM INFORMATION_SCHEMA.TABLES couldn't we?
 
So what does the DATA_FREE column tell us? It tells us the free data within InnoDB for that particular table. A table can share a tablespace with multiple other tables.
 
The tablespace which is used by a table depends on whether the innodb_file_per_table was enabled during table creation and/or at the last time the table was rebuild (e.g. by OPTIMIZE TABLE).
 
If innodb_file_per_table was always disabled then this query probably reports the correct free space:

SELECT DATA_FREE FROM INFORMATION_SCHEMA.TABLES WHERE ENGINE='InnoDB' LIMIT 1;


This is because all tables will share 1 tablespace. …

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