Document how to determine which Aria tables are encrypted

You can query information_schema.INNODB_TABLESPACES_ENCRYPTION to determine which InnoDB tables are encrypted:

https://mariadb.com/kb/en/library/encrypting-data-for-innodb-xtradb/#seeing-which-tables-are-encrypted

https://mariadb.com/kb/en/library/information-schema-innodb_tablespaces_encryption-table/

Aria tables can also be encrypted if aria_encrypt_tables is enabled:

https://mariadb.com/kb/en/library/aria-system-variables/#aria_encrypt_tables

However, I can't tell if there is actually any way to determine which Aria tables are encrypted.

The only way I've been able to come up with is by finding out which Aria tables use the row_format PAGE:

SELECT TABLE_SCHEMA, TABLE_NAME 

FROM information_schema.TABLES 

WHERE ENGINE='Aria' 

AND ROW_FORMAT='PAGE'

AND TABLE_SCHEMA != 'information_schema';

And then finding some data from a particular table:

MariaDB [(none)]> SELECT * FROM db1.aria_tab LIMIT 1;

+----+------+

| id | str  |

+----+------+

|  1 | str1 |

+----+------+

1 row in set (0.00 sec)

And then checking whether you can find some plain text from that data in the data file:

$ sudo strings /var/lib/mysql/db1/aria_tab.MAD | grep "str1"

str1

However, it would probably make sense to have an information_schema table that says which Aria tables are encrypted.

Also, we should document whether Aria tables are also encrypted/decrypted by InnoDB's background encryption threads (configured by innodb_encryption_threads and innodb_encryption_rotate_key_age) or if they are encrypted/decrypted by some other means. My tests seem to indicate that Aria tables are not encrypted/decrypted by InnoDB's background threads. In fact, it appears that the encryption status of an Aria table depends solely on the value of aria_encrypt_tables at the time that it was created. See MDEV-17267.