dbencoding = 'utf8mb4'

Dear Sir.

After Altering DB, I altered DB Table one by one from php MyAdmin.

ran SQL like following

ALTER TABLE
    elgg_config
    CONVERT TO CHARACTER SET utf8mb4
    COLLATE utf8mb4_unicode_ci;

It appears that all tables have updated except the following three tables.

elgg_config
elgg_datalists 
elgg_users_sessions

I get an error msg with all the above three tables. The error msg is:

#1071 - Specified key was too long; max key length is 1000 bytes

Before Collation was utf8_general_cim, but agter altering it now shows utf8mb4_unicode_ci for each table except the above three. 

What should I do? Could anyone help me?