MyBB Community Forums

As in wordpress SELECT, INSERT, UPDATE and DELETE are the required privileges for most task other privileges are required only for tasks like upgrading or some plugins may require extra. 

As per wordpress hardening instructions minimizing privileges helps with security http://codex.wordpress.org/Hardening_WordPress

so for mybb what should be the optimal mysql privileges and will it help with security ? 

Once the installation is complete (including all plugins, etc) you should only need SELECT, INSERT, UPDATE, DELETE. Installing plugins will often require new tables to be created or dropped using CREATE TABLE, etc.

Usually this won't improve security too much, as if an attacker has access to your database, you've pretty much already lost. I would just make sure the MySQL user for your forum has access only to the forum database and no others.

Task- thread views update requires drop command. - I checked error in my task log

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if it doesn't affect security much I will grant all privileges to avoid unknown errors