How can I delete cache from database in [messages table] ???????

[newsfish]

Hi,
In my Roundcube web mail database, I have a table named "messages". As you know, this table behaves as a cache table and store a copy of messages in it. but I have a problem,

This table permanently becomes full and causes that my web mail database becomes slow, and in this condition I have to delete all content of this table for improving my web mail.

How can I solve this issue?

These are all the config related to cache in main.inc.php

$rcmail_config['enable_caching'] = true;

$rcmail_config['message_cache_lifetime'] = '1h';