Hi,
you can convert your table into a MEMORY (formely called HEAP) table with
ALTER TABLE forum_useronline ENGINE=MEMORY;
You could have created it as a MEMORY table from the beginning by using ENGINE=MEMORY instead of TYPE=MyISAM in the CREATE TABLE statement.
After every server start, memory tables are empty.
Your memory table will have a maximum size of 16MB by default.
You can change the default with SET GLOBAL max_heap_table_size=value; (before ALTER/CREATE, this requires SUPER privilege) or set the maximum size of an individual table with
ALTER/CREATE TABLE ... AVG_ROW_LENGTH=value MAX_ROWS=value;
You can combine this with the ENGINE=MEMORY table option.
Read more about MEMORY tables in
http://dev.mysql.com/doc/refman/4.1/en/memory-storage-engine.html
Regards
Ingo Strüwing, Senior Software Developer - Storage Engines
MySQL AB, www.mysql.com