MyISAM uses a special tree-like cache to make bulk inserts faster for INSERT ... SELECT INSERT ... VALUES (...) (...) ... and LOAD DATA INFILE when adding data to non-empty tables. This variable limits the size of the cache tree in bytes per thread. Setting it to 0 disables this optimization. The default value is 8MB. Before MySQL 4.0.3. this variable was named myisam_bulk_insert_tree_size.