Copying to tmp table and big wp_usermeta

Hi,
each time I search for a user in the ACP (by username, by email an so on) this query is executed:

SELECT SQL_CALC_FOUND_ROWS DISTINCT(wp_users.ID) FROMwp_users,wp_usermetaWHEREwp_users.ID=wp_usermeta.user_idAND ( (wp_users.user_loginLIKE '%test%' ORwp_users.user_nicenameLIKE '%test%' ORwp_users.display_nameLIKE '%test%' ORwp_users.user_emailLIKE '%test%' ORwp_users.user_urlLIKE '%test%') OR ((wp_usermeta.meta_key= 'first_name' ORwp_usermeta.meta_key= 'last_name') ANDwp_usermeta.meta_valueLIKE '%test%') OR (wp_usermeta.meta_key= 'wp_s2member_subscr_id' ANDwp_usermeta.meta_valueLIKE '%test%') OR (wp_usermeta.meta_key= 'wp_s2member_custom' ANDwp_usermeta.meta_valueLIKE '%test%') OR (wp_usermeta.meta_key= 'wp_s2member_custom_fields' ANDwp_usermeta.meta_valueLIKE '%test%')) AND wp_users.user_status != 2 ORDER BY user_login ASC LIMIT 0, 200

The wp_usermeta table is about 1,7 Gb. To make a test I tried to change both max_heap_table_size and tmp_table_size to 4192M. But this does not solve.

Any suggestion for me?

Thank you!!!