如何提高COUNT（DISTINCT field1）... GROUP BY field2的性能？

I have the following query

我有以下查询

EXPLAIN SELECT COUNT(DISTINCT ip_address) as ip_address, exec_date
    FROM requests
    GROUP BY exec_date;

id  select_type table       type        possible_keys   key         key_len ref      rows   Extra
1   SIMPLE      requests    range       NULL            daily_ips   263     NULL    488213  Using index for group-by (scanning)
EXPLAIN S