Mysql 大于小于查询count速度慢

SQL优化：

SQL如下：

SELECT
    count(0) 
FROM
    a 
WHERE
    a.del_flag = '0' 
    AND 
        a.start_time >= '2022-12-01 00:00:00'
    
    AND 
        a.start_time <= '2022-12-31 24:00:00'

查询时间16秒，怎么能优化下，我加了索引了不管用

求大家帮忙看看

采用between。

SELECT
    count(0) 
FROM
    a 
WHERE
    a.del_flag = '0' 
    AND 
        a.start_time between '2022-12-01 00:00:00'  AND '2022-12-31 24:00:00'

EXPLAIN 查看下执行计划，分析一下原因

求大佬帮忙看看啊