在表中创建索引不会降低成本

Question

我有这张桌子：

accident_info
(
    accident_index varchar(20),
    first_road_class varchar(20),
    accident_severity varchar(20),
    date date,
    urban_or_rural_area varchar(20),
    weather_conditions varchar(40),
    year int,
    inscotland varchar(20)
);

针对此表，我执行以下查询：

select count(accident_index)as hits, first_road_class 
from accident_info 
group by first_road_class;

没有索引。

我想创建一个索引来降低我的总成本，但是到目前为止我做的似乎无效。这是：

create index on accident_info(accident_index, first_road_class);

First ten Rows of my table

Answer 1

对于此查询：

select count(accident_index) as hits, first_road_class 
from accident_info 
group by first_road_class;

您可以尝试在accident_info(first_road_class, accident_index)上建立索引。列的顺序很重要。