關於mysql的聯合索引，覆蓋索引一直濛濛噠，所以寫點東西來熟悉一下
首先建立乙個表orders，結構如下：
create
table orders(
order_id int unsigned auto_increment,
order_status int
notnull,
total_price int unsigned,
settle_type int,
store_id int,
buyer_id int,
goods varchar(20),
create_time datetime default
current_timestamp,
primary
key(order_id)
);
drop procedure
ifexists
load_orders;
delimiter //
create
procedure
load_orders
(count int unsigned)
begin
declare
status
intdefault 0;
declare type int default
0;declare store int default
133;
declare price int default
85;declare buyer int default
1001;
declare goods varchar(20) ;
declare i int unsigned default
0;set goods= 'al';
while ido
set status=mod (status,77);
settype=mod(type,29);
if buyer>2000
then
set buyer=1001; end
if;if price>1000
then
set price=200; end
if;if length(goods)>18
then
set goods= 'al'; end
if;insert into orders(order_status,total_price,settle_type,store_id,goods,buyer_id) values (status, price, type, store, goods, buyer);
set count=count+1;
set status=status+1;
settype=type+1;
set store=store+1;
set price=price+43;
set buyer=buyer+1;
set goods=concat(goods, char(round(rand()*25)+97));
set i=i+1;
endwhile;
end//
delimiter ;
call load_orders(99999);#呼叫儲存過程
select
count(store_id) as cnt from orders where settle_type = 1
and order_status = 2
and buyer_id=1990;
select
count(order_id) as cnt from orders where settle_type = 1
and order_status = 2
and buyer_id=1990;
select
count(*) as cnt from orders where settle_type = 1
and order_status = 2
and buyer_id=1990;
我們發現，此時count(普通列)，count(主鍵索引列)，count(*)耗時都是差不多的，需要7秒多，當然還是count(*)最快。
從查詢計畫中我們可以看到是全表掃瞄的，using where。
我們改變表的結構，給where後的三列新增乙個聯合索引：
alter
table orders add
key union_key(settle_type,order_status,buyer_id);
我們發現，此時count(普通列)需要用時最久，
count(主鍵索引列)，count(*)耗時都是很短。
由上圖我們看到當使用索引的第二列和第三列時根本就用不到索引。
聯合索引中是放著主鍵索引的值的。
上面的using index condition就是使用到了icp喲，where會在儲存引擎中過濾後再傳遞給server層。
desc select settle_type from orders where settle_type = 1
and order_status = 2
and buyer_id=1990;
desc select settle_type from orders where settle_type = 1
and buyer_id=1990;
desc select settle_type from orders where order_status = 2
and buyer_id=1990;
desc select buyer_id from orders where settle_type = 1
and order_status = 2
and buyer_id=1990;
desc select buyer_id from orders where settle_type = 1
and buyer_id=1990;
desc select buyer_id from orders where order_status = 2
and buyer_id=1990;
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