今天沒有事驗證一下複合索引在不同的數據庫中的使用情況，僅是測試而已，以一個例子測試，結果如下
首先在oracle，sqlserver， mysql建立表testpayorderinfo 表結構一樣，然後插入20w數據
分別創建複合索引（testpayorderid,connid,customerid的複合索引）
以複合索引中的不同條件字段的組合作爲條件進行測試
說明：
testpayorderid 對應a
connid 對應b
customerid 對應c
測試結果如下：
sql server：（版本sql server 2008）
select * from testpayorderinfo where testpayorderid=194027 --a 走
select * from testpayorderinfo where connid=257277 --b 走
select * from testpayorderinfo where customerid=1304349 --c 走
select * from testpayorderinfo where testpayorderid=194027 and connid=257277 --ab 走
select * from testpayorderinfo where testpayorderid=194027 and customerid=1304349 --ac 走
select * from testpayorderinfo where testpayorderid=194027 and connid=257277 and customerid=1304349 --abc 走
select * from testpayorderinfo where connid=257277 and customerid=1304349 --bc 走
select testpayorderid,connid,customerid from testpayorderinfo where connid=257277 --b 走
select testpayorderid,connid,customerid from testpayorderinfo where customerid=1304349 --c 走
select testpayorderid,connid,customerid from testpayorderinfo where connid=257277 and customerid=1304349 --bc 走
oracle：（oracle Release 12.1.0.2.0 ）
create index ix_com_acc on testpayorderinfo(testpayorderid,connid,customerid)
select * from testpayorderinfo where testpayorderid=194027 --a 走
select * from testpayorderinfo where connid=257277 --b 不走
select * from testpayorderinfo where customerid=1304349 --c 不走
select * from testpayorderinfo where testpayorderid=194027 and connid=257277 --ab 走
select * from testpayorderinfo where testpayorderid=194027 and customerid=1304349 --ac 走
select * from testpayorderinfo where testpayorderid=194027 and connid=257277 and customerid=1304349 --abc 走
select * from testpayorderinfo where connid=257277 and customerid=1304349 --bc 不走
select testpayorderid,connid,customerid from testpayorderinfo where connid=257277 --b 走
select testpayorderid,connid,customerid from testpayorderinfo where customerid=1304349 --c 走
select testpayorderid,connid,customerid from testpayorderinfo where connid=257277 and customerid=1304349 --bc 走
mysql ：（Server version: 5.5.47-log MySQL Community Server (GPL)）
select * from testpayorderinfo where testpayorderid=194027 --a 走
select * from testpayorderinfo where connid=257277 --b 不走
select * from testpayorderinfo where customerid=1304349 --c 不走
select * from testpayorderinfo where testpayorderid=194027 and connid=257277 --ab 走
select * from testpayorderinfo where testpayorderid=194027 and customerid=1304349 --ac 走
select * from testpayorderinfo where testpayorderid=194027 and connid=257277 and customerid=1304349 --abc 走
select * from testpayorderinfo where connid=257277 and customerid=1304349 --bc 不走
select testpayorderid,connid,customerid from testpayorderinfo where connid=257277 --b 走
select testpayorderid,connid,customerid from testpayorderinfo where customerid=1304349 --c 走
select testpayorderid,connid,customerid from testpayorderinfo where connid=257277 and customerid=1304349 --bc 走
測試結果：
oracle和mysql 走的索引是一樣的，sql server 只要是複合索引中的任何一個字段都走索引
