问题
How to use indexes with more than one column
The original index has an index on block_id, but is it necesarry when it's already in the unique index with two column?
Indexes with more than one column
(a,b,c)
- you can search for a, b and c
- you can search for a and b
- you can search for a
- you can not search for a and c
Does this apply to unique indexes too?
table
id
block_id
account_id
name
indexes origin
PRIMARY KEY (`id`)
UNIQUE KEY `block_id` (`block_id`,`account_id`)
KEY `block_id` (`block_id`),
KEY `account_id` (`account_id`),
indexes alternative
PRIMARY KEY (`id`)
UNIQUE KEY `block_id` (`block_id`,`account_id`)
KEY `account_id` (`account_id`),
回答1:
The rules you describe above have to my knowledge always held whether an index is unique or not. You might run explain on the query you have in mind and observe when the index is used and when it is not used under various circumstances.
来源：https://stackoverflow.com/questions/13321332/mysql-and-indexes-with-more-than-one-column