MySql | 关系数据库与非关系数据库的性能

Question

What i want to ask, if we define relations, one-to-one, one-to-many etc will that increase the performance in comparison to if we dont create relations but do join the table on the go like

select * from employee inner join user on user.user_id = employee.user_id

i know this question has been asked before and most answers i have got saying that performance don't get affected by not using relations.

But i have also heard that creating indexes makes the query faster, so is it possible to create indexes on tables for foreign keys without creating relations. I'm little confused about index.

and what if we have large database like 100+ tables plus alot of records will the relations matter in terms of database query performace??

im using mysql and php..

Answer 1

Foreign keys are basically used for data integrity.

Of course, indexing boosts performance.

Regarding the performance with or without foreign keys, when it's said they improve performance is because when you define a foreign key you are implicitly defining an index. Such an index is created on the referencing table automatically if it does not exist.

Answer 2

Relations are used to maintain the referential integrity of the database. They do not affect performance of the "select" query at all. They do reduce performance of "insert", "update" and "delete" queries, but you rarely want a relational database without referntial integrity.

Indexes are what makes the "select" query run faster. They also make insert and update queries significantly slower. To know more about how the indexes work go to use-the-index-luke . This is by far the best site about this topic that I have found.

That said, databases usually make indexes automatically when you declare a primary key, and some of them (MySql in particular) make indexes automatically even when you define a foreign key. You can read all about why they do that on the above site.