How to properly index fields on a Rails join table migration?

Question

Rails 4 introduced a feature for generating join table migrations:

bin/rails generate migration CreateTeamsUsersJoinTable team user

This re

Answer 1

To complete xlembouras answer, you make your choice based on how you query your table.

For example, if you have a view that shows the users of a given team, the query will look like this :

SELECT user.name FROM team_users tu 
INNER JOIN users u 
ON tu.user_id = u.id
WHERE tu.team_id = X

This benefits from t.index [:team_id, :user_id] but not t.index [:user_id, :team_id]

The database will only use one index seek on the first index to retrieve the user ids

If you have a view that shows the teams of a given user, the query would be something like the following :

SELECT team.name FROM team_users tu 
INNER JOIN team t 
ON tu.team_id = t.id
WHERE u.user_id = X

This benefits from t.index [:user_id, :team_id] but not t.index [:team_id, :user_id]

The database will only use one index seek on the first index to retrieve the team ids

That's because of the way composed index are stored :

Answer 2

I would have to say that the intention behind this is to let you choose the strategy that fits you better. That is usually a decision with two criteria

• how much overhead does indexing adds
• how you'll query your db

So they can not make that decision for you and add the indexing overhead as a convention.

In order to make that decision you need to know how you intend to query your database. In MySQL composite indexes can be of use for a set of queries. multiple-column index

An index on column_a, column_b, column_c will be used for queries on

• all three fields together
• column_a alone
• column_a and column_b together

So there is a difference between

t.index [:team_id, :user_id]

and

t.index [:user_id, :team_id]

the actual full set of indexes that you 'll need is

t.index [:team_id, :user_id]
t.index :user_id

or

t.index [:user_id, :team_id]
t.index :team_id

In order to handle with an index all three cases.

example

if you find your app to use more frequently queries like

user.teams which translate to

select * from teams where user_id = X

an index on user_id (or user_id, team_id) can be handy

so t.index [:user_id, :team_id] should be your choice