Should I use indexes for a many-to-many database table?

Question

does it make sense to create indexes for a table called user_movies with the following columns:

user_id movie_id

There will be much more reading than inserting or updating on this table but I'm not sure what to do. Also: Is it adequate to omit a primary key in this situation?

Answer 1

The correct definition for this table is as follows:

CREATE TABLE user_movies (
  user_id INT NOT NULL,
  movie_id INT NOT NULL,
  PRIMARY KEY (user_id, movie_id),
  FOREIGN KEY (user_id) REFERENCES users(user_id),
  FOREIGN KEY (movie_id) REFERENCES movies(movie_id)
) ENGINE=InnoDb;

Notice "primary key" is a constraint, not a column. It's best practice to have a primary key constraint in every table. Do not confuse primary key constraint with an auto-generated pseudokey column.

In MySQL, declaring a foreign key or a primary key implicitly creates an index. Yes, these are beneficial.

Answer 2

我会单独索引两列，是的，你可以消除主键。

Answer 3

I have always heard that you should create a unique index on BOTH columns, first one way ( user_id + movie_id ) then the other way ( movie_id + user_id ). It DOES work slightly faster (not much, about 10-20%) in my application with some quick and dirty testing.

It also makes sure you can't have two rows that tie the same movie_id to the same user_id (which could be good, but perhaps not always).

Answer 4

如果您使用这样的“连接表”，您可能会在查询中使用某些连接 - 这些连接可能会受益于这两列中每一列的索引（这意味着两个单独的索引）。