Is using SERIAL fine for the primary key when creating a table of all the comments made by users?

I am trying to learn postgres after working with mongodb for a while, and I'm wondering how to have a unique ID for each comment.

I have a foreign key userId for which user created the comment, but for primary key I need some kind of commentId. Is it fine to use SERIAL for my commentId? Or, is there a better approach like UUIDs? I don't know if I will ever have to migrate the data.

Solution

Since the actual value of the comment id does not interest you (just the fact that it's there and it's unique), serial is a good choice for such a column. Note that in modern PostgreSQL databases (since 7.3), creating a serial does not automatically mean it will have a unique constraint, so you'd have to handle that explicitly. E.g.:

CREATE TABLE comments (
    comment_id SERIAL PRIMARY KEY,
    user_id INT REFERENCES users(id), -- You should probably also index it
    comment VARCHAR(200) -- Or any other reasonable size
)

EDIT:
To answer the question in the comments, a similar behavior could be created for a UUID column by giving it a default value of a newly generated UUID.

First, you'd have to install the postgres-contrib` package (if you don't have it installed yet). E.g., On Red Hat based linuxes, you could run (as root):

$ dnf install postgresql-contrib

Then, from a privileged user, you need to create the extension:

CREATE EXTENSION IF NOT EXISTS "uuid-ossp";

This will create a generate_uuid_v1 function you could use:

CREATE TABLE comments (
    comment_id UUID DEFAULT UUID_GENERATE_V1() PRIMARY KEY,
    user_id INT REFERENCES users(id), -- You should probably also index it
    comment VARCHAR(200) -- Or any other reasonable size
)