I don't understand how postgresql's nextval() work, can someone explain?

Question

I\'m just starting to wade into backend development after my first few months on the job as a front end dev. I\'m working with postgreSQL and can\'t seem to wrap my head around

Answer 1

NEXTVAL is a function to get the next value from a sequence.

Sequence is an object which returns ever-increasing numbers, different for each call, regardless of transactions etc.

Each time you call NEXTVAL, you get a different number.

This is mainly used to generate surrogate primary keys for you tables.

You can create a table like this:

CREATE SEQUENCE mysequence;
CREATE TABLE mytable (id BIGINT NOT NULL PRIMARY KEY, value INT);

and insert values like this:

INSERT
INTO    mytable (id, value)
VALUES
        (NEXTVAL('mysequence'), 1),
        (NEXTVAL('mysequence'), 2);

and see what you get:

SELECT * FROM mytable;
 id | value
----+-------
  1 |     1
  2 |     2

PostgreSQL offers a nice syntax sugar for this:

CREATE TABLE mytable (id BIGSERIAL PRIMARY KEY, value INT);

which is equivalent to

CREATE SEQUENCE mytable_id_seq; -- table_column_'seq'
CREATE TABLE mytable (id BIGINT NOT NULL PRIMARY KEY DEFAULT NEXTVAL('mytable_id_seq'), value INT); -- it's not null and has a default value automatically

and can be used like this:

INSERT
INTO    mytable (value)
VALUES  (1),
        (2);  -- you can omit id, it will get filled for you.

Note that even if you rollback your insert statement or run concurrent statements from two different sessions, the returned sequence values will never be the same and never get reused (read the fine print in the docs though under CYCLE).

So you can be sure all the values of your primary keys will be generated unique within the table.