Postgresql Sequence vs Serial

Question

I was wondering when it is better to choose sequence, and when it is better to use serial.

What I want is returning last value after insert using

SELECT LASTVAL();

I read this question PostgreSQL Autoincrement

I never use serial before.

Answer 1

Check out a nice answer about Sequence vs. Serial.

Sequence will just create sequence of unique numbers. It's not a datatype. It is a sequence. For example:

create sequence testing1;
select nextval('testing1');  -- 1
select nextval('testing1');  -- 2

You can use the same sequence in multiple places like this:

create sequence testing1;
create table table1(id int not null default nextval('testing1'), firstname varchar(20));
create table table2(id int not null default nextval('testing1'), firstname varchar(20));

insert into table1 (firstname) values ('tom'), ('henry');
insert into table2 (firstname) values ('tom'), ('henry');

select * from table1;

| id | firstname |
|----|-----------|
|  1 |       tom |
|  2 |     henry |

select * from table2;

| id | firstname |
|----|-----------|
|  3 |       tom |
|  4 |     henry |

Serial is a pseudo datatype. It will create a sequence object. Let's take a look at a straight-forward table (similar to the one you will see in the link).

create table test(field1 serial);

This will cause a sequence to be created along with the table. The sequence name's nomenclature is <tablename>_<fieldname>_seq. The above one is the equivalent of:

create sequence test_field1_seq;
create table test(field1 int not null default nextval('test_field1_seq'));

Also see: http://www.postgresql.org/docs/9.3/static/datatype-numeric.html

You can reuse the sequence that is auto-created by serial datatype, or you may choose to just use one serial/sequence per table.

create table table3(id serial, firstname varchar(20));
create table table4(id int not null default nextval('table3_id_seq'), firstname varchar(20));

(The risk here is that if table3 is dropped and we continue using table3's sequence, we will get an error)

create table table5(id serial, firstname varchar(20));    
insert into table3 (firstname) values ('tom'), ('henry');
insert into table4 (firstname) values ('tom'), ('henry');
insert into table5 (firstname) values ('tom'), ('henry');

select * from table3;
| id | firstname |
|----|-----------|
|  1 |       tom |
|  2 |     henry |
        
select * from table4; -- this uses sequence created in table3
| id | firstname |
|----|-----------|
|  3 |       tom |
|  4 |     henry |
        
select * from table5;
| id | firstname |
|----|-----------|
|  1 |       tom |
|  2 |     henry |    

Feel free to try out an example: http://sqlfiddle.com/#!15/074ac/1

Answer 2

2021 answer using identity

I was wondering when it is better to choose sequence, and when it is better to use serial.

Not an answer to the whole question (only the part quoted above), still I guess it could help further readers. You should not use sequence nor serial, you should rather prefer identity columns:

create table apps (
    id integer primary key generated always as identity
);

See this detailed answer: https://stackoverflow.com/a/55300741/978690 (and also https://wiki.postgresql.org/wiki/Don%27t_Do_This#Don.27t_use_serial)