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auto-increment postgresql sql

Postgresql Sequence vs Serial

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.

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