Question

I have the following table in postgres:

CREATE TABLE "test" (
    "id" serial NOT NULL PRIMARY KEY,
    "value" text
)

I am doing following insertions:

insert into test (id, value) values (1, 'alpha')
insert into test (id, value) values (2, 'beta')

insert into test (value) values ('gamma')

In the first 2 inserts I am explicitly mentioning the id. However the table’s auto increment pointer is not updated in this case. Hence in the 3rd insert I get the error:

ERROR:  duplicate key value violates unique constraint "test_pkey"
DETAIL:  Key (id)=(1) already exists.

I never faced this problem in Mysql in both MyISAM and INNODB engines. Explicit or not, mysql always update autoincrement pointer based on the max row id.

What is the workaround for this problem in postgres? I need it because I want a tighter control for some ids in my table.

UPDATE:
I need it because for some values I need to have a fixed id. For other new entries I dont mind creating new ones.

I think it may be possible by manually incrementing the nextval pointer to max(id) + 1 whenever I am explicitly inserting the ids. But I am not sure how to do that.

Answer 1

That’s how it’s supposed to work – next_val('test_id_seq') is only called when the system needs a value for this column and you have not provided one. If you provide value no such call is performed and consequently the sequence is not "updated".

You could work around this by manually setting the value of the sequence after your last insert with explicitly provided values:

SELECT setval('test_id_seq', (SELECT MAX(id) from "test"));

The name of the sequence is autogenerated and is always tablename_columnname_seq.