postgresql duplicate key violates unique constraint

Question

Hi i have a question i know this was posted many times but i didn't find an answer to my problem. The problem is that i have a table and a column "id" i want it to be unique number just as normal. This type of column is serial and the next value after each insert is comming from a sequence so everything seems to be all right but it still sometimes showing this error. I dont know why ? In the documentation it is writtne the sequence is fool prof and always works. If I add a UNIQUE constraint to that column will it thelp ? I worked before many times on Postres but this error is showing for me for the first time. I did everything as normal and i never had this problem before. Can you help me to find the answer that can be used in the future for all tables that will be created ? Lets say we have something easy like this:

CREATE TABLE comments
(
  id serial NOT NULL,
  some_column text NOT NULL,
  CONSTRAINT id_pkey PRIMARY KEY (id)
)
WITH (
  OIDS=FALSE
);
ALTER TABLE interesting.comments OWNER TO postgres;

If i add:

ALTER TABLE comments ADD CONSTRAINT id_id_key UNIQUE(id)

Will if be enought or is there some other thing that should be done ?

Answer 1

This article explains that your sequence might be out of sync and that you have to manually bring it back in sync.

An excerpt from the article in case the URL changes:

If you get this message when trying to insert data into a PostgreSQL database:

ERROR:  duplicate key violates unique constraint

That likely means that the primary key sequence in the table you're working with has somehow become out of sync, likely because of a mass import process (or something along those lines). Call it a "bug by design", but it seems that you have to manually reset the a primary key index after restoring from a dump file. At any rate, to see if your values are out of sync, run these two commands:

SELECT MAX(the_primary_key) FROM the_table;   
SELECT nextval('the_primary_key_sequence');

If the first value is higher than the second value, your sequence is out of sync. Back up your PG database (just in case), then run this command:

SELECT setval('the_primary_key_sequence', (SELECT MAX(the_primary_key) FROM the_table)+1);

That will set the sequence to the next available value that's higher than any existing primary key in the sequence.