I am writing an idempotent schema change script for a Postgres 12 database. However I noticed that if I include the IF NOT EXISTS in an ADD COLUMN statement then even if the column already exists it is adding duplicate Indexes for the uniqueness constraint which already exists. Simple example:
-- set up base table
CREATE TABLE IF NOT EXISTS test_table
(id SERIAL PRIMARY KEY
);
-- statement intended to be idempotent
ALTER TABLE test_table
ADD COLUMN IF NOT EXISTS name varchar(50) UNIQUE;
Running this script creates a new index test_table_name_key[n] each time it is run. I can't find anything in the Postgres documentation and don't understand why this is allowed to happen? If I break it into two parts eg:
ALTER TABLE test_table
ADD COLUMN IF NOT EXISTS name varchar(50);
ALTER TABLE
ADD CONSTRAINT test_table_name_key UNIQUE (name);
Then the transaction fails because Postgres rejects the creation of a constraint which already exists (which I can then catch in a DO EXCEPTION block). As far as I can tell this is because doing it by this approach I am forced to give the constraint a name. This constrasts with the ALTER COLUMN SET NOT NULL which can be run multiple times without error or side effects as far as I can tell.
Question: why does it add a duplicate unique constraint and are there any problems with having multiple identical indexes on a table column? (I think this is a subtle 'error' and only spotted it by chance so am concerned it may arise in a production situation)
You can create multiple unique constraints on the same column as long as they have different names, simply because there is nothing in the PostgreSQL code that forbids that. Each unique constraint will create a unique index with the same name, because that is how unique constraints are implemented.
This can be a valid use case: for example, if the index is bloated, you could create a new constraint and then drop the old one.
But normally, it is useless and does harm, because each index will make data modifications on the table slower.
This question already has answers here:
What is a PRIMARY KEY
(4 answers)
Closed 2 years ago.
I currently have a table:
userID | color | quantity
-------------------------
where userID is the primary key. My problem is when I try to insert to the DB (that already has one item from the same ID) I get the error: pq: duplicate key value violates unique constraint I am using Go with lib/pq package to insert.
I am unsure whether I have the wrong idea of what to use a PK for, or if I don't understand what kind of table I need to make
Primary key is a key that uniquely identifies each single row in the table and therefore needs to be unique. If you need more rows with same userID in your table then userID cannot be a primary key.
When you specify column (or group of columns) as a primary key PostgreSQL will put uniqueness constraint on it so it cannot happen that two rows in table have same contents of that column - that's why you see constraint violation error.
You can solve this problem by adding another ID column that will have unique value for each row (e.g. autoincremented sequence) and making it primary key instead of userID.
Here is a detailed tutorial on primary key in Postgres to give you a better understanding of primary key usage.
I built a db with serial type for the pks, I migrated to another server and the pk columns are now integer and as a result I cannot add new data due to the not null restriction of a pk. Is there any Alter command which can fix this?
SERIAL is not a data type in postgresql just a convenience word when creating tables that makes the column an integer type and adds auto-incrementing. All you have to do is add back auto-incrementing (a sequence) to the column and make sure its next value is greater than anything in the table.
This question covers adding serial to an existing column
This answer explains how to reset the counter
I made a mistake by the creation of my table. The primary key was incorrect. I delete the constraint and now I don't have a primary key in my table, only the field with the data. Now I want to set again this field as auto_increment primary key without losing my data. How I can do this?
I tryed this:
ALTER TABLE name_table ADD COLUMN name_column serial primary key;
But with this I am losing my data and creating a new column, that I don't want
try this
ALTER TABLE table_name ADD CONSTRAINT some_name primary key (name_column);
For my suggestion,
backup your database first in sql or csv or xml or excel something
restore-able.
Then alter your table structure, column data type, from UI or command
Then if data recorded on your table are lost or gone, restore your
backup data only, (not the structure of table)
After that you have changed column data type and also get your required data. I hope it will work.
Hi guys I was trying several ways and I found this one and maybe also somebody later can use:
Create a sequenz: Sequenz is the way that Postgresql implement to generate auto_increment fields. Ones we have a auto_increment is also a primary key. Should not be like this, is not a rule, but in most of the cases a auto_increment field is a primary key.
To create a sequenz is like this:
CREATE SEQUENCE exemplo_id_seq
INCREMENT 1 --the increment upgrate will be made 1 + 1
MINVALUE 1
MAXVALUE
START 1 --the start counting is in 1
CACHE 1;
After this is only to give this sequenz to the affected field using NEXTVAL, like this:
ALTER TABLE table_name ALTER COLUMN id SET DEFAULT NEXTVAL("exemplo_id_seq"::regclass);
Is working good without losing the data from old errors
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 coming from a sequence so everything seems to be all right but it still sometimes shows this error. I don't know why. In the documentation, it says the sequence is foolproof and always works. If I add a UNIQUE constraint to that column will it help? 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? Let's 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 enough or is there some other thing that should be done?
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.
Intro
I also encountered this problem and the solution proposed by #adamo was basically the right solution. However, I had to invest a lot of time in the details, which is why I am now writing a new answer in order to save this time for others.
Case
My case was as follows: There was a table that was filled with data using an app. Now a new entry had to be inserted manually via SQL. After that the sequence was out of sync and no more records could be inserted via the app.
Solution
As mentioned in the answer from #adamo, the sequence must be synchronized manually. For this purpose the name of the sequence is needed. For Postgres, the name of the sequence can be determined with the command PG_GET_SERIAL_SEQUENCE. Most examples use lower case table names. In my case the tables were created by an ORM middleware (like Hibernate or Entity Framework Core etc.) and their names all started with a capital letter.
In an e-mail from 2004 (link) I got the right hint.
(Let's assume for all examples, that Foo is the table's name and Foo_id the related column.)
Command to get the sequence name:
SELECT PG_GET_SERIAL_SEQUENCE('"Foo"', 'Foo_id');
So, the table name must be in double quotes, surrounded by single quotes.
1. Validate, that the sequence is out-of-sync
SELECT CURRVAL(PG_GET_SERIAL_SEQUENCE('"Foo"', 'Foo_id')) AS "Current Value", MAX("Foo_id") AS "Max Value" FROM "Foo";
When the Current Value is less than Max Value, your sequence is out-of-sync.
2. Correction
SELECT SETVAL((SELECT PG_GET_SERIAL_SEQUENCE('"Foo"', 'Foo_id')), (SELECT (MAX("Foo_id") + 1) FROM "Foo"), FALSE);
Replace the table_name to your actual name of the table.
Gives the current last id for the table. Note it that for next step.
SELECT MAX(id) FROM table_name;
Get the next id sequence according to postgresql. Make sure this id is higher than the current max id we get from step 1
SELECT nextVal('"table_name_id_seq"');
if it's not higher than then use this step 3 to update the next sequence.
SELECT setval('"table_name_id_seq"', (SELECT MAX(id) FROM table_name)+1);
The primary key is already protecting you from inserting duplicate values, as you're experiencing when you get that error. Adding another unique constraint isn't necessary to do that.
The "duplicate key" error is telling you that the work was not done because it would produce a duplicate key, not that it discovered a duplicate key already commited to the table.
For future searchs, use ON CONFLICT DO NOTHING.
Referrence - https://www.calazan.com/how-to-reset-the-primary-key-sequence-in-postgresql-with-django/
I had the same problem try this:
python manage.py sqlsequencereset table_name
Eg:
python manage.py sqlsequencereset auth
you need to run this in production settings(if you have)
and you need Postgres installed to run this on the server
From http://www.postgresql.org/docs/current/interactive/datatype.html
Note: Prior to PostgreSQL 7.3, serial implied UNIQUE. This is no longer automatic. If you wish a serial column to be in a unique constraint or a primary key, it must now be specified, same as with any other data type.
In my case carate table script is:
CREATE TABLE public."Survey_symptom_binds"
(
id integer NOT NULL DEFAULT nextval('"Survey_symptom_binds_id_seq"'::regclass),
survey_id integer,
"order" smallint,
symptom_id integer,
CONSTRAINT "Survey_symptom_binds_pkey" PRIMARY KEY (id)
)
SO:
SELECT nextval('"Survey_symptom_binds_id_seq"'::regclass),
MAX(id)
FROM public."Survey_symptom_binds";
SELECT nextval('"Survey_symptom_binds_id_seq"'::regclass) less than MAX(id) !!!
Try to fix the proble:
SELECT setval('"Survey_symptom_binds_id_seq"', (SELECT MAX(id) FROM public."Survey_symptom_binds")+1);
Good Luck every one!
I had the same problem. It was because of the type of my relations. I had a table property which related to both states and cities. So, at first I had a relation from property to states as OneToOne, and the same for cities. And I had the same error "duplicate key violates unique constraint". That means that: I can only have one property related to one state and city. But that doesnt make sense, because a city can have multiple properties. So the problem is the relation. The relation should be ManyToOne. Many properties to One city
Table name started with a capital letter if tables were created by an ORM middleware (like Hibernate or Entity Framework Core etc.)
SELECT setval('"Table_name_Id_seq"', (SELECT MAX("Id") FROM "Table_name") + 1)
WHERE
NOT EXISTS (
SELECT *
FROM (SELECT CURRVAL(PG_GET_SERIAL_SEQUENCE('"Table_name"', 'Id')) AS seq, MAX("Id") AS max_id
FROM "Table_name") AS seq_table
WHERE seq > max_id
)
try that CLI
it's just a suggestion to enhance the adamo code (thanks a lot adamo)
SELECT setval('tableName_columnName_seq', (SELECT MAX(columnName) FROM tableName));
For programatically solution at Django. Based on Paolo Melchiorre's answer, I wrote a chunk as a function to be called before any .save()
from django.db import connection
def setSqlCursor(db_table):
sql = """SELECT pg_catalog.setval(pg_get_serial_sequence('"""+db_table+"""', 'id'), MAX(id)) FROM """+db_table+""";"""
with connection.cursor() as cursor:
cursor.execute(sql)
I have similar problem but I solved it by removing all the foreign key in my Postgresql