In database dump created with pg_dump, some tables have DEFAULTs in the CREATE TABLE statement, i.e.:
CREATE TABLE test (
f1 integer DEFAULT nextval('test_f1_seq'::regclass) NOT NULL
);
But others have an additional ALTER statement:
ALTER TABLE ONLY test2 ALTER COLUMN f1 SET DEFAULT nextval('test2_f1_seq'::regclass);
What is the reason of this? All sequential fields were created with type SERIAL, but in the dump they look different, and I can't guess any rule for this.
The difference must be that in the first case, the sequence is “owned” by the table column.
You can specify this dependency using the OWNED BY clause when you create a sequence. A sequence that is owned by a column will automatically be dropped when the column is.
If a sequence is implicitly created by using serial, it will be owned by the column.
Related
Say we have a schema named : "test", a table in it as below:
CREATE TABLE IF NOT EXISTS test.details
(
id integer NOT NULL DEFAULT nextval('test.details_id_seq'::regclass),
username character varying(50) COLLATE pg_catalog."default" NOT NULL
)
as we could see id column is a Sequence which is already been created in this schema.
now if we create a new schema named "check" and altered details table schema as
create schema check
alter table test.details set schema check
will the sequence be auto created in check schema?
The sequence is an independent database object. If you change the schema of the table, the sequence will remain in the old schema, and the table will continue to use it. You can use ALTER SEQUENCE to change the sequence's schema as well, and the default value will continue to work (PostgreSQL stores the parsed default expression rather than the text).
All that becomes easier if you use identity columns to generate keys: there, you don't have to take care of the sequence. It is created automatically and changes schema with the table.
I have some tables in PostgreSQL 12.9 that were declared as something like
-- This table is written in old style
create table old_style_table_1 (
id bigserial not null primary key,
...
);
-- This table uses new feature
create table new_style_table_2 (
id bigint generated by default as identity,
...
);
Second table seems to be declared using the identity flag introduced in 10th version.
Time went by, and we have partitioned the old tables, while keeping the original sequences:
CREATE TABLE partitioned_old_style_table_1 (LIKE old_style_table_1 INCLUDING DEFAULTS) PARTITION BY HASH (user_id);
CREATE TABLE partitioned_new_style_table_2 (LIKE new_style_table_2 INCLUDING DEFAULTS) PARTITION BY HASH (user_id);
DDL for their id columns seems to be id bigint default nextval('old_style_table_1_id_seq') not null and id bigint default nextval('new_style_table_2_id_seq') not null.
Everything has worked fine so far. Partitioned tables proved to be a great boon and we decided to retire the old tables by dropping them.
DROP TABLE old_style_table_1, new_style_table_2;
-- [2BP01] ERROR: cannot drop desired object(s) because other objects depend on them
-- Detail: default value for column id of table old_style_table_1 depends on sequence old_style_table_1_id_seq
-- default value for column id of table new_style_table_2 depends on sequence new_style_table_2_id_seq
After some pondering I've found out that sequences may have owners in postgres, so I opted to change them:
ALTER SEQUENCE old_style_table_1_id_seq OWNED BY partitioned_old_style_table_1.id;
DROP TABLE old_style_table_1;
-- Worked out flawlessly
ALTER SEQUENCE new_style_table_2_id_seq OWNED BY partitioned_new_style_table_2.id;
ALTER SEQUENCE new_style_table_2_id_seq OWNED BY NONE;
-- Here's the culprit of the question:
-- [0A000] ERROR: cannot change ownership of identity sequence
So, apparently the fact that this column has pg_attribute.attidentity set to 'd' forbids me from:
• changing the default value of the column:
ALTER TABLE new_style_table_2 ALTER COLUMN id SET DEFAULT 0;
-- [42601] ERROR: column "id" of relation "new_style_table_2" is an identity column
• dropping the default value:
ALTER TABLE new_style_table_2 ALTER COLUMN id DROP DEFAULT;
-- [42601] ERROR: column "id" of relation "new_style_table_2" is an identity column
-- Hint: Use ALTER TABLE ... ALTER COLUMN ... DROP IDENTITY instead.
• dropping the identity, column or the table altogether (new tables already depend on the sequence):
ALTER TABLE new_style_table_2 ALTER COLUMN id DROP IDENTITY IF EXISTS;
-- or
ALTER TABLE new_style_table_2 DROP COLUMN id;
-- or
DROP TABLE new_style_table_2;
-- result in
-- [2BP01] ERROR: cannot drop desired object(s) because other objects depend on them
-- default value for column id of table partitioned_new_style_table_2 depends on sequence new_style_table_2_id_seq
I've looked up the documentation, it provides the way to SET IDENTITY or ADD IDENTITY, but no way to remove it or to change to a throwaway sequence without attempting to drop the existing one.
➥ So, how am I able to remove an identity flag from the column-sequence pair so it won't affect other tables that use this sequence?
UPD: Tried running UPDATE pg_attribute SET attidentity='' WHERE attrelid=16816; on localhost, still receive [2BP01] and [0A000]. :/
Though I managed to execute the DROP DEFAULT value bit, but it seems like a dead end.
I don't think there is a safe and supported way to do that (without catalog modifications). Fortunately, there is nothing special about sequences that would make dropping them a problem. So take a short down time and:
remove the default value that uses the identity sequence
record the current value of the sequence
drop the table
create a new sequence with an appropriate START value
use the new sequence to set new default values
If you want an identity column, you should define it on the partitioned table, not on one of the partitions.
I am trying to add an identity column in a table through alter query using Ingres DB. While creating the table, i am able to define the identity column but not when i am trying to add it through alter query. Kindly Suggest me an alter query for it.
It's not as straightforward as you might think, "alter table" has a a number of restrictions which make this a multi-step operation. Try this:
create table something(a integer, b varchar(20)) with page_size=8192;
alter table something add column c integer not null with default;
modify something to reconstruct;
alter table something alter column c integer not null generated always as identity;
modify something to reconstruct;
I have a table with existing data. Is there a way to add a primary key without deleting and re-creating the table?
(Updated - Thanks to the people who commented)
Modern Versions of PostgreSQL
Suppose you have a table named test1, to which you want to add an auto-incrementing, primary-key id (surrogate) column. The following command should be sufficient in recent versions of PostgreSQL:
ALTER TABLE test1 ADD COLUMN id SERIAL PRIMARY KEY;
Older Versions of PostgreSQL
In old versions of PostgreSQL (prior to 8.x?) you had to do all the dirty work. The following sequence of commands should do the trick:
ALTER TABLE test1 ADD COLUMN id INTEGER;
CREATE SEQUENCE test_id_seq OWNED BY test1.id;
ALTER TABLE test1 ALTER COLUMN id SET DEFAULT nextval('test_id_seq');
UPDATE test1 SET id = nextval('test_id_seq');
Again, in recent versions of Postgres this is roughly equivalent to the single command above.
ALTER TABLE test1 ADD COLUMN id SERIAL PRIMARY KEY;
This is all you need to:
Add the id column
Populate it with a sequence from 1 to count(*).
Set it as primary key / not null.
Credit is given to #resnyanskiy who gave this answer in a comment.
To use an identity column in v10,
ALTER TABLE test
ADD COLUMN id { int | bigint | smallint}
GENERATED { BY DEFAULT | ALWAYS } AS IDENTITY PRIMARY KEY;
For an explanation of identity columns, see https://blog.2ndquadrant.com/postgresql-10-identity-columns/.
For the difference between GENERATED BY DEFAULT and GENERATED ALWAYS, see https://www.cybertec-postgresql.com/en/sequences-gains-and-pitfalls/.
For altering the sequence, see https://popsql.io/learn-sql/postgresql/how-to-alter-sequence-in-postgresql/.
I landed here because I was looking for something like that too. In my case, I was copying the data from a set of staging tables with many columns into one table while also assigning row ids to the target table. Here is a variant of the above approaches that I used.
I added the serial column at the end of my target table. That way I don't have to have a placeholder for it in the Insert statement. Then a simple select * into the target table auto populated this column. Here are the two SQL statements that I used on PostgreSQL 9.6.4.
ALTER TABLE target ADD COLUMN some_column SERIAL;
INSERT INTO target SELECT * from source;
ALTER TABLE test1 ADD id int8 NOT NULL GENERATED ALWAYS AS IDENTITY;
I have a table linelevelpmts with a column seq (Int4) which is to be used as a sequence.
I know I can delete the column and recreate it as type serial, but can I modify the existing column to be used as a sequence.
ALTER TABLE "public"."linelevelpmts" ALTER COLUMN "seq" SET DEFAULT nextval('linelevelpmts_seq_seq'::regclass);
This code generates an error: Relation linelevelpmts_seq_seq does not exist.
This code generates an error: Relation linelevelpmts_seq_seq does not exist.
Well you need to first create the sequence you want to use for the default value:
create sequence linelevelpmts_seq_seq;
ALTER TABLE public.linelevelpmts
ALTER COLUMN seq SET DEFAULT nextval('linelevelpmts_seq_seq'::regclass);
If you want the same effect as if it was created as serial you also need to change the "owner" of the sequence:
alter sequence linelevelpmts_seq_seq owned by linelevelpmts.seq;
Edit
Igor's comment is a good one: if you already have values in the column seq you should adjust the starting value of the sequence:
select setval('linelevelpmts_seq_seq', (select max(seq) from linelevelpmts));