I created a table in PostgreSQL with a foreign key constraint.
I dropped the table to which the foreign key belongs. Now how to alter the table or how to defer the foreign key present in the table?
To clarify:
I have a table named test. It has a column called subjectName, which is a foreign key of subject Table. Now I dropped subject table. How to remove the FK constaint on table test
Assuming the following tables:
create table subject
(
name varchar(10) primary key
);
create table test
(
some_column integer,
subject_name varchar(10) not null references subject
);
there are two scenarios what could have happened when you dropped the table subject:
1. you didn't actually drop it:
drop table subject;
ERROR: cannot drop table subject because other objects depend on it
Detail: constraint test_subject_name_fkey on table test depends on table subject
Hint: Use DROP ... CASCADE to drop the dependent objects too.
2. you did drop it, then the foreign key is gone as well.
drop table subject cascade;
NOTICE: drop cascades to constraint test_subject_name_fkey on table test
which tells you that the foreign key constraint was automatically dropped.
Perhaps your question in not exactly what you mean. Are you wanting to remove the which was a foreign key from the table. As amply indicated if you dropped the parent table then the FK is also dropped. However the column itself is not dropped from the child table. To remove that you need to alter the table.
alter table test drop column subject_name;
See demo here
Related
This question already has an answer here:
ERROR: unique constraint on partitioned table must include all partitioning columns
(1 answer)
Closed last month.
I'm trying to create a partitioned table which refers to itself, creating a doubly-linked list.
CREATE TABLE test2 (
id serial NOT NULL,
category integer NOT NULL,
time timestamp(6) NOT NULL,
prev_event integer,
next_event integer
) PARTITION BY HASH (category);
Once I add primary key I get the following error.
alter table test2 add primary key (id);
ERROR: unique constraint on partitioned table must include all partitioning columns
DETAIL: PRIMARY KEY constraint on table "test2" lacks column "category" which is part of the partition key.
Why does the unique constrain require all partitioned columns to be included?
EDIT: Now I understand why this is needed: https://www.postgresql.org/docs/current/ddl-partitioning.html#DDL-PARTITIONING-DECLARATIVE-LIMITATIONS
Once I add PK with both columns it works.
alter table test2 add primary key (id, category);
But then adding the FK to itself doesn't work.
alter table test2 add foreign key (prev_event) references test2 (id) on update cascade on delete cascade;
ERROR: there is no unique constraint matching given keys for referenced table "test2"
Since PK is not just id but id-category I can't create FK pointing to id.
Is there any way to deal with this or am I missing something?
I would like to avoid using inheritance partitioning if possible.
EDIT2: It seems this is a known problem. https://www.reddit.com/r/PostgreSQL/comments/di5mbr/postgresql_12_foreign_keys_and_partitioned_tables/f3tsoop/
Seems that there is no straightforward solution. PostgreSQL simply doesn't support this as of v14. One solution is to use triggers to enforce 'foreign key' behavior. Other is to use multi-column foreign keys. Both are far from optimal.
I have a requirement of deleting records from the Postgres database tables.
We have a Customer table which is the main table, this table contains a primary key which is used in so many other tables as a FOREIGN KEY, I want to delete one of the customers as well as its reference used in other tables. Is there any way to delete the customer from main table as well as from other tables which contains foreign key.
Thanks in Advance.
In the other tables, you want a cascading delete foreign key reference. You can create one in the database using:
alter table othertable add constraint fk_othertable_customerid
foreign key (customerid) references customers(customerid)
on delete cascade;
Note: This assumes that customerid is the name of the column in both tables and that it is already defined in the other tables.
A cascading foreign key constraint does exactly what you specify. When a row is deleted in the reference table, then all related rows are deleted.
If you already have foreign key constraints on customerid, then drop the existing constraint and add the cascading version.
When trying to ALTER TABLE in Postgres 9.5 to create foreign key constraint: from product_template.product_brand_id to product_brand.id
ALTER TABLE public.product_template
ADD CONSTRAINT product_template_product_brand_id_fkey
FOREIGN KEY (product_brand_id)
REFERENCES public.product_brand (id) MATCH SIMPLE
ON UPDATE NO ACTION
ON DELETE SET NULL;
Returns error
ERROR: insert or update on table "product_template" violates foreign key constraint "product_template_product_brand_id_fkey"
DETAIL: Key (product_brand_id)=(12) is not present in table "product_brand".
STATEMENT: ALTER TABLE "product_template" ADD FOREIGN KEY ("product_brand_id") REFERENCES "product_brand" ON DELETE set null
Im confused why postgres is trying to find product_brand.product_brand_id, when the fkey is from product_template.product_brand_id to product_brand.id
Any ideas?
The error message simply states that there is at least one row in the table product_template that contains the value 12 in the column product_brand_id
But there is no corresponding row in the table product_brand where the column id contains the value 12
Key (product_brand_id)=(12) relates the source column of the foreign key, not the target column.
In simple terms, the value of FOREIGN KEY(product_brand_id) provided in your ALTER statement is not present in the source (product_brand) table.
If I create a new schema on the current database (management), why does it complain about cross-database references?
management=# create schema mgschema;
CREATE SCHEMA
management=# alter table clients add column task_id int null references mgschema.tasks.id;
ERROR: cross-database references are not implemented: "mgschema.tasks.id"
alter table clients add column task_id int null references mgschema.tasks.id;
The REFERENCES syntax in not correct, you should use:
REFERENCES reftable [ ( refcolumn ) ]
A simple references only expects a table name. The foreign key will then automatically point to the primary key of that table, e.g.
alter table clients add column task_id int null references mgschema.tasks;
Another alternative is to to specify the table and columns, but not with a single identifier:
alter table clients add column task_id int null references mgschema.tasks (id);
The second format is needed if the target table has multiple unique constraints.
How can I alter the reference to a table in PostgreSQL when the table name has been changed?
Say I have:
CREATE TABLE example1 (
id serial NOT NULL PRIMARY KEY,
name varchar(100)
);
CREATE TABLE example2 (
id serial NOT NULL PRIMARY KEY,
example1fk integer REFERENCES example1 (id) DEFERRABLE INITIALLY DEFERRED
);
Later I do:
ALTER TABLE example1 RENAME TO example3;
How to change the definition of the foreign key constraint?
example1fk integer REFERENCES example1 (id) DEFERRABLE INITIALLY DEFERRED,
Internal dependencies between tables and / or other objects are never bound to the object name. Internally, every object is stored in a catalog table and the OID (internal primary key) of the object is used for everything else.
Accordingly, a FOREIGN KEY reference is stored in the catalog tables pg_constraint (the constraint itself incl. its name) and pg_depend. Changing table names will not impair functionality at all.
The name of the constraint remains unchanged. You can ignore that, or you may want to rename the constraint so it's not misleading.
However, since you did not specify a constraint name at creation time, the system picked a default, which is example2_example1fk_fkey in your case unless the name was taken. No reference to the referenced table name. But the column name will likely have to change in your example, too. And that is used in the constraint name.
ALTER TABLE example2 RENAME example1fk TO example3fk; -- rename column
In Postgres 9.2 or later you can just rename the constraint as well (as dequis commented):
ALTER TABLE example2 RENAME CONSTRAINT example2_example1fk_fkey TO example2_example3fk_fkey;
In older versions, you have to drop and recreate the constraint to rename it, best in a single statement:
ALTER TABLE example2 -- rename constraint
DROP CONSTRAINT example2_example1fk_fkey
, ADD CONSTRAINT example2_example3fk_fkey FOREIGN KEY (example3fk)
REFERENCES example3 (id) DEFERRABLE INITIALLY DEFERRED;
Details in the manual.