Delete rows with foreign key in PostgreSQL - sql

Question

I would like to delete rows which contain a foreign key, but when I try something like this:
DELETE FROM osoby WHERE id_osoby='1'
I get this statement:
ERROR: update or delete on table "osoby" violates foreign key constraint "kontakty_ibfk_1" on table "kontakty"
DETAIL: Key (id_osoby)=(1) is still referenced from table "kontakty".
How can I delete these rows?

Answer 1

To automate this, you could define the foreign key constraint with ON DELETE CASCADE.
I quote the the manual for foreign key constraints:
CASCADE specifies that when a referenced row is deleted, row(s)
referencing it should be automatically deleted as well.
Look up the current FK definition like this:
SELECT pg_get_constraintdef(oid) AS constraint_def
FROM pg_constraint
WHERE conrelid = 'public.kontakty'::regclass -- assuming public schema
AND conname = 'kontakty_ibfk_1';
Then add or modify the ON DELETE ... part to ON DELETE CASCADE (preserving everything else as is) in a statement like:
ALTER TABLE kontakty
DROP CONSTRAINT kontakty_ibfk_1
, ADD CONSTRAINT kontakty_ibfk_1
FOREIGN KEY (id_osoby) REFERENCES osoby (id_osoby) ON DELETE CASCADE;
There is no ALTER CONSTRAINT command. Drop and recreate the constraint in a single ALTER TABLE statement to avoid possible race conditions with concurrent write access.
You need the privileges to do so, obviously. The operation takes an ACCESS EXCLUSIVE lock on table kontakty and a SHARE ROW EXCLUSIVE lock on table osoby.
If you can't ALTER the table, then deleting by hand (once) or by trigger BEFORE DELETE (every time) are the remaining options.

Answer 2

One should not recommend this as a general solution, but for one-off deletion of rows in a database that is not in production or in active use, you may be able to temporarily disable triggers on the tables in question.
In my case, I'm in development mode and have a couple of tables that reference one another via foreign keys. Thus, deleting their contents isn't quite as simple as removing all of the rows from one table before the other. So, for me, it worked fine to delete their contents as follows:
ALTER TABLE table1 DISABLE TRIGGER ALL;
ALTER TABLE table2 DISABLE TRIGGER ALL;
DELETE FROM table1;
DELETE FROM table2;
ALTER TABLE table1 ENABLE TRIGGER ALL;
ALTER TABLE table2 ENABLE TRIGGER ALL;
You should be able to add WHERE clauses as desired, of course with care to avoid undermining the integrity of the database.
There's some good, related discussion at http://www.openscope.net/2012/08/23/subverting-foreign-key-constraints-in-postgres-or-mysql/

Answer 3

You can't delete a foreign key if it still references another table.
First delete the reference
delete from kontakty
where id_osoby = 1;
DELETE FROM osoby
WHERE id_osoby = 1;

Answer 4

It's been a while since this question was asked, hope can help.
Because you can not change or alter the db structure, you can do this. according the postgresql docs.
TRUNCATE -- empty a table or set of tables.
TRUNCATE [ TABLE ] [ ONLY ] name [ * ] [, ... ]
[ RESTART IDENTITY | CONTINUE IDENTITY ] [ CASCADE | RESTRICT ]
Description
TRUNCATE quickly removes all rows from a set of tables. It has the same effect as an unqualified DELETE on each table, but since it does not actually scan the tables it is faster. Furthermore, it reclaims disk space immediately, rather than requiring a subsequent VACUUM operation. This is most useful on large tables.
Truncate the table othertable, and cascade to any tables that reference othertable via foreign-key constraints:
TRUNCATE othertable CASCADE;
The same, and also reset any associated sequence generators:
TRUNCATE bigtable, fattable RESTART IDENTITY;
Truncate and reset any associated sequence generators:
TRUNCATE revinfo RESTART IDENTITY CASCADE ;

Answer 5

It means that in table kontakty you have a row referencing the row in osoby you want to delete. You have do delete that row first or set a cascade delete on the relation between tables.
Powodzenia!

Answer 6

One can achieve this by issueing an extra SQL script that deletes the records related via the FK.
For this, using subselect in WHERE clause of a DELETE command can simply do what is needed.
Something similar to:
DELETE FROM kontakty
WHERE fk_column_from_kontakty_matching_id_osoby IN (
SELECT id_osoby FROM osoby WHERE id_osoby = '1'
);
DELETE FROM osoby WHERE id_osoby = '1';
the example assumes the FK column from kontakty matching osoby.id_osoby is called fk_column_from_kontakty_matching_id_osoby as it cannot be interpolated from the error message provided
For the case of OP's the query could by simplified a lot, but I have decided to leave it like this for it demonstrates accomplishing more complex scenarios
This approach is quite useful for SQL migrations, where one cannot depend on an assigned id and where ids cannot be easily fetched first and acted upon later.