How does one write a DELETE CASCADE for postgres?

问题:

I'm manually constructing a DELETE CASCADE statement for postgres.

I have a 'transaction' and a 'slice' table, related as shown below:

    Table "public.slice"   Column  | Type | Modifiers  ----------+------+-----------  id       | text | not null  name     | text |  Referenced by:     TABLE "transaction" CONSTRAINT "transaction_slice_id_fkey" FOREIGN KEY (slice_id) REFERENCES slice(id)  Table "public.transaction"   Column  | Type | Modifiers  ----------+------+-----------  id       | text | not null  slice_id | text |  Referenced by:     TABLE "classification_item" CONSTRAINT "classification_item_transaction_id_fkey" FOREIGN KEY (transaction_id) REFERENCES transaction(id) Table "public.classification_item"      Column     | Type | Modifiers  ----------------+------+-----------  id             | text | not null  transaction_id | text |  Foreign-key constraints:     "classification_item_transaction_id_fkey" FOREIGN KEY (transaction_id) REFERENCES transaction(id) 

Say I want to delete all transactions and classification_items referenced by the slice whose name is 'my_slice'. What do I need to write?

=# delete from classification_item where transaction_id= #...?  =# delete from transaction where slice_id= #...?  =# delete from slice where name='my_slice'; 

回答1:

In case you can't do what others have suggested:

begin; delete from classification_item where transaction_id in (select id from "transaction" where slice_id = (select id from slice where name = 'my_slice')); delete from "transaction" where slice_id in (select id from slice where name='my_slice'); delete from slice where name='my_slice'; commit; 

回答2:

Postgres foreign keys support the CASCADE deletes:

slice_id integer REFERENCES slice(id) ON DELETE CASCADE 

etc

回答3:

It's soemthing that is defined in the table rather than the DELETE Query. Example (look at order_id):

CREATE TABLE order_items (     product_no integer REFERENCES products ON DELETE RESTRICT,     order_id integer REFERENCES orders ON DELETE CASCADE,     quantity integer,     PRIMARY KEY (product_no, order_id) ); 

回答4:

You should use CASCADE deletes, and it should be possible to do so even if you inherited a database schema. You would just modify the constraints to add the CASCADE deletes to the schemas:

1. Drop and re-create the constraints to add CASCADE deletes:

   ALTER TABLE ONLY "transaction"      DROP CONSTRAINT transaction_slice_id_fkey; ALTER TABLE ONLY "transaction"      ADD CONSTRAINT transaction_slice_id_fkey      FOREIGN KEY (slice_id) REFERENCES slice(id) ON DELETE CASCADE;  ALTER TABLE ONLY "classification_item"      DROP CONSTRAINT classification_item_transaction_id_fkey; ALTER TABLE ONLY "classification_item"      ADD CONSTRAINT classification_item_transaction_id_fkey      FOREIGN KEY (transaction_id) REFERENCES transaction(id) ON DELETE CASCADE; 

2. Now the following query will delete not just the my_slice record from table slice, but also all records from tablestransaction and classification_item referencing it:

   DELETE FROM slice WHERE name='my_slice'; 

That procedure will work even if the original schema is created by an object-relational mapper like SQLAlchemy. However in such a case, take care to re-apply that "patch" whenever the schema changes or is re-created. Only if that can't be implemented automatically, it might not be a good idea after all …

回答5:

It can be delegated to DBMS by set a constraint property 'On delete' = CASCADE. Please see an example.

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