[ADMIN] Circular Dependency in Tables and Deletion of Data
Hello, Note: I know that its not a very good db design. But what if a senario comes in front of you? I have created an uncommon database design. Which is as follows: Created 2 Tables :- Emp1 and Emp2 TableA with 2 Attributes A1 and A2. A1 is the primary key. TableB with 2 Attributes B1 and B22. B1 is primary key B2 is refereing to Emp1.Pr1 Now I have altered table for addeing one more foreign key constraint. The constraint is on TableA.A2 refering to TableB.B1 This creates the circular dependency. Now I have add one record in each table then Will it allow me to delete without schema change??If yes then how? I can not delete the tables also?? Is this a useless functionality given in DB or where is this used? Can postgres detect it automatically and restrict it to do so? Is any other DB restricts this kind of functionality or not? Sample code is given below for trial purpose create table TableA ( A1 integer primary key, A2 integer ); create table TableB ( B1 integer primary key, B2 integer references TableA(A1) ); ALTER TABLE TableA ADD CONSTRAINT distfk FOREIGN KEY (A2) REFERENCES TableB(B1) MATCH FULL; Insert INTO TableA values (1,null); Insert INTO TableB values (100,1); Update TableA SET A2 = 100 WHERE A1 = 1; Delete FROM TableA; Isnt it a crazy thing?? Regards, Mans ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [ADMIN] Circular Dependency in Tables and Deletion of Data
Am Montag, 24. Juli 2006 14:53 schrieb Mans: Isnt it a crazy thing?? Yes, but with deferrable constraints and transactions blocks, it should be no problem to manage the data. -- Peter Eisentraut http://developer.postgresql.org/~petere/ ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [ADMIN] Circular Dependency in Tables and Deletion of Data
Hi Peter, Can you please give some detailed information about it? Regards, Mans Peter Eisentraut wrote: Am Montag, 24. Juli 2006 14:53 schrieb Mans: Isnt it a crazy thing?? Yes, but with deferrable constraints and transactions blocks, it should be no problem to manage the data. -- Peter Eisentraut http://developer.postgresql.org/~petere/ ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [ADMIN] Circular Dependency in Tables and Deletion of Data
On Mon, 24 Jul 2006, Mans wrote: Hello, Note: I know that its not a very good db design. But what if a senario comes in front of you? I have created an uncommon database design. Which is as follows: Created 2 Tables :- Emp1 and Emp2 TableA with 2 Attributes A1 and A2. A1 is the primary key. TableB with 2 Attributes B1 and B22. B1 is primary key B2 is refereing to Emp1.Pr1 Now I have altered table for addeing one more foreign key constraint. The constraint is on TableA.A2 refering to TableB.B1 This creates the circular dependency. Now I have add one record in each table then Will it allow me to delete without schema change??If yes then how? I can not delete the tables also?? Is this a useless functionality given in DB or where is this used? Can postgres detect it automatically and restrict it to do so? Is any other DB restricts this kind of functionality or not? Sample code is given below for trial purpose create table TableA ( A1 integer primary key, A2 integer ); create table TableB ( B1 integer primary key, B2 integer references TableA(A1) ); ALTER TABLE TableA ADD CONSTRAINT distfk FOREIGN KEY (A2) REFERENCES TableB(B1) MATCH FULL; You need to have one or both of the constraints marked at least as deferrable. If you make the constraint initially deferred, then it should automatically wait until transaction end to check that constraint (thus you could use a second statement in the same transaction to remove the related rows). If you leave it as initially immediate, then you can use SET CONSTRAINTS to make it temporarily deferred. Also, the schema you gave should not prevent you from removing rows whose referencing column is NULL, which is potentially useful for some people. ---(end of broadcast)--- TIP 6: explain analyze is your friend
