Re: [HACKERS] [PATCHES] Work-in-progress referential action trigger
Added to TODO: o Fix problem when cascading referential triggers make changes on cascaded tables, seeing the tables in an intermediate state http://archives.postgresql.org/pgsql-hackers/2005-09/msg00174.php http://archives.postgresql.org/pgsql-hackers/2005-09/msg00174.php --- Stephan Szabo wrote: [Hackers now seems more appropriate] On Thu, 1 Sep 2005, Stephan Szabo wrote: On Tue, 23 Aug 2005, Stephan Szabo wrote: Here's my current work in progress for 8.1 devel related to fixing the timing issues with referential actions having their checks run on intermediate states. I've only put in a simple test that failed against 8.0 in the regression patch and regression still passes for me. There's still an outstanding question of whether looping gives the correct result in the presence of explicit inserts and set constraints immediate in before triggers. As Darcy noticed, the patch as given does definately still have problems with before triggers. I was able to construct a case that violates the constraint with an update in a before delete trigger. I think this might be why the spec has the wierd timing rules for before triggers on cascaded deletes such that the deletions happen before the before triggers. We have a similar problem for before triggers that update the rows that are being cascade updated. The following seems to violate the constraint for me on 8.0.3: drop table pk cascade; drop table fk cascade; drop function fk_move(); create table pk(a int primary key); create table fk(a int references pk on delete cascade on update cascade, b int); create function fk_move() returns trigger as ' begin raise notice '' about to move for % '', old.b; update fk set b=b-1 where b old.b; return new; end;' language 'plpgsql'; create trigger fkmovetrig before update on fk for each row execute procedure fk_move(); insert into pk values(1); insert into pk values(2); insert into fk values(1,1); insert into fk values(1,2); insert into fk values(2,3); select * from pk; select * from fk; update pk set a = 3 where a = 1; select * from pk; select * from fk; This gives me (3,1), (1,1) and (2,2) as the rows in fk where the (1,1) row is invalid. This is obviously wrong, but the question is, what is the correct answer? Should the update in the before trigger trying to change b on a row that no longer has a reference have errored? Well, the spec seems to get out of this simply. I read SQL2003's trigger execution information (specifically 14.27 GR5g*) to say that before triggers that call data changing statements are invalid. We can't do that for compatibility reasons, but it would allow us to say that modifying a row in a before trigger that is also a row selected in the outer statement is an error for this update case. It'd presumably be an error for a normal delete as well, although I think it might be relaxable for cascaded deletes because the spec seems to say that the before triggers for deletions caused by the cascade are actually run after the removals. I'm not sure whether we could easily differentiate this case from any other cases where the row was modified twice either yet. --- * If TR is a BEFORE trigger and if, before the completion of the execution of an SQL procedure statement simply contained in TSS, an attempt is made to execute an SQL-data change statement or an SQL-invoked routine that possibly modifies SQL-data, then an exception condition is raised: prohibited statement encountered during trigger execution. ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org -- Bruce Momjian http://candle.pha.pa.us EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] [PATCHES] Work-in-progress referential action trigger
On Friday 09 September 2005 08:46, Stephan Szabo wrote: On Fri, 9 Sep 2005, Tom Lane wrote: Stephan Szabo [EMAIL PROTECTED] writes: Is there a case other than a before trigger updating a row we will want to act upon later in the statement where we'll get a row with xmax of our transaction and cmax greater than the current command? The greater-cmax case could occur via any kind of function, not only a trigger, ie update tab set x = foo(x) where ... where foo() is a volatile function that internally updates the tab table. I *thought* I was missing a case, I just couldn't figure out what. I suppose you could say that this is horrible programming practice and anyone who tries it deserves whatever weird behavior ensues ... but it's not the case that every such situation involves a trigger. Well, the change I was thinking of would have made it an error if foo(x) updated a row that was then later selected by the update rather than the current behavior which I think would have ignored the already updated row, so that's probably not going to work. I see that this still is not addressed fulling in beta 3. Can anybody give a quick overview of where this is sitting, and if it's likely to make it's way into 8.1 gold ? ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq -- Darcy Buskermolen Wavefire Technologies Corp. http://www.wavefire.com ph: 250.717.0200 fx: 250.763.1759 ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] [PATCHES] Work-in-progress referential action trigger
On Fri, 2 Sep 2005, Stephan Szabo wrote: [Hackers now seems more appropriate] On Thu, 1 Sep 2005, Stephan Szabo wrote: On Tue, 23 Aug 2005, Stephan Szabo wrote: Here's my current work in progress for 8.1 devel related to fixing the timing issues with referential actions having their checks run on intermediate states. I've only put in a simple test that failed against 8.0 in the regression patch and regression still passes for me. There's still an outstanding question of whether looping gives the correct result in the presence of explicit inserts and set constraints immediate in before triggers. As Darcy noticed, the patch as given does definately still have problems with before triggers. I was able to construct a case that violates the constraint with an update in a before delete trigger. I think this might be why the spec has the wierd timing rules for before triggers on cascaded deletes such that the deletions happen before the before triggers. We have a similar problem for before triggers that update the rows that are being cascade updated. The following seems to violate the constraint for me on 8.0.3: drop table pk cascade; drop table fk cascade; drop function fk_move(); create table pk(a int primary key); create table fk(a int references pk on delete cascade on update cascade, b int); create function fk_move() returns trigger as ' begin raise notice '' about to move for % '', old.b; update fk set b=b-1 where b old.b; return new; end;' language 'plpgsql'; create trigger fkmovetrig before update on fk for each row execute procedure fk_move(); insert into pk values(1); insert into pk values(2); insert into fk values(1,1); insert into fk values(1,2); insert into fk values(2,3); select * from pk; select * from fk; update pk set a = 3 where a = 1; select * from pk; select * from fk; This gives me (3,1), (1,1) and (2,2) as the rows in fk where the (1,1) row is invalid. This is obviously wrong, but the question is, what is the correct answer? Should the update in the before trigger trying to change b on a row that no longer has a reference have errored? We can't do that for compatibility reasons, but it would allow us to say that modifying a row in a before trigger that is also a row selected in the outer statement is an error for this update case. It'd presumably be an error for a normal delete as well, although I think it might be relaxable for cascaded deletes because the spec seems to say that the before triggers for deletions caused by the cascade are actually run after the removals. I'm not sure whether we could easily differentiate this case from any other cases where the row was modified twice either yet. Is there a case other than a before trigger updating a row we will want to act upon later in the statement where we'll get a row with xmax of our transaction and cmax greater than the current command? ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] [PATCHES] Work-in-progress referential action trigger timing
Stephan Szabo [EMAIL PROTECTED] writes: Is there a case other than a before trigger updating a row we will want to act upon later in the statement where we'll get a row with xmax of our transaction and cmax greater than the current command? The greater-cmax case could occur via any kind of function, not only a trigger, ie update tab set x = foo(x) where ... where foo() is a volatile function that internally updates the tab table. I suppose you could say that this is horrible programming practice and anyone who tries it deserves whatever weird behavior ensues ... but it's not the case that every such situation involves a trigger. regards, tom lane ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] [PATCHES] Work-in-progress referential action trigger
On Fri, 9 Sep 2005, Tom Lane wrote: Stephan Szabo [EMAIL PROTECTED] writes: Is there a case other than a before trigger updating a row we will want to act upon later in the statement where we'll get a row with xmax of our transaction and cmax greater than the current command? The greater-cmax case could occur via any kind of function, not only a trigger, ie update tab set x = foo(x) where ... where foo() is a volatile function that internally updates the tab table. I *thought* I was missing a case, I just couldn't figure out what. I suppose you could say that this is horrible programming practice and anyone who tries it deserves whatever weird behavior ensues ... but it's not the case that every such situation involves a trigger. Well, the change I was thinking of would have made it an error if foo(x) updated a row that was then later selected by the update rather than the current behavior which I think would have ignored the already updated row, so that's probably not going to work. ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
