Foreign key referential actions
От | Stephan Szabo |
---|---|
Тема | Foreign key referential actions |
Дата | |
Msg-id | 20011112182220.B76772-100000@megazone23.bigpanda.com обсуждение исходный текст |
Ответы |
Re: Foreign key referential actions
(Tom Lane <tgl@sss.pgh.pa.us>)
Re: Foreign key referential actions (Bruce Momjian <pgman@candle.pha.pa.us>) |
Список | pgsql-hackers |
Right now, referential actions get deferred along with normal checks and run against the state of the database at that time. I think this violates SQL92 11.8 General Rules 4-6 and have some reasoning and proposed ideas towards making it more complient although I don't actually have an implementation in mind for the most correct version. :( Here are my interpretations: GR 4 says that the matching rows (unique and non-unique) are determined immediately before the execution of an SQL statement. We can ignore the fluff about non-unique matching rows for now because I believe that applies to match partial only.GR 5 says when there's a delete rule and a row of the referenced table is marked for deletion (if it's not already marked such) then do something based on the action, for example mark matching rows for deletion if it is cascade. This seems to imply the action is supposed to occur immediately, since AFAICS the rows aren't marked for deletion on the commit but rather on the delete itself.GR 6 seems to be pretty much the same for update. I think the correct course of action would be if I'm right: *Make referential actions (other than no action) not deferrable and thus initially immediate. This means that you see thecascaded (or nulled or defaulted) results immediately, but I think that satisfies GRs 5 and 6. It also makes the problemsof what we can see a little less problematic, but doesn't quite cure them. *To fix the visibility issues I think we'd need to be able to see what rows matched immediately before the statement andthen reference those rows later, even if the values that we're keying on have changed. I'm really not sure how we'd dothis without a great deal of extra work. An intermediate step towards complience would probably be making sure the rowexisted before this statement (I think for the fk constraints this means if it was created by another statement or acommand before this one) which is wrong if a row that matched before this statement was modified by this statement to anew value that we won't match. Most of these cases would be errors by sql anyway (I think these'd probably be real triggereddata change violations) and would be wrong by our current implementation as well. I'm not sure that the intermediate step on the second is actually worthwhile over just waiting and trying to do it right, but if I'm right in what it takes, it's reasonably minimal.
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