On 05/16/2014 08:06 AM, Blagoj Petrushev wrote: > Hi, > > I'm thinking of an extension to trigger functionality like this: > > CREATE TRIGGER trigger_name > AFTER event > ON table > CONCURRENTLY EXECUTE PROCEDURE trigger_fc > > This would call the trigger after the end of the transaction.
If "after the end of the transaction" is what you mean by "concurrently", then that's the wrong word to choose. "AFTER COMMIT" ? The concept of running a trigger "concurrently" just doesn't make sense in PostgreSQL, because the backend is single threaded. You wouldn't be able to run any SQL commands until the trigger finished. It isn't possible to do anything useful without a transaction, so PostgreSQL would need to start a transaction for the trigger and commit the transaction at the end, as if you'd run SELECT my_procedure();. Because it's outside the scope of the transaction it probably wouldn't be possible to do FOR EACH ROW with a NEW and OLD var, unless you stashed them as materialized rows in the queue of pending "AFTER COMMIT" triggers. Finally, because it's after transaction commit, you couldn't easily guarantee that the trigger would really run. If the backend crashed / the server was shut down / etc after the commit but before your trigger finished, you'd have a committed transaction but the trigger would not run. To fix that you'd need to somehow make the trigger queue WAL-logged and run it during replay, which from my rather limited understanding of this area would be ... "interesting" to do. It'd also mean the trigger couldn't have any session context. This isn't easy, if it's practical at all. > I have a big table with big text column article and a nullable > tsvector column fts_article. On each insert or update that changes the > article, I trigger-issue 'NOTIFY article_changed row_id', then, with a > daemon listener, I catch the notification and update fts_article > accordingly with my_fts_fc(article). The reason I don't do this > directly in my trigger is because my_fts_fc is slow for big articles, > fts_article has a gin index, and also, on heavy load, my listener can > do these updates concurrently. Now, with a concurrent execution of > triggers, I can just call my_fts_fc inside the trigger instead of the > notify roundtrip. I don't think that really fits. It seems like you want to run the trigger procedure in the background on another back-end. That'd be quite cool, but also not trivial to do, especially if you wanted to guarantee that it happened reliably and in a crash-safe manner. -- Craig Ringer http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
