OK, this is the problem: I want to write a bit of SQL that if a row exists in a table, then update it, otherwise it will update it. Problem is, there is a very high chance of simultaneous execute of this query on the same row (the rows have a unique index).
So, strategy one: begin; update row; if (no rows affected) insert row; commit; Problem - race condition! If the two transactions run at the same time, the second will end up doing an insert on a unique row which will cause query failure Strategy two: begin; select row for update; if (row returned) update; else insert; commit; Problem - race condition. The row-level locking doesn't allow me to lock 'potential rows', so if the row does not yet exists and two transactions run simultaneously then the second with die with a unique violation; Strategy three: begin; lock table in exclusive mode; update row; if (no rows affected) insert row; commit; Problem - Works, but this table needs high concurrency. Every time a member hits a page of the site that needs authentication, this function is called. In particular, the login transaction can take a little time sometimes and we can't halt everyone else's activites for that duration... So what is the solution??? I'm not sure if acquiring a ROW EXCLUSIVE MODE lock will help at all. Also, I can't try the insert and then the update because the INSERT, in Postgres, will cause an outright transaction failure. What the heck is the solution?? Chris ---------------------------(end of broadcast)--------------------------- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
