On Fri, 13 Feb 2004, Tom Lane wrote: > Stephan Szabo <[EMAIL PROTECTED]> writes: > > One thing is that IIRC we're going to ask for only one row when we do the > > SPI_execp_current. However, unless I misremember, the behavior of for > > update and limit means that saying limit 1 is potentially unsafe (if you > > block on a row that goes away). Is there anyway for us to let the planner > > know this? > > I was looking at that last night. It seems like we could add a LIMIT at > least in some contexts. In the case at hand, we're just going to error > out immediately if we find a matching row, and so there's no need for > FOR UPDATE, is there?
I think there still is, because a not yet committed transaction could have deleted them all in which case I think the correct behavior is to wait and if that transaction commits allow the action and if it rolls back to error. Really we'd want a different behavior where we're only blocking in these cases if all the matching rows are locked by other transactions. > However, I'm not sure it would help the OP anyway. With the stats he > had, the planner would still take a seqscan, because it's going to > expect that it can find a match by probing the first ten or so rows of > the first page. With anything close to the normal cost parameters, > that's going to look more expensive than an index probe. Possibly if > the table had a few more values it would work. Hmm, that's true. It also doesn't help the real actions (cascade, set *) since those really do need to get at all the rows, but it probably helps in a reasonable number of cases. ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]