--- Tom Lane <tgl@xxxxxxxxxxxxx> wrote: > "Jim C. Nasby" <jnasby@xxxxxxxxxxxxx> writes: > > I believe that's not necessarily true. If you > select a tuple and it's > > ctid and it's updated more than once with a vacuum > in-between I believe > > it could end up back in the same position, which > would mean the same > > ctid. > > This is the reason for the recommendation that you > don't trust a TID for > longer than one transaction. If you select a row > and see it has TID > such and such, and then later try to > fetch/update/delete that row by > TID, the worst that can happen is that you'll not > find the row because > some other xact has already updated or deleted it. > You can not find > a different row in the TID slot, because VACUUM will > not have removed > a row that is possibly still visible to your > transaction. (VACUUM > has no idea whether you're running under > SERIALIZABLE rules or not, > and so it takes the conservative approach that any > row you could ever > possibly have seen as good is still interesting.) > But this guarantee > only lasts as long as your current transaction. > > regards, tom lane > Just in case anyone following this thread gets a little confused, my response was somewhat tangential to the main discussion; I was talking of fetching the record by primary key or such, and then comparing the ctid values. Agreed that any other valid use of ctid is dubious. __________________________________ Yahoo! Mail for Mobile Take Yahoo! Mail with you! Check email on your mobile phone. http://mobile.yahoo.com/learn/mail ---------------------------(end of broadcast)--------------------------- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to majordomo@xxxxxxxxxxxxxx so that your message can get through to the mailing list cleanly
