Michael Fuhr <mike@xxxxxxxx> writes: > On Tue, Feb 08, 2005 at 12:58:34AM -0500, Tom Lane wrote: >> I'm too tired to work out an example, but I think this probably doesn't >> work in general: the xmax on the version of the row you can see might >> not correspond to a live transaction, but that doesn't mean someone >> else doesn't hold a lock on the latest committed version of the row. > If you could point me in the right direction I'll try to work out > an example where my suggestion fails. I'm thinking about a multiple-update situation: your snapshot includes row version A, which was superseded by version B, which was superseded by version C. By the time you are looking, the transaction that committed version B is gone so the xmax you see (B's xact) isn't locked anymore. But the "frontmost" version of the row is still locked (by C or some later heir) so if you tried to update you'd block. Like I said, I'm pretty tired and I might be missing something... regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to majordomo@xxxxxxxxxxxxxx
