"Tom Lane" <[EMAIL PROTECTED]> writes: >>> set_current_snapshot() would have to sanity check that the xmin of the new >>> snapshot isn't older than the current globaloldestxmin. > >> That would solve the backend to backend IPC problem nicely. > > But it fails on the count of making sure that globaloldestxmin doesn't > advance past the snap you want to use. And exactly how will you pass > a snap through a table? It won't become visible until you commit ... > whereupon your own xmin isn't blocking the advance of globaloldestxmin.
Hm, good point. You could always do it in a separate connection, but that starts to get annoying. I was more envisioning passing it around out-of-band though, something like: $db->execute(SET TRANSACTION ISOLATION LEVEL SERIALIZABLE); $snap = $db->execute(select current_snapshot()); for each db { if (fork()) $slave[i] = $db->connect(); $slave[i]->execute(select set_snapshot($snap)); $slave[i]->execute(copy table[i] to file[i]); } I'm also wondering about something like: $db->execute(SET TRANSACTION ISOLATION LEVEL SERIALIZABLE); $snap = $db->execute(select current_snapshot()); if (fork()) $slave = $db->connect(); $slave->execute(select set_snapshot($snap); $slave->execute(copy tab from hugefile); signal parent } else { while(no signal yet) { $rows_loaded_so_far = $db->execute(select count(*) from tab); display_progress($rows_loaded_so_far); sleep(60); } } Sorry for the vaguely perlish pseudocode but it's the clearest way I can think to write it. I don't think it would make much sense to try to do anything like this in plpgsql; I think you really do want to be doing it in a language outside the database where it's easier to open multiple connections and handle IPC. I realize the second idea might take more hackery than just setting the snapshot... In particular as written above it wouldn't work because the slave would be writing with a new xid that isn't actually in the snapshot. -- Gregory Stark EnterpriseDB http://www.enterprisedb.com ---------------------------(end of broadcast)--------------------------- TIP 5: don't forget to increase your free space map settings