"Pavan Deolasee" <[EMAIL PROTECTED]> writes: > On 9/12/07, Tom Lane <[EMAIL PROTECTED]> wrote: >> XXX doesn't the above completely break the anti-wraparound guarantees? >> And why couldn't we avoid the problem by pruning the previous tuple, >> which is surely dead?
> We preserve anti-wraparound guarantees by not letting the relfrozenxid > advance past the minimum of cut-off xid and xmin/xmax of not-yet-frozen > tuples. Given that this is required to address corner case of DEAD tuple > following a RD tuple, the final relfrozenxid would be very close to the > cut-off xid. Isn't it ? > We could have actually pruned the preceding RD tuples (as we do in > vacuum full), but we were worried about missing some corner case > where someone may still want to follow the chain from the RD tuple. This seems all wrong to me. We'd only be considering freezing a tuple whose xmin precedes the global xmin. If it has a predecessor, that predecessor has xmax equal to this one's xmin, therefore also preceding global xmin, therefore it would be seen as DEAD not RECENTLY_DEAD. So we should never need to freeze a tuple that isn't the start of its HOT chain. Also, if you find a DEAD tuple after a RECENTLY_DEAD one, you can certainly prune both, because what this tells you is that both tuples are in fact dead to all observers. regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 6: explain analyze is your friend