> I think the word "line pointer" is causing some confusion > here. Let me explain the idea again: Each page has a set of > line pointers OR item-ids as they are referred in the code (I > shall use the word item-id here after). > The item-id stores the offset(15 bits), length (15 bits) and > two flags, LP_USED and LP_DELETE (2 bits). > > The root tuple is pointed to by some item-id (say, I1) on the > page. When the tuple is hot updated, a heap-only tuple is > added which is linked to the root tuple by its t_ctid and is > pointed by another item-id I2 on the page. > > Now, when the root tuple subsequently becomes DEAD, I am > proposing to store I2 in the offset field of I1. The length > field in I1 can be set to zero (or some other special value) > to mark that I1 is now just a redirection to I2 and does not > point to any real tuple, dead or live.
Oh, thanks for explaining. I was really misunderstanding. Is that possible ? I thought item-id's (slots) need to be in physical order. > If we do reuse dead tuples without vacuum we should probably, > as already > > suggested, > > disconnect the "what is dead enough for reuse/vacuum" from > global xmin > > right from the start. > > > I did not get that completely. Can you elaborate on that ? > > Once we find a heap-only DEAD tuple, we remove it from the > tuple-chain (and thus remove its only reference) and set LP_DELETE > on its item-id. When we run out of free-space in a page, we search for > an item-id whose LP_DELETE is set and whose length is atleast equal > to the new tuple length. We reuse that item-id and the associated > storage for storing the new tuple. With the recent discussion about flashback (explicitly setting a snapshot to 5 min ago) and forensics, I think we should have a way to delay what is considered "DEAD and available for immediate reuse". I know this can reduce the efficiency of HOT when delayed too far, but I think we should have that possibility if we reuse without vacuum. Andreas ---------------------------(end of broadcast)--------------------------- TIP 2: Don't 'kill -9' the postmaster
