> Ãhel kenal päeval, E, 2006-06-26 kell 09:10, kirjutas Mark Woodward: >> > ÃÅhel kenal päeval, R, 2006-06-23 kell 17:27, kirjutas Bruce >> Momjian: >> >> Jonah H. Harris wrote: >> >> > On 6/23/06, Tom Lane <[EMAIL PROTECTED]> wrote: >> >> > > What I see in this discussion is a huge amount of "the grass must >> be >> >> > > greener on the other side" syndrome, and hardly any recognition >> that >> >> > > every technique has its downsides and complications. >> >> > >> >> > I'm being totally objective. I don't think we should abandon >> >> > PostgreSQL's overall design at all, because we do perform INSERTs >> and >> >> > DELETEs much better than most systems. However, I've looked at >> many >> >> > systems and how they implement UPDATE so that it is a scalable >> >> > operation. Sure, there are costs and benefits to each >> implementation, >> >> > but I think we have some pretty brilliant people in this community >> and >> >> > can come up with an elegant design for scalable UPDATEs. >> >> >> >> I think the UPDATE case is similar to the bitmap index scan or >> perhaps >> >> bitmap indexes on disk --- there are cases we know can not be handled >> >> well by our existing code, so we have added (or might add) these >> >> features to try to address those difficult cases. >> > >> > Not really. Bitmap index scan and bitmap index are both new additions >> > working well with existing framework. >> > >> > While the problem of slowdown on frequent updates is real, the >> suggested >> > fix is just plain wrong, as it is based on someones faulty assumption >> on >> > how index lookup works, and very much simplified view of how different >> > parts of the system work to implement MVCC. >> >> Yes, the suggestion was based on MVCC concepts, not a particular >> implementation. > > On the contrary - afaik, it was loosely based on how Oracle does it with > its rollback segments, only assuming that rollback segments are kept in > heap and that indexes point only to the oldest row version :p
Well, give me a little more credit than that. Yes, Oracle did play small part in my thinking, but only in as much as "they can't do it, why can't we?" The problem was how to get the most recent tuple to be more efficient and not have tuples that will never be used impact performance without excessive locking or moving data around. It was a just a quick idea. Bruce's solution, you have to admit, is somewhat similar. > >> > The original fix he "suggests" was to that imagined behaviour and thus >> > ignored all the real problems of such change. >> >> The original suggestion, was nothing more than a hypothetical for the >> purpose of discussion. >> >> The problem was the steady degradation of performance on frequent >> updates. >> That was the point of discussion. I brought up "one possible way" to >> start a "brain storm." The discussion then morphed into critisizing the >> example and not addressing the problem. > > The problem is heatedly discussed every 3-4 months. And yet, here we are again. > >> Anyway, I think some decent discussion about the problem did happen, and >> that is good. > > Agreed. > > Maybe this _was_ the best way to bring up the discussion again. I have a way, for better or worse, I guess, of stirring up the pot. :-) Cry as we may about MySQL, but I have a sneaking suspicion that this is one of the issues that puts PostgreSQL at a serious disadvantage. While heavily updated rows are a single type of problem, these days I think *most* database deployments are as back-ends for web sites. This problem is *very* critical to that type of application, consequently probably why PostgreSQL has difficulty in that space. If PostgreSQL can be made *not* to suffer performance degradation on heavily updated rows, then that is realy the last issue in the way of it being a completely creadible medium to large enterprise back end. This combined with its amazing pragramability, should make it unstoppable. ---------------------------(end of broadcast)--------------------------- TIP 5: don't forget to increase your free space map settings
