Dne 7.5.2011 04:02, Robert Haas napsal(a): > 2011/4/30 Tomas Vondra <t...@fuzzy.cz>: >> I've been digging in the sources, and I've noticed the MaxOffsetNumber >> is defined (in storage/off.h) like this >> >> (BLCKSZ / sizeof(ItemIdData)) >> >> I guess it might be made a bit more precise by subtracting the header >> like this >> >> (BLCKSZ - offsetof(PageHeaderData, pd_linp) / sizeof(ItemIdData)) >> >> although the difference is negligible (2048 vs 2042 for 8kB pages). > > I guess we could do that, but I'm not sure there's much point. It's > also not entirely clear that this would actually work out to a win, > because of the issues discussed in the "When can/should we prune or > defragment?" section of src/backend/access/heap/README.HOT > > We could probably figure this out with some careful testing, but I'm > not sure it's worth the effort.
No idea if it's worth the effort and if something can be broken by this change. I've noticed this when trying to implement a more thorough check of table contents vs. index. I was asking 'how many items can be stored on a page' and I've noticed this. Anyway I don't see how this could affect HOT updates? AFAIK this has nothing to do with evaluation of free space on a page. It only affects checking if the offset number may be valid (OffseNumberIsValid macro), and code that needs to keep info about items on a page, as it usually does something like this: OffsetNumber unused[MaxOffsetNumber]; But yes, the amount of memory saved is negligible (12B) so the only noticeable benefit might be catching some invalid offset numbers. regards Tomas -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
