Nicolas Barbier wrote: > 2011/5/11, Bruce Momjian <br...@momjian.us>: > > > FYI, because the visibility map is only one _bit_ per page, it is 8000 * > > 8 or 64k times smaller than the heap, e.g. one 8k page covers 64MB of > > heap pages. > > Actually, that would be "one 8kB block covers 512MB of heap": 1 block > of visibility map (8kB) = 64k visibility bits = covers 64k blocks = > covers 512MB of heap. The cost of keeping the visibility map in cache > is therefore totally negligible, only the cost of WAL logging changes > to it is of interest.
Ah, yes, thanks, even better. > > This is important because we rely on this compactness in hope that > > the WAL logging of this information will not be burdensome. > > The size of on entry in the map (1 bit) is not very related to the WAL > overhead required per change of such a bit (i.e., the log record for a > 1 bit change will certainly be way more than 1 bit). True. -- Bruce Momjian <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + It's impossible for everything to be true. + -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
