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. > 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). Nicolas -- A. Because it breaks the logical sequence of discussion. Q. Why is top posting bad? -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers