On Fri, Oct 23, 2015 at 7:37 AM, Jim Nasby wrote: > What I'm wondering is how compressible a 'normal' FPI is. Certainly if the > hole is zero'd out and the page is mostly empty you'll get great > compression. What about other workloads? For reference, if a 'FPI > placeholder' WAL record is 16 bytes, that's 51,200% compression. If it's 12 > bytes, it's 68,200% compression. (I'm assuming we write the hole too, but > maybe that's not true?)
Well, to begin with FPI usually avoid to include the page hole in the middle. Now, regarding the compressibility of a page taken without its hole, that's highly schema-dependent. Based on some measurements I did some time ago a page with repetitive data could compress up to 40%, with less compressible stuff like UUID I recall it to be 20~25%. I hacked out for the FPW compression patch a module able to work directly on raw pages to test their compressibility: https://github.com/michaelpq/pg_plugins/tree/master/compress_test get_raw_page() has been taken from pageinspect and I added to it an option to remove the hole in the middle of the page. Using that you are able to guess how much pages can get compressed. -- Michael -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
