On Wed, Feb 24, 2010 at 11:05 AM, Gokulakannan Somasundaram <gokul...@gmail.com> wrote: >> I think you're a barking up the wrong tree. AFAIUI, the need for the >> visibility map has not very much to do with whether the table has >> indices, and everything to do with avoiding unnecessary VACUUMs. In >> any event, you've not shown that the visibility map HAS any overhead, >> so talking about skipping it seems entirely premature. Keep in mind >> that the visibility map is quite small. > > OK! i am not saying to remove the visibility map, if i am misunderstood. All > i am saying here is to remove the index only scan processing of visibility > map. If it is being used only for vacuums, you need not make it crash safe > and no WAL comes into picture.
So basically you want to have index-only scans, but you want them to be really slow? >> The point of the visibility map as far as index-only scans are >> concerned is that if all the needed column values can be extracted >> from the index, we still need to read the heap page to check tuple >> visibility - unless, of course, we already know from the visibility >> map that all the tuples on that heap page are guaranteed to be visible >> to all transactions. On a read-only or read-mostly table, this will >> reduce the cost of checking tuple visibility by several orders of >> magnitude. >> > I understand that. As i suggested above, if you have no indexes for a table, > why do you need to spend the extra effort in making it crash safe for that > table? Hope i am clear. Tables without indices don't need to be crash safe? News to me. ...Robert -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
