On 6/12/2004 3:45 PM, Tom Lane wrote:
Jan Wieck <[EMAIL PROTECTED]> writes:But a per relation bitmap that tells if a block is a) free of dead tuples and b) all remaining tuples in it are frozen could be used to let vacuum skip them (there can't be anything to do). The bit would get reset whenever the block is marked dirty. This would cause vacuum to look at mainly recently touched blocks, likely to be found in the buffer cache anyway and thus dramatically reduce the amount of IO and thereby make high frequent vacuuming less expensive.
I don't think it would help very much to define a bit like that --- I can't believe that very many pages would contain only frozen tuples, unless you were to adopt an aggressive policy of using VACUUM FREEZE a lot.
I thought this implies an aggressive policy of freezing everything by default. But I guess there is something I am not aware of that makes aggressive freezing a bad thing.
It might be interesting though to have some kind of "fast vacuum" mode that doesn't worry about freezing tuples, but only reclaiming dead ones. This could look at only recently touched pages, with perhaps the cooperation of the bgwriter to keep track of candidate pages. (You'd still have to do full-database scans for freezable tuples occasionally ... but not very often.)
Wouldn't that screw the current FSM population mechanisms? Not that my suggestions above wouldn't do that either :-)
The main thing I don't see how to handle efficiently is getting rid of the index entries for dead rows. The present indexam API is designed for getting rid of index entries wholesale, but you'd need something that works better for retail removal to approach vacuuming this way.
Which is a problem so fundamentally embedded in the index implementation itself that is sounds more like a wholesale replacement of the index am than some nifty changes.
Jan
-- #======================================================================# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #================================================== [EMAIL PROTECTED] #
---------------------------(end of broadcast)--------------------------- TIP 7: don't forget to increase your free space map settings
