Re: Vacuum only with 20% old tuples

> We can't "drop and recreate" without a solution to the relation
> versioning issue (unless you are prepared to accept a nonfunctional
> database after a failure partway through index rebuild on a system
> table).  I think we should do this, but it's not all that simple...
> 
> I do not see what your 20% idea has to do with this, though, nor
> why it's a good idea.  If I've told the thing to vacuum I think
> it should vacuum.  20% of a big table could be a lot of megabytes,
> and I don't want some arbitrary decision in the code about whether
> I can reclaim that space or not.

Well, I think we should do a sequential scan before starting vacuum to
find the number of expired rows.

Now that we are removing indexes, doing that to remove a few tuples is a
major waste.  The user can not really know if the table is worth
vacuuming in normal use.  They are just going to use the default.  Now,
I think a FORCE option would be good, or the ability to change the 20%
default.

Remember, commercial db's don't even return unused space if you remove
all the rows in a table.  At least Informix doesn't and I am sure there
are others.  I like vacuum, but let's not make it do major hurtles for
small gain.

--  Bruce Momjian                        |  http://candle.pha.pa.us pgman@candle.pha.pa.us               |  (610)
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