Re: Dead Space Map

Greg Stark wrote:
>
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
>
> > I should be clearer. Suppose you have a table with a single index on
> > the primary key. You are updating the row over and over again (a
> > typical case). You create the first row, commit, then it is updated
> > (two copies), commit, then you update it again. That first created row
> > might not be visible to anyone, but has the same index value as the new
> > row you are about to add. Why not reused that heap tuple?
>
> If you commit each update then your tuple might well be visible to other
> transactions, how would you check that?

You check the xmin/xmax using standard visibility rules.

> I originally thought you meant if you are repeatedly updating the same record
> within the same transaction. In that case sure you could reuse the space but
> a) only if it's big enough for the new record and b) how often do you really
> do that?

Right, that is a rare case.

--
Bruce Momjian http://candle.pha.pa.us
SRA OSS, Inc. http://www.sraoss.com

+ If your life is a hard drive, Christ can be your backup. +