Heikki,
Thanks for providing this summary. As someone unfamiliar with the
domain (both HOT development specifically and tuple management in
general), I found it easy to follow.
On Jul 13, 2007, at 8:31 , Heikki Linnakangas wrote:
> Pruning
> -------
> To reclaim the index-visible (i.e. first) tuple in a HOT chain, the
> line pointer is turned into a redirecting line pointer that points
> to the line pointer of the next tuple in the chain. To keep track
> of the space occupied by the dead tuple, so that we can reuse the
> space, a new line pointer is allocated and marked with LP_DELETE to
> point to the dead tuple. That means its tid changes, but that's ok
> since it's dead.
> Row-level fragmentation
> -----------------------
> If there's no LP_DELETEd tuples large enough to fit the new tuple
> in, the row-level fragmentation is repaired in the hope that some
> of the slots were actually big enough, but were just fragmented.
> That's done by mapping the offsets in the page, and enlarging all
> LP_DELETEd line pointers up to the beginning of the next tuple.
Would it make sense to enlarge the LP_DELETEd line pointers up to the
beginning of the next tuple at the time the tuple is marked LP_DELETE?
> Vacuum
> ------
> Vacuum prunes all HOT chains, and removes any LP_DELETEd tuples,
> making the space available for any use.
In the case of a fragmented row, am I right to assume vacuum reclaims
all space up to the next (live) tuple?
Michael Glaesemann
grzm seespotcode net
