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
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