Re: HOT documentation README

Pavan Deolasee wrote:
> It would be worth mentioning that columns appearing in predicates
> of partial indexes and expressions of expression indexes are also
> checked. If any of these columns are changed, HOT update is not done.

Added now:

Tuples are checked against expression and
partial indexes to be sure no referenced columns change.

Something about those was in the original version but not the new one I
used.

> > When the last live tuple in an update chain becomes dead (after a DELETE
> > or a cold update), the redirecting line pointer is marked as redirected
> > dead. That allows us to immediately reuse the heap space (but not the
> > line pointer itself).
>
>
>
> A lazy vacuum is required to reclaim redirect-dead line pointers.

Updated:

When the last live tuple in an update chain becomes dead (after a DELETE
or a cold update), the redirecting line pointer is marked as redirected
dead. That allows us to immediately reuse the heap space, while VACUUM
can reclaim the line pointer space.

> To limit the damage in the worst case, and to
> > keep numerous arrays as well as the bitmaps in bitmap scans reasonably
> > sized, the maximum number of line pointers (MaxHeapTuplesPerPage) is
> > arbitrarily capped at twice its previous maximum.
>
>
>
> With the latest patch, we have reverted it back to the original value.

Updated:

We have effectively implemented the "truncate dead tuples to just line
pointer" idea that has been proposed and rejected before because of fear
of line pointer bloat. To limit the damage in the worst case, and to
keep numerous arrays as well as the bitmaps in bitmap scans reasonably
sized, the maximum number of line pointers (MaxHeapTuplesPerPage) is
arbitrarily capped.

> VACUUM FULL
> > -----------
>
>
>
> It might be worth mentioning that vacuum full also removes
> redirected line pointers by making them directly point to
> the first tuple in the HOT chain. We can do so, because vacuum
> full works with an exclusive lock on the relation.

Updated:

To make vacuum full work, any DEAD tuples in the middle of an update
chain need to be removed (see comments at the top of
heap_prune_hotchain_hard() for details). Vacuum full performs a more
aggressive pruning that not only removes dead tuples at the beginning of
an update chain, but scans the whole chain and removes any intermediate
dead tuples as well. It also removes redirected line pointers by making
them directly point to the first tuple in the HOT chain. This is
possible because vacuum full works with an exclusive lock on the
relation.

> VACUUM
> > ------
> >
> > There is little change to regular vacuum. It removes dead HOT tuples,
> > like pruning does, and cleans up any redirected dead line pointers.
>
>
>
> One change that is worth mentioning is that with HOT it needs vacuum
> strength
> lock in the first phase (currently it works with SHARE lock if no tuples
> need
> freezing or EXCLUSIVE lock otherwise). We can improve it a bit by first
> checking if there is really a need for pruning and then only go for cleanup
> lock. But thats probably not worth the efforts (atleast for large tables
> where
> we should usually get cleanup lock rather easily).
>
>
>
> Statistics
> > ----------
> >
> > XXX: How do HOT-updates affect statistics? How often do we need to run
> > autovacuum?
>
>
>
> As the latest patch stands, we track dead-space in the relation and trigger
> autovacuuum based on the percentage of dead space in the table. We
> don't have any mechanism to account for index bloat yet. Autoanalyze
> does not change.

OK.

Current README.HOT version:

ftp://momjian.us/pub/postgresql/mypatches/README.HOT

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://www.enterprisedb.com

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