On Sun, Jun 25, 2006 at 09:13:48PM +0300, Heikki Linnakangas wrote:
> >If you can't expire the old row because one of the indexed columns was
> >modified, I see no reason to try to reduce the additional index entries.
>
> It won't enable early expiration, but it means less work to do on update.
> If there's a lot of indexes, not having to add so many index tuples can be
> a significant saving.
While catching up on this thread, the following idea came to me that I
think would allow for not updating an index on an UPDATE if it's key
doesn't change. If I understand Bruce's SITC proposal correctly, this
would differ in that SITC requires that no index keys change.
My idea is that if an UPDATE places the new tuple on the same page as
the old tuple, it will not create new index entries for any indexes
where the key doesn't change. This means that when fetching tuples from
the index, ctid would have to be followed until you found the version
you wanted OR you found the first ctid that pointed to a different page
(because that tuple will have it's own index entry) OR you found a tuple
with a different value for the key of the index you're using (because
it'd be invalid, and there'd be a different index entry for it). I
believe that the behavior of the index hint bits would also have to
change somewhat, as each index entry would now essentially be pointing
at all the tuples in the ctid chain that exist on a page, not just
single tuple.
In the case of an UPDATE that needs to put the new tuple on a different
page, our current behavior would be used. This means that the hint bits
would still be useful in limiting the number of heap pages you hit. I
also believe this means that we wouldn't suffer any additional overhead
from our current code when there isn't much free space on pages.
Since SITC allows for in-page space reuse without vacuuming only when no
index keys change, it's most useful for very heavily updated tables such
as session handlers or queue tables, because those tables typically have
very few indexes, so it's pretty unlikely that an index key will change.
For more general-purpose tables that have more indexes but still see a
fair number of updates to a subset of rows, not having to update every
index would likely be a win. I also don't see any reason why both
options couldn't be used together.
--
Jim C. Nasby, Sr. Engineering Consultant jnasby@pervasive.com
Pervasive Software http://pervasive.com work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461
