Re: CREATE INDEX and HOT - revised design

Ah, sounds like you have the idea clearly now. Great.

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Heikki Linnakangas wrote:
> Sorry, I was a bit too quick to respond. I didn't understand at first
> how this differs from Pavan's/Simon's proposals.
>
> Let me answer my own questions.
>
> Heikki Linnakangas wrote:
> > Bruce Momjian wrote:
> >> A different idea is to flag the _index_ as using HOT for the table or
> >> not, using a boolean in pg_index. The idea is that when a new index is
> >> created, it has its HOT boolean set to false and indexes all tuples and
> >> ignores HOT chains. Then doing lookups using that index, the new index
> >> does not follow HOT chains. We also add a boolean to pg_class to
> >> indicate no new HOT chains should be created and set that to false once
> >> the new index is created. Then, at some later time when all HOT chains
> >> are dead, we can enable HOT chain following for the new index and allow
> >> new HOT chains to be created.
> >
> > When exactly would all HOT chains be dead? AFAICS, that would be after
> > the xid of CREATE INDEX gets older than oldest xmin, and VACUUM is run
> > to prune and pointer-swing all HOT chains.
>
> I still think that's true.
>
> > Would we have to wait after setting the new forbid_hot_updates-flag in
> > pg_class, to make sure everyone sees the change? What if CREATE INDEX
> > crashes, would we need a vacuum to reset the flag?
>
> You wouldn't need to do any extra waits to set the forbid_hot_updates
> flag, CREATE INDEX locks the table and already sends a relcache
> invalidations to make the new index visible. CREATE INDEX CONCURRENTLY
> waits already.
>
> >> A more sophisticated idea would be to place an xid, rather than a
> >> boolean, in pg_index to indicate which chains were created after the
> >> index was created to control whether the index should follow that HOT
> >> chain, or ignore it. The xmax of the head of the HOT chain can be used
> >> as an indicator of when the chain was created. Transactions started
> >> before the pg_index xid could continue following the old rules and
> >> insert into the _new_ index for HOT chain additions, and new
> >> transactions would create HOT chains that could skip adding to the new
> >> index. Cleanup of the hybrid HOT chains (some indexes take part, some
> >> do not) would be more complex.
> >
> > What xid would you place in pg_index? Xid of the transaction running
> > CREATE INDEX, ReadNewTransactionId() or what?
>
> Apparently ReadNewTransactionId to make sure there's no existing tuples
> with an xmax smaller than that.
>
> > How does that work if you have a transaction that begins before CREATE
> > INDEX, and updates something after CREATE INDEX?
>
> You actually explained that above...
>
> The HOT_UPDATED flag on a tuple would basically mean that all indexes
> with xid < xmax doesn't contain an index pointer for the tuple, and all
> others do. When inserting new updated tuples, we'd also need to maintain
> that invariant.
>
> >> I know we have xid wrap-around, but I think the VACUUM FREEZE could
> >> handle it by freezing the pg_index xid column value when it does the
> >> table.
> >
> > I don't think you can freeze the xid-column, we went through a similar
> > discussion on pg_class.relfrozenxid. But you can move it forward to
> > oldest xmin.
>
> You could actually "freeze" the column, because unlike relfrozenid we
> never need to unfreeze it.
>
> --
> Heikki Linnakangas
> EnterpriseDB http://www.enterprisedb.com
>
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> TIP 6: explain analyze is your friend

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

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