Re: CREATE INDEX and HOT - revised design - Mailing list pgsql-hackers
From | Heikki Linnakangas |
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Subject | Re: CREATE INDEX and HOT - revised design |
Date | |
Msg-id | 4601565A.1080705@enterprisedb.com Whole thread Raw |
In response to | Re: CREATE INDEX and HOT - revised design (Heikki Linnakangas <heikki@enterprisedb.com>) |
Responses |
Re: CREATE INDEX and HOT - revised design
(Bruce Momjian <bruce@momjian.us>)
|
List | pgsql-hackers |
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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