Re: BRIN vs. HOT

Robert Haas wrote:
> If I understand correctly, we currently deem an update to be non-HOT
> whenever any indexed column is updated. This is because tuple
> versions created by HOT updates can later be removed by HOT pruning,
> which means that they must not be referenced by index entries.
> Otherwise, after HOT pruning removed the tuple, the index entries
> would at best be pointing at nothing and at worse be pointing at some
> completely unrelated tuple.
>
> But what about index types that do not store TIDs - i.e. BRIN?

Oh, I had a note to get back to the topic of HOT updates and it fell
through the cracks :-( You're right, this needs to be addressed.

> If the
> indexed column is updated, we can't actually create a Heap Only Tuple
> (HOT), because then the index might be wrong. But we could create a
> Heap Mostly Tuple[1]. We'd construct the update chain in the heap
> page just as we would for HOT, and set all the same flags. But then
> we'd also insert new index entries for any TID-free indexes, currently
> just BRIN. For BRIN, that would have the effect of updating the
> summary data for that page in such a way that it would encompass both
> the old and new values.

Sounds reasonable.

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Álvaro Herrera http://www.2ndQuadrant.com/
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