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.

Álvaro Herrera      
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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