Re: HOT updates in index-less tables

On Sat, Nov 13, 2010 at 12:13 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Hannu Krosing <hannu@2ndQuadrant.com> writes:
>> On Sat, 2010-11-13 at 10:51 -0500, Tom Lane wrote:
>>> If a table has no indexes, we will always decide that any same-page
>>> update operation is a HOT update, since obviously it isn't modifying
>>> any indexed columns.  But is there any benefit to doing so?
>
>> If we do the in-page "mini vacuum" even without HOT, then there should
>> be no benefit from index-less HOT updates.
>
> AFAICS we do: heap_update marks the page as prunable whether it's a HOT
> update or not.  The only difference between treating the update as HOT vs
> not-HOT is that if there was more than one HOT update, the intermediate
> tuples could be completely reclaimed by page pruning (ie, their line
> pointers go away too).  With not-HOT updates, the intermediate line
> pointers would have to remain in DEAD state until vacuum, since page
> pruning wouldn't know if there were index entries pointing at them.
> But that seems like a pretty tiny penalty.

I'm not at all convinced that's a tiny penalty.

--
Robert Haas
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