From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Hannu Krosing <hannu(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: HOT updates in index-less tables |
Date: | 2010-11-13 22:29:15 |
Message-ID: | AANLkTin=pDzGo1mFxi8wUSV0E5FWOd4BWsOyueRy-D+p@mail.gmail.com |
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Lists: | pgsql-hackers |
On Sat, Nov 13, 2010 at 12:13 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Hannu Krosing <hannu(at)2ndQuadrant(dot)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
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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