On 4 August 2016 at 18:17, Andres Freund <andres@anarazel.de> wrote:
> On 2016-08-04 18:11:17 +0100, Simon Riggs wrote:
>> On 4 August 2016 at 17:31, Andres Freund <andres@anarazel.de> wrote:
>> > Hi,
>> >
>> > On 2016-08-04 16:29:09 +0530, Pavan Deolasee wrote:
>> >> Indexes whose values do not change do not require new index pointers. Only
>> >> the index whose key is being changed will need a new index entry. The new
>> >> index entry will be set to the CTID of the root line pointer.
>> >
>> > That seems to require tracing all hot-chains in a page, to actually
>> > figure out what the root line pointer of a warm-updated HOT tuple is,
>> > provided it's HOT_UPDATED itself. Or have you found a smart way to
>> > figure that out?
>>
>> Hmm, sorry, I did think of that point and I thought I had added it to the doc.
>>
>> So, yes, I agree - re-locating the root is the biggest downside to
>> this idea. Perhaps Pavan has other thoughts?
>>
>> I think its doable, but it will be fiddly.
>
> I'm less afraid of the fiddlyness of finding the root tuple, than the
> cost. It's not cheap to walk through, potentially, all hot chains to
> find the root ctid.
>
> Have you considered what it'd take to allow index pointers to allow to
> point to "intermediate" root tuples? I.e. have some indexes point into
> the middle of an update-chain, whereas others still point to the
> beginning? There's certainly some complications involved with that, but
> it'd also have the advantage in reducing the amount of rechecking that
> needs to be done.
"Intermediate root tuples" was my first attempt at this and it didn't
work. I'll dig up the details, some problem in VACUUM, IIRC.
--
Simon Riggs http://www.2ndQuadrant.com/
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