On 8/10/16 12:48 PM, Claudio Freire wrote:
> On Tue, Aug 9, 2016 at 11:39 PM, Jim Nasby <Jim.Nasby@bluetreble.com> wrote:
>> On 8/9/16 6:44 PM, Claudio Freire wrote:
>>>
>>> Since we can lookup all occurrences of k1=a index=0 and k2=a index=0,
>>> and in fact we probably did so already as part of the update logic
>>
>>
>> That's a change from what currently happens, right?
>>
>> The reason I think that's important is that dropping the assumption that we
>> can't safely re-find index entries from the heap opens up other
>> optimizations, ones that should be significantly simpler to implement. The
>> most obvious example being getting rid of full index scans in vacuum. While
>> that won't help with write amplification, it would reduce the cost of vacuum
>> enormously. Orders of magnitude wouldn't surprise me in the least.
>>
>> If that's indeed a prerequisite to WARM it would be great to get that
>> groundwork laid early so others could work on other optimizations it would
>> enable.
>
> I can do that. I've been prospecting the code to see what changes it
> would entail already.
>
> But it's still specific to btree, I'm not sure the same optimizations
> can be applied to GIN (maybe, if the posting list is sorted) or GIST
> (probably, since it's like a btree, but I don't know the code well
> enough).
>
> Certainly hash indexes won't support it.
Why not? If this is all predicated on re-finding index keys based on
heap data then this is just another index lookup, no?
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
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