Re: Index Skip Scan

Hi Dmitry,

On 06/19/2018 06:01 AM, Dmitry Dolgov wrote:
>> On 18 June 2018 at 19:31, Alexander Korotkov <a.korotkov@postgrespro.ru> wrote:
>> Is skip scan only possible for index-only scan?  I guess, that no.  We
>> could also make plain index scan to behave like a skip scan.  And it
>> should be useful for accelerating DISTINCT ON clause.  Thus, we might
>> have 4 kinds of index scan: IndexScan, IndexOnlyScan, IndexSkipScan,
>> IndexOnlySkipScan.  So, I don't think I like index scan nodes to
>> multiply this way, and it would be probably better to keep number
>> nodes smaller.  But I don't insist on that, and I would like to hear
>> other opinions too.
> 
> In one of patches I'm working on I had similar situation, when I wanted to
> split one node into two similar nodes (before I just extended it) and logically
> it made perfect sense. But it turned out to be quite useless and the advantage
> I've got wasn't worth it - and just to mention, those nodes had more differences
> than in this patch. So I agree that probably it would be better to keep using
> IndexOnlyScan.
> 

I looked at this today, and creating a new node (T_IndexOnlySkipScan) 
would make the patch more complex.

The question is if the patch should create such a node such that future 
patches didn't have to deal with refactoring to a new node to cover 
additional functionality.

Thanks for your feedback !

Best regards,
  Jesper