Subject: Re: Index Skip Scan
From: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
To: 9erthalion6@gmail.com
Cc: florisvannee@Optiver.com, pg@bowt.ie, jesper.pedersen@redhat.com,
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Date: Thu, 6 Feb 2020 22:56:40 +0900
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Sorry, I forgot to write more significant thing.

On 2020/02/06 21:22, Kyotaro Horiguchi wrote:
> At Thu, 6 Feb 2020 11:57:07 +0100, Dmitry Dolgov <9erthalion6@gmail.com> wrote in
>>> On Thu, Feb 06, 2020 at 10:24:50AM +0900, Kyotaro Horiguchi wrote:
>>> At Wed, 5 Feb 2020 17:37:30 +0100, Dmitry Dolgov <9erthalion6@gmail.com> wrote in
>>> We could add an additional parameter "in_cursor" to
>>> ExecSupportBackwardScan and let skip scan return false if in_cursor is
>>> true, but I'm not sure it's acceptable.
>> I also was thinking about whether it's possible to use
>> ExecSupportBackwardScan here, but skip scan is just a mode of an
>> index/indexonly scan. Which means that ExecSupportBackwardScan also need
>> to know somehow if this mode is being used, and then, since this
>> function is called after it's already decided to use skip scan in the
>> resulting plan, somehow correct the plan (exclude skipping and try to
>> find next best path?) - do I understand your suggestion correct?

No. I thought of the opposite thing. I meant that
IndexSupportsBackwardScan returns false if Index(Only)Scan is
going to do skip scan. But I found that the function doesn't have
access to plan node nor executor node.  So I wrote as the follows.

>> I didn't thought so hardly, but a bit of confirmation told me that
>> IndexSupportsBackwardScan returns fixed flag for AM.  It seems that
>> things are not that simple.

regards.

-- 

Kyotaro Horiguchi