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List: postgis-users
Subject: Re: [postgis-users] Will it work, or am I overlooking something?
From: hubert depesz lubaczewski <depesz () gmail ! com>
Date: 2014-06-18 12:57:03
Message-ID: CAKrjmhdRyoKL=9ONKMDPeGjkTx9qfH8ogLhCKHLt4XHqOfo=7A () mail ! gmail ! com
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On Tue, Jun 17, 2014 at 3:06 AM, =C3=85smund Tokheim <asmundto@gmail.com> w=
rote:
> Sorry, but I fail to see such case - if point is close, it will be also
> close on at least one of 3 planes (x,y), (x,z), (y,z)
> Let's say that your table has the points (1, 1, 1000), (1, 2000, 1),
> (2000,1,1) and (2, 2, 2). Unless I'm completely mistaken, a 1. nearest
> neighbour search from (0, 0, 0) using your algorithm would return (1, 1,
> 1000) as the nearest neighbour. The correct point should however be (2, 2=
,
> 2), which is somewhat close in all planes, but not very close in any plan=
e.
>
Hmm ... I see now the problem - I was working under assumption that correct
point would be "overwritten" by bad one. But in this case - on every
mapping (xy, xz, yz) - the overwrite happens by different point. Good
catch, thanks.
> My only concern is that the <-> operator won't use the index when you
apply an arbitrary function like translate_xz to its input.
It will if I'll index the function, and not the point.
Regards,
depesz
[Attachment #5 (text/html)]
<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Tue, Jun 17, 2014 \
at 3:06 AM, Åsmund Tokheim <span dir="ltr"><<a href="mailto:asmundto@gmail.com" \
target="_blank">asmundto@gmail.com</a>></span> wrote:<br> <blockquote \
class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid \
rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div dir="ltr"><div \
class="gmail_extra"><div class="gmail_quote"><div>Sorry, but I fail to see such case \
- if point is close, it will be also close on at least one of 3 planes (x,y), (x,z), \
(y,z) <br> </div></div></div></div><div><div class="gmail_extra"><div \
class="gmail_quote"><div class=""><div></div>
</div><div>Let's say that your table has the points (1, 1, 1000), (1, 2000, 1), \
(2000,1,1) and (2, 2, 2). Unless I'm completely mistaken, a 1. nearest neighbour \
search from (0, 0, 0) using your algorithm would return (1, 1, 1000) as the nearest \
neighbour. The correct point should however be (2, 2, 2), which is somewhat close in \
all planes, but not very close in any plane.</div> \
</div></div></div></div></blockquote><div><br></div>Hmm ... I see now the problem - I \
was working under assumption that correct point would be "overwritten" by \
bad one. But in this case - on every mapping (xy, xz, yz) - the overwrite happens by \
different point. Good catch, thanks.<br> <br>> My only concern is that the \
<-> operator won't use the index when you apply an arbitrary function like \
<span style="font-family:arial,sans-serif;font-size:13px">translate_xz to its \
input.<br><br></span></div> <div class="gmail_quote"><span \
style="font-family:arial,sans-serif;font-size:13px">It will if I'll index the \
function, and not the point.<br><br></span></div><div class="gmail_quote"><span \
style="font-family:arial,sans-serif;font-size:13px">Regards,<br> \
<br>depesz<br></span></div></div></div>
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