[postgis-devel] Recheck on Points broken

[Regina Obe]

Okay I did some more snooping of the distance outputs.

 

Using this query that consistently fails:

 

create table test (geo geometry);

insert into test values 

  ('0101000020E61000007D91D0967329E4BF6631B1F9B8D64A40'::geometry), 

  ('0101000020E6100000E2AFC91AF510C1BFCDCCCCCCCCAC4A40'::geometry);

 

set enable_seqscan = false;

select *,  '0101000020E610000092054CE0D6DDE5BFCDCCCCCCCCAC4A40'::geometry <-> geo , ST_Distance('0101000020E610000092054CE0D6DDE5BFCDCCCCCCCCAC4A40'::geometry, geo) 

from test ORDER BY geo <->

('0101000020E610000092054CE0D6DDE5BFCDCCCCCCCCAC4A40'::geometry);

 

     ?column?      |    st_distance

-------------------+-------------------

0.331823813642119 | 0.331823813642119

              0.55 |              0.55

(2 rows)

 

The issue seems to be with the second record that the approximate distance 0.5500000111921  is just a wee bit bigger than the actual distances.

Which as you said should never happen.

 

NOTICE:  bbox2df_distance: (0.331818528656)

NOTICE:  bbox2df_distance: (0.550000011921)

NOTICE:  index returned tuples in wrong order. cmp: -1

 

Does this create beaming lights in your head?

 

From: postgis-devel [mailto:postgis-devel-bounces at lists.osgeo.org] On Behalf Of Paul Ramsey
Sent: Tuesday, January 03, 2017 10:00 AM
To: PostGIS Development Discussion <postgis-devel at lists.osgeo.org>
Subject: Re: [postgis-devel] Recheck on Points broken

 

Are we 100% sure this is a float/double issue? The distance between two float boxes should always be *smaller* than the actual distance between two objects, since the float box extrema are always moved *away* from the original object when the float boxes are created. So from a "theoretical mechanics" perspective this shouldn't actually be the source of the problem.

 

>From a real world point-of-view, how could it still be the problem?

- maybe the box generation code has a short circuit for points which doesn't do the correct thing? Pretty sure it doesn't, but.

- something else?

P

 

On Tue, Jan 3, 2017 at 4:54 AM, Regina Obe <lr at pcorp.us <mailto:lr at pcorp.us> > wrote:


>> Anyrate if there is nothing that can be done about ensuring index
>> distance is always less or equal to actual distance

> I remember we were all happy about having *real* distance with a more recent PostgreSQL. If that's not the case (as the user reports) we should try to find a way to provide both mechanisms.
> Two different operators, maybe ?
> --strk;

We've already got two different operators.  We have <#> which doesn't use recheck and that's what I told the user to use, until we resolve this.

The thing is the <-> I think pretty much for all reasonable purposes returns the right order for points (and in fact in this case yields the same value and <-> I think in all cases it will).

So it's just the float4/float8 mess why it screws up and it Errors noisily in PostgreSQL land where we cannot control.

I hate telling people to if you have point / point use <#> and if you have others you can safely use  <->.  That's sooo unfriendly.

I think the best thing is just to not have point-point checks use recheck.  Can we do that easily? That's what PostgreSQL point/point does.
That way we don't fall into the PostgreSQL box trap of

"Hey -- how come your index distance is bigger than your computed distance.  What kind of monster are you? Screw you!"

Thanks,
Regina


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