[Mobilitydb-dev] Update about my experiments integrating PostGIS 3.1.3 and MobilityDB

[Esteban Zimanyi]

I always solved that problem with the setPrecision functions
https://docs.mobilitydb.com/MobilityDB/develop/ch04s05.html#box_setPrecision
https://docs.mobilitydb.com/MobilityDB/develop/ch05s06.html#tpoint_setPrecision
You can find numerous setPrecision calls in all the tests. I needed to do
that when switching to PosgreSQL 13 because the output of the floating
point values were different between versions.

The difference now is that in PostGIS 3 they started using the ryu library
for output coordinates. The difference wrt the other tests is that we need
to output a geometry/geography as result of the trajectory function.

I have almost finished today to write a simple setPrecision function for
geometry/geography so that for example, the test
 SELECT ST_AsText(trajectory(tnpoint 'Npoint(1, 0.5)@2000-01-01'));
will be replaced by
 SELECT ST_AsText(setPrecision(trajectory(tnpoint 'Npoint(1,
0.5)@2000-01-01'), 6));
and this will solve the multiplatform variation for the tests.



------------------------------------------------------------
Prof. Esteban Zimanyi
Department of Computer & Decision Engineering  (CoDE) CP 165/15
Universite Libre de Bruxelles
Avenue F. D. Roosevelt 50
B-1050 Brussels, Belgium
fax: + 32.2.650.47.13
tel: + 32.2.650.31.85
e-mail: esteban.zimanyi at ulb.be
Internet: http://cs.ulb.ac.be/members/esteban/
------------------------------------------------------------


On Mon, Aug 23, 2021 at 6:42 PM Vicky Vergara <vicky at georepublic.de> wrote:

> Yeah, I have that problem also when testing on different
> platforms/compilers/postgres
> But I resolved by using on some tests that were having precision problems
> with:
> SET extra_float_digits = -3;
>
> https://github.com/pgRouting/pgrouting/blob/main/pgtap/withPoints/withPoints_oneToOne-compare-dijkstra.test.sql#L30
> Documentation about that can be found here:
> https://www.postgresql.org/docs/12/datatype-numeric.html
>
> In another case I had to resolve floating point comparison internally with:
>
> https://github.com/pgRouting/pgrouting/blob/main/src/common/xy_vertex.cpp#L36
> Which I got from here:
> https://en.cppreference.com/w/cpp/types/numeric_limits/epsilon
>
> Allways keep in mind floating point
> Remember for example that:
> mathematically: 1 == 1/3 + 1/3 + 1/3
> But computationally: 1 != 1/3 + 1/3 + 1/3
> The reason is floating point
>
> I like this article
>
> https://randomascii.wordpress.com/2012/02/25/comparing-floating-point-numbers-2012-edition/
>
>
> On Sun, Aug 22, 2021 at 7:56 AM Esteban Zimanyi <estebanzimanyi at gmail.com>
> wrote:
>
>> Dear Vicky and Regina
>>
>> Following the recommendations we received at the PostGIS Dev meeting this
>> August 20, I started to experiment the embedding of liblwgeom from PostGIS
>> 3.1.3 inside MobilityDB. I was able to run the MobilityDB tests but
>> obviously things have changed from versions 2.5.5 to 3.1.3.
>>
>> You will find enclosed a diff of the results of one of the tests. As you
>> can see there are two types of differences. The first one concerns a very
>> small floating point precision difference, e.g.,
>>
>>  SELECT ST_AsText(trajectory(tnpoint 'Npoint(1, 0.5)@2000-01-01'));
>> -                 st_astext
>> ---------------------------------------------
>> - POINT(48.71866291282778 77.76407051015086)
>> +                st_astext
>> +------------------------------------------
>> + POINT(48.7186629128278 77.7640705101509)
>>  (1 row)
>>
>> Another type of error concerns deprecated functions
>>
>>  SELECT setPrecision(atGeometry(tnpoint '{Npoint(1, 0.3)@2000-01-01,
>> Npoint(1, 0.5)@2000-01-02, Npoint(1, 0.5)@2000-01-03}', geometry
>> 'SRID=5676;Polygon((50 50,50 100,100 100,100 50,50 50))'), 6);
>> -ERROR
>> -DETAIL:  Library function 'intersects' was deprecated in PostGIS 3.0.0
>> -HINT:  Consider running: SELECT postgis_extensions_upgrade()
>> +              setprecision
>> +----------------------------------------
>> + {NPoint(1,0.3)@2000-01-01 00:00:00+00}
>> +(1 row)
>> +
>>
>> In the forthcoming days I will be modifying the develop branch so that
>> the tests can be run in both versions PostGIS 2.5.5 and 3.1.3.
>>
>> Furthermore, as a consequence of the problem I reported on the ticket
>> https://trac.osgeo.org/postgis/ticket/4979#comment:5
>> it is not possible to mix PostGIS 2.5.5 and 3.1.3 (e.g., use liblwgeom.so
>> from PostGIS 2.5.5 with PostGIS 3.1.3 which was one of the tests planned)
>> since all the lwgeom struct definitions have been changed.
>>
>> Regards
>>
>> Esteban
>>
>> ------------------------------------------------------------
>> Prof. Esteban Zimanyi
>> Department of Computer & Decision Engineering  (CoDE) CP 165/15
>> Universite Libre de Bruxelles
>> Avenue F. D. Roosevelt 50
>> B-1050 Brussels, Belgium
>> fax: + 32.2.650.47.13
>> tel: + 32.2.650.31.85
>> e-mail: esteban.zimanyi at ulb.be
>> Internet: http://cs.ulb.ac.be/members/esteban/
>> ------------------------------------------------------------
>>
>
>
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