Sphere based calculations for geography don't appear to be faster?

[Marco Boeringa]

So you are actually saying that geographiclib doesn't have a fast 
optimized calculation path for the spherical case (or the current 
PostGIS implementation of this library doesn't use it if it exists in 
the library?), but likely uses exactly the same code (path) for both 
spherical and spheroidal mode, so the option to use sphere is 
essentially without real use in the PostGIS case? Because why would you 
choose spherical over spheroid, if both have exactly the same 
performance and spheroid gives the more accurate result?

Would seem a missed optimization option in geographiclib or PostGIS... 
but it does seem consistent with the matching timing results.

Marco

Op 1-10-2024 om 22:20 schreef Paul Ramsey:
>
>> On Oct 1, 2024, at 1:18 PM, Regina Obe <lr at pcorp.us> wrote:
>>
>> This does not surprise me.  I think Paul wrote that note 15 years ago and even then I don't recall it making a significant difference and back then we weren't even using geographylib and think we are now even for sphere.
>> We should probably take that not out of the docs.
>>
>> Paul you have anything to say about the below? Does it make a difference maybe if you have a narly multipolygon with 5,000,000 points?
> No idea. But I bet if you trace the code you’ll find that area uses the geographiclib code in both spherical and spheroidal mode, it just sets the major and semi-major axes equal for the spherical mode. I haven’t confirmed that, but my best guess.
>
> P
>
>>> -----Original Message-----
>>> From: Marco Boeringa
>>> Subject: Sphere based calculations for geography don't appear to be faster?
>>>
>>> Hi,
>>>
>>> After the PostGIS 3.4.3 release, that fixed an issue with sphere based
>>> calculations for geography type, I decided to run a quick test to see if sphere
>>> based calculations would give a performance benefit for my workflow. Due to
>>> the existing bug, I had been using spheroid based calculations exclusively up to
>>> now.
>>>
>>> According to the Help (https://postgis.net/docs/ST_Area.html), the sphere
>>> based calculations should be faster. However, looking at the timings listed
>>> below, it appears there is no significant difference?
>>>
>>> SQL used:
>>>
>>> DROP MATERIALIZED VIEW IF EXISTS atest;
>>> CREATE MATERIALIZED VIEW atest AS SELECT
>>> ST_AREA(way::geography,true/false) FROM planet_osm_polygon
>>>
>>> The test data was the Geofabrik Italy extract, and used the polygon table of an
>>> osm2pgsql import containing +/- 19M records, containing polygons of varying
>>> sizes up to country size. The tests were run on a local dedicated system with
>>> plenty of IO and CPU.
>>>
>>> Marco
>>>
>>> *** SPHEROID ***:
>>> Start time    2024-10-01 08:07:05.984
>>> Finish time    2024-10-01 08:07:15.248
>>>
>>> Start time    2024-10-01 08:08:13.881
>>> Finish time    2024-10-01 08:08:24.362
>>>
>>> Start time    2024-10-01 08:08:42.772
>>> Finish time    2024-10-01 08:08:51.998
>>>
>>> Start time    2024-10-01 08:09:16.137
>>> Finish time    2024-10-01 08:09:26.484
>>>
>>> *** SPHERE ***
>>> Start time    2024-10-01 08:10:35.261
>>> Finish time    2024-10-01 08:10:45.846
>>>
>>> Start time    2024-10-01 08:11:05.856
>>> Finish time    2024-10-01 08:11:16.110
>>>
>>> Start time    2024-10-01 08:11:35.278
>>> Finish time    2024-10-01 08:11:44.499
>>>
>>> Start time    2024-10-01 08:12:18.769
>>> Finish time    2024-10-01 08:12:29.020