[postgis-users] st_intersects and st_disjoint inconsistent results

[Emily Gouge]

Here you go.  Thanks!


SELECT ST_AsHEXEWKB (geometry) FROM test.eflowpath WHERE id = 
'889105be-5782-43f1-b50c-5a5825c83875'

01020000200912000007000000642F25DC75A24CC0E4DE5740FCB34840A7CEFE9B72A24CC09DA85B2CFBB34840B5519D0E64A24CC091FAA188FBB34840FA449E245DA24CC054C2137AFDB34840F4ACFFCE51A24CC09FEB562A03B448405328C1D144A24CC09A3DD00A0CB44840404C10C03CA24CC0EA07FE6910B44840

SELECT ST_AsHEXEWKB (geometry) FROM test.eflowpath WHERE id = 
'e5703673-a995-472e-b4b4-0280143eba0c'

0102000020091200000E00000004BE47A23CA24CC098A1F14410B448409871AEBC3FA24CC078341F2114B448400858AB764DA24CC09D0546031DB448406BFD3E2D50A24CC0BEDDEDD522B4484004824AA654A24CC02DC9A60128B44840EE377FB850A24CC0FA18BD642DB44840CCAF8B474EA24CC02CCCE78134B44840D7158E7B4EA24CC01D7C17A53AB44840ACFA01B452A24CC02688BA0F40B44840DB508C8752A24CC006CDF80846B44840A1F31ABB44A24CC0C891730756B44840009AF7EE45A24CC06B7649415CB448408C2ECAC749A24CC0CE57248161B44840A74302A150A24CC07DD00E1368B44840

On 2022-02-10 5:57 p.m., Paul Ramsey wrote:
> 
> 
>> On Feb 10, 2022, at 4:55 PM, Emily Gouge <egouge at refractions.net> wrote:
>>
>> I have a linear dataset on which I was building a query to find edges that are “very close” but don’t touch. While working on this query I found some unexpected results with the st_intersects and st_disjoint functions. As outlined below, the query returned true for both st_instersects and st_disjoint for a few geometries comparisons, but ONLY when a where clause was used to filter the geometries spatially. When unique identifiers were used to filter geometries only st_disjoint returns true.
>>
>> Versions:
>> Except where noted otherwise the results below reference testing on these versions:
>> POSTGIS="3.2.0 3.2.0" [EXTENSION] PGSQL="140" GEOS="3.10.1-CAPI-1.16.0" PROJ="7.2.1" LIBXML="2.9.9" LIBJSON="0.12" LIBPROTOBUF="1.2.1" WAGYU="0.5.0 (Internal)"
>> PostgreSQL 14.1, compiled by Visual C++ build 1914, 64-bit
>>
>> When I run this query:
>> select a.id, b.id,
>> st_intersects(a.geometry, b.geometry),
>> st_intersects(b.geometry, a.geometry),
>> st_disjoint(a.geometry, b.geometry),
>> st_disjoint(b.geometry, a.geometry)
>> from test.eflowpath a, test.eflowpath b
>> where a.id != b.id
>> and st_dwithin(a.geometry, b.geometry, 0.00001)
>> and st_disjoint(a.geometry, b.geometry);
>>
>> PostGIS 3.2: 50 rows were returned, but there are three rows that return true for both st_disjoint and st_intersects. Given the query this in itself is a bit odd as you'd expect reciprocal results for the pairs of the geometry (so at least 4 rows).
>> -------
>> 889105be-5782-43f1-b50c-5a5825c83875
>> e5703673-a995-472e-b4b4-0280143eba0c
>> true
>> true
>> true
>> true
>> -------
>> 0d05aabb-9ff3-4d8f-b6c7-b2b44b0868c8
>> 3a09b2af-5932-4e36-9e3e-d8109e5463fa
>> true
>> true
>> true
>> true
>> -------
>> 3a09b2af-5932-4e36-9e3e-d8109e5463fa
>> 0d05aabb-9ff3-4d8f-b6c7-b2b44b0868c8
>> true
>> true
>> true
>> true
>> -------
>> ...
>>
>> Note: In PostGIS 3.1.2 50 rows were returned by only two rows returned true for both disjoint and intersects.
>>
>> HOWEVER,
>> When I compare one pair of those edges specifically using the ids they are only disjoint (which is the result I would expect to see)
>>
>> select a.id, b.id,
>> st_intersects(a.geometry, b.geometry),
>> st_intersects(b.geometry, a.geometry),
>> st_disjoint(a.geometry, b.geometry),
>> st_disjoint(b.geometry, a.geometry)
>> from test.eflowpath a, test.eflowpath b
>> where b.id = '889105be-5782-43f1-b50c-5a5825c83875' and a.id = 'e5703673-a995-472e-b4b4-0280143eba0c'
>>
>> Results:
>> -------
>> e5703673-a995-472e-b4b4-0280143eba0c
>> 889105be-5782-43f1-b50c-5a5825c83875
>> false
>> false
>> true
>> true
>> -------
>>
>> I thought perhaps this had something to do with the indexes so I removed all geometry indexes from the table and re-ran the initial query. In postgis 3.2 this returned the same results as the indexed query.
>> Note: In PostGIS 3.1.2 this also returned three rows with intersects and disjoint true. As noted above with indexes in 3.1.2 only 2 rows were returned where intersects and disjoint were true.
>>
>> Similar results occurred if st_dwithin from the where statement was increased to 0.01.
>>
>> Test Data: When I made a table with only the edges in question all queries returned expected results: st_intersects is false and st_disjoint is true. As a result providing a small test case for this issue doesn’t seem possible. But I am happy to provide all the data - there are 27,444 rows.
>>
>> While this isn’t a problem for me, I find it unexpected that the results from st_intersects and st_disjoint of two geometries would be different based on the where clause in the query (and the data in the table).
>>
>> Thoughts?
> 
> The fact that a single test case returns one result, but results in a larger set returns another says to me that likely the issue in different code lines because of different cache behaviour. When you do the single test case you get a brute force intersects. When you do several in a batch, you get prepared geometry (at least, for the cases that happen after caching).
> 
> If you can provide the HEXWKB of the two geometries that showed disagreement (where b.id = '889105be-5782-43f1-b50c-5a5825c83875' and a.id = 'e5703673-a995-472e-b4b4-0280143eba0c') we can set up a test case in GEOS that compares the normal and prepared geometry calls and see if that's the problem.
> 
> P.
> 
> 
>>
>> Thanks,
>> Emily
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> 
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