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<p class="MsoNormal"><span lang="EN-US">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Is it normal that ST_Polygonize() gives two different results depending on if the linestrings are ST_Union() before calling it or not?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">In the following example, test 1 (no union) and 3 (ST_Collect) give the same result, constructing two overlapping polygons. Test 2 (ST_Union) on the other side constructs 3 disjoint polygons removing overlaps. All tests
provide, however, the same basic set of linestrings to ST_Polygonize.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">WITH polys AS (<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> SELECT ST_GeomFromText('POLYGON((1 1, 3 2, 3 0, 1 1))') geom<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> UNION ALL<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> SELECT ST_GeomFromText('POLYGON((2 1, 4 2, 4 0, 2 1))') geom<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">), extrings AS (<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">SELECT 1 test, ST_ExteriorRing(geom) geom<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">FROM polys<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">UNION ALL<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">SELECT 2 test, ST_Union(ST_ExteriorRing(geom)) geom<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">FROM polys<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">UNION ALL<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">SELECT 3 test, ST_Collect(ST_ExteriorRing(geom)) geom<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">FROM polys<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">SELECT test, ST_AsText(ST_Polygonize(geom))<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">FROM extrings<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">GROUP BY test;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">It seems to me that only test 2 (ST_Union) returns the expected result which is to reconstruct planar polygons from a set of linestrings however they are provided.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">It bothers me because I would like to get the same result as test 2 without relying on ST_Union() which seems to sometime slightly shift vertexes in order to union geometries together (true or false?).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Thanks for any explanation.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Pierre<o:p></o:p></span></p>
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