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Method works correct in QGIS 2. I found some differences between pyqgis2 API: qgis.org/api/2.18/qgsvectorlayer_8cpp_source.html (QGIS2 :line 2732) and pyqgis3 API: qgis.org/api/2.18/qgsvectorlayer_8cpp_source.html (QGIS2 :line 2732) And one more interesting fact: Working time increase proportionally to the number of user-selected features (Working_time ≈ n*5 min, where n - number of user-selected features). And I still did not find solution this problem. If someone have some idea please share with me.
