diff --git a/app/server/datasource/mongodb/type_mapping.go b/app/server/datasource/mongodb/type_mapping.go
index c12bd8a0..5a67e642 100644
--- a/app/server/datasource/mongodb/type_mapping.go
+++ b/app/server/datasource/mongodb/type_mapping.go
@@ -81,22 +81,19 @@ func bsonToYqlColumn(docs []bson.Raw, doSkipUnsupported bool, logger *zap.Logger
 			}
 
 			prevType, prevTypeExists := deducedTypes[key]
-			prevTypeString := prevType.String()
 
 			t, err := typeMap(elem.Value(), logger)
 			if err != nil {
 				if errors.Is(err, errNull) {
-					if !prevTypeExists {
-						ambiguousFields[key] = struct{}{}
-					}
+					ambiguousFields[key] = struct{}{}
 
 					continue
 				} else if errors.Is(err, errEmptyArray) {
 					if prevTypeExists && prevType.GetListType() == nil {
 						deducedTypes[key] = common.MakePrimitiveType(Ydb.Type_UTF8)
 
-						logger.Debug(fmt.Sprintf("bsonToYqlColumn: keeping serialized %v. prev: %v curr: []", key, prevTypeString))
-					} else if !prevTypeExists {
+						logger.Debug(fmt.Sprintf("bsonToYqlColumn: keeping serialized %v. prev: %v curr: []", key, prevType.String()))
+					} else {
 						ambiguousFields[key] = struct{}{}
 					}
 
@@ -116,23 +113,21 @@ func bsonToYqlColumn(docs []bson.Raw, doSkipUnsupported bool, logger *zap.Logger
 
 			tString := t.String()
 
-			if !prevTypeExists {
+			if prevTypeExists && prevType.String() != tString {
+				deducedTypes[key] = common.MakePrimitiveType(Ydb.Type_UTF8)
+
+				logger.Debug(fmt.Sprintf("bsonToYqlColumn: keeping serialized %v. prev: %v curr: %v", key, prevType.String(), tString))
+			} else {
 				deducedTypes[key] = t
 
 				logger.Debug(fmt.Sprintf("bsonToYqlColumn: new column %v %v", key, tString))
-			} else if prevTypeString != tString {
-				deducedTypes[key] = common.MakePrimitiveType(Ydb.Type_UTF8)
-
-				logger.Debug(fmt.Sprintf("bsonToYqlColumn: keeping serialized %v. curr: %v prev: %v", key, prevTypeString, tString))
 			}
 		}
 	}
 
-	if !doSkipUnsupported {
-		for field := range ambiguousFields {
-			if _, ok := deducedTypes[field]; !ok {
-				deducedTypes[field] = common.MakePrimitiveType(Ydb.Type_UTF8)
-			}
+	for field := range ambiguousFields {
+		if _, ok := deducedTypes[field]; !ok {
+			deducedTypes[field] = common.MakePrimitiveType(Ydb.Type_UTF8)
 		}
 	}
 
@@ -140,7 +135,7 @@ func bsonToYqlColumn(docs []bson.Raw, doSkipUnsupported bool, logger *zap.Logger
 
 	for columnName, deducedType := range deducedTypes {
 		if columnName == idColumn {
-			columns = append(columns, &Ydb.Column{Name: columnName, Type: deducedTypes[columnName]})
+			columns = append(columns, &Ydb.Column{Name: columnName, Type: deducedType})
 		} else {
 			columns = append(columns, &Ydb.Column{Name: columnName, Type: common.MakeOptionalType(deducedType)})
 		}