st_linemerge:：将输入的MultiLineString连接成一个或多个LineString。

Description

Returns a LineString or MultiLineString formed by joining together the line elements of a MultiLineString. Lines are joined at their endpoints at 2-way intersections. Lines are not joined across intersections of 3-way or greater degree.

http://postgis.net/docs/ST_LineMerge.html

https://help.aliyun.com/document_detail/408017.html