Merge reversed linestring in correct order in postGIS

Question

I want to merge linestring in the correct order in postGIS, but sometime they are reversed so something like this :

select astext(linemerge(ST_union(
ST_GeomFromText('LINESTRING(3 3, 2 2)'),
ST_GeomFromText('LINESTRING(1 1, 2 2)'))));

returns this :

"LINESTRING(1 1,2 2,3 3)"

But I want this :

"LINESTRING(3 3, 2 2, 1 1)"

I really don't know what to do.

Answer 1

If you just want to reverse a geometry you can just use ST_Reverse (returns the geometry with vertex order reversed).

If the problem is you want to keep some arbitrary order, I don't think you can do it using ST_Union (the definition for ST_Union being point set union, as in ordering is not guaranteed).

Answer 2

Give it to diciu. I just want to add an example:

select ST_AsText(ST_Reverse(ST_GeomFromText('LINESTRING(3 3, 2 2)')));