qgis - Creating polygons from line segments using PostgreSQL and PostGIS

Is there a PostGIS function that creates polygons from multiple line segments?

I want to convert the pictured thirteen lines to four polygons. All lines belong to the table "boundary" (gid, geom). I've categorized them by the gid column for a better illustration.

Is ST_MakePolygon what I'm looking for? Can anyone help me with the SQL syntax that QGIS requires for loading the polygons?

Edit: changed "closed line geometries" to "multiple line segments"

Answer

ST_Polygonize will do the job:

CREATE VIEW boundarypolygons AS
SELECT 
   g.path[1] as gid, 
   g.geom::geometry(polygon, 31492) as geom 
FROM
   (SELECT 

     (ST_Dump(ST_Polygonize(geom))).* 
   FROM boundary
) as g;