PostGIS: ERROR: lwpoly_from_lwlines: shell must have at least 4 points

Question

I have achieved to close lines in order to build a polygon with:

st_addpoint(
    st_makeline(geom ORDER BY .cd_noseq), 
    st_pointn(
        st_makeline(geom ORDER BY cd_noseq), 
    1)
)

but then, when I try to make a polygon with st_makepolygon like this:

st_makepolygon(
    st_addpoint(
        st_makeline(geom ORDER BY cd_noseq), 
        st_pointn(
            st_makeline(geom ORDER BY cd_noseq), 
        1)
    )
) as geom

I get the following error:

ERROR:  lwpoly_from_lwlines: shell must have at least 4 points
********** Error **********

ERROR: lwpoly_from_lwlines: shell must have at least 4 points
SQL state: XX000

Technology:

PostgreSQL 9.2.7 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-4), 64-bit" "POSTGIS="2.1.1 r12113" GEOS="3.4.2-CAPI-1.8.2 r3921" PROJ="Rel. 4.8.0, 6 March 2012" GDAL="GDAL 1.9.2, released 2012/10/08" LIBXML="2.7.6" LIBJSON="UNKNOWN" (core procs from "2.1.0 r11822" need upgrade) TOPOLOGY (topology procs from "2.1.0 r11822" need upg (…)

Answer 1

Your query looks good. According to the source the error message means exactly what it says. One of your linestrings doesn't have at least 4 points.

Check your results with ST_NumPoints!