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LineString decomposition

This process is also similar to MultiPoint decomposition. The points can be extracted one at a time or dumped into a geometry dump. Aside from these generic methods, there are also special functions for finding the start or end point of a line.

First, let's find the count of the vertices in a LineString. A generic ST_NPoints function can be used for that:

        SELECT
        ST_NPoints(ST_MakeLine(ST_MakePoint(20,50),
        ST_MakePoint(19.95,49.98)));

        2

The ST_NPoints works for any geometry type--not just for LineStrings, but also LinearRings, polygons, their multi-variants, and even for points, in which case it will return 1. There is also a ST_NumPoints function, which works for LineStrings only.

The individual points are extracted from a LineString by the ST_PointN function, which is not unlike ST_GeometryN used for Multi-geometry decomposition.

        SELECT 
        ST_AsText(ST_PointN(ST_MakeLine(ARRAY[
        ST_MakePoint(20,50),ST_MakePoint(19.95,49.98),     
        ST_MakePoint(19.90,49.96)]),2));

        POINT(19.95 49.98)

Dumping the points into a set of rows is also similar to dumping a Multi-geometry. The only difference is that the function used is named ST_DumpPoints. The geometry dump compound type is a direct output from this function:

        SELECT
        ST_DumpPoints(ST_MakeLine(ARRAY[
        ST_MakePoint(20,50),ST_MakePoint(19.95,49.98),   
        ST_MakePoint(19.90,49.96)]));

        st_dumppoints                     
        --------------------------------------------------
       ({1},010100000000000000000034400000000000004940)
       ({2},01010000003333333333F333403D0AD7A370FD4840)
       ({3},01010000006666666666E633407B14AE47E1FA4840)

And for accessing the vertices' geometry, we can do the following:

         SELECT 
         (ST_DumpPoints(ST_MakeLine(ARRAY[
         ST_MakePoint(20,50),ST_MakePoint(19.95,49.98),    
         ST_MakePoint(19.90,49.96)]))).geom;
                            geom                   
        --------------------------------------------
        010100000000000000000034400000000000004940
        01010000003333333333F333403D0AD7A370FD4840
        01010000006666666666E633407B14AE47E1FA4840

Finally, there are specialized methods for finding the first and last vertices of LineStrings. They are named ST_StartPoint and ST_EndPoint:

        SELECT
        ST_AsText(ST_StartPoint(ST_MakeLine(ARRAY[
        ST_MakePoint(20,50),ST_MakePoint(19.95,49.98),   
        ST_MakePoint(19.90,49.96)])));

        POINT(20 50)

        SELECT
        ST_AsText(ST_EndPoint(ST_MakeLine(ARRAY[
        ST_MakePoint(20,50),ST_MakePoint(19.95,49.98), 
        ST_MakePoint(19.90,49.96)])));

        POINT(19.9 49.96)