The ST_IsSimple() function

The table issimple_test is created with two columns. The pid column is a SMALLINT containing the unique identifier for each row. The g1 ST_Geometry column stores the simple and nonsimple geometry samples:

CREATE TABLE issimple_test (pid smallint,
                            g1  ST_Geometry);

The following INSERT statements insert two records into the issimple_test table. The first is a simple linestring because it does not intersect its interior. The second is non-simple because it does intersect its interior:

INSERT INTO issimple_test VALUES(
   1,
   ST_LineFromText('linestring (10 10, 20 20, 30 30)',1000)
);

INSERT INTO issimple_test VALUES(
   2,
   ST_LineFromText('linestring (10 10,20 20,20 30,10 30,10 20,20 10)',1000)
);

The query returns the results of the ST_IsSimple() function. The first record returns t because the linestring is simple, while the second record returns f because the linestring is not simple:

SELECT pid, ST_IsSimple(g1) Is_it_simple 
   FROM issimple_test;

   pid is_it_simple 

     1            t
     2            f