PostGIS the spatial extension to the PostgreSQL database provides a host of functions for querying, creating and manipulating geometries within the database. ST_Dump allows you to disaggregate collection and multi geometries such as MultiPolygon and MultiLineSting easily. The example below demonstrates starting with a table that contains one row with a MultiLineString geometry and another with a LineString and running a query to disaggregate returning three rows all with a LineString geometry.

Create a table for our demo data:

CREATE TABLE complex
(
   id serial,
   "name" text,
   geom geometry
);

Insert one row with a MultiLineString with two parts ('Bob') and another with a single LineString ('Harry')

INSERT INTO complex (name, geom)
  VALUES (
    'Bob',
    ST_GeomFromEWKT('MULTILINESTRING((498376.89 651569.6,498372.28 651571.89),(498372.28 651571.89,498371.77 651576.05))')
  );

INSERT INTO complex (name, geom)
  VALUES (
    'Harry',
    ST_GeomFromEWKT('LINESTRING(598376.89 751569.6,398372.75 658771.03)')
  );

Query to show we have two rows one of which has a MultiLineString geometry:

SELECT *, ST_GeometryType(geom) as geom_type, ST_AsEWKT(geom) as geom_wkt from complex;

Results:

Use the ST_Dump function to split the row with a MultiLineString into two rows each with a LineString and generate a decimal id composed of the original rows id and the LineString's position in the MultiLineString as the fractional part so that each row still has a unique id that's derived from it's original id:

SELECT
  COALESCE((simple.id || '.' || simple.path[1]::text)::float, simple.id) as id,
  simple.name,
  simple.simple_geom as geom,
  ST_GeometryType(simple.simple_geom) as geom_type,
  ST_AsEWKT(simple.simple_geom) as geom_wkt
FROM (
  SELECT
    dumped.*,
    (dumped.geom_dump).geom as simple_geom,
    (dumped.geom_dump).path as path
  FROM (
    SELECT *, ST_Dump(geom) AS geom_dump FROM complex
  ) as dumped
) AS simple;

Results: