Merging features in PostgreSQL

Question

I am using PostGIS on pgAdmin3 (PostgreSQL 9.3 e PostGIS 2.2). I have a shapefile with more than 500.000 features and I want to merge them into one.

The geometries are MultiPolygon.

I would like to have as output a shapefile with the same geometry's complexity like the input. I Have to simply the geometry after a merge...

I used several codes but none of them seem to work.

All these queries do not return what I would aspect: I have a table with the same number of feature than before.

CREATE TABLE test1 AS 
    SELECT gid, fid, (ST_DUMP(geom)).geom AS geom FROM land_polygons;

or

CREATE TABLE test2 AS
SELECT
  1::INT as gid,
  ST_Buffer(ST_Collect(geom),0)::geometry(MultiPolygon,4326)
FROM
  land_polygons;

or

CREATE TABLE test3 AS
SELECT gid, fid, ST_GeometryN(geom, generate_series(1, ST_NumGeometries(geom))) AS geom 
FROM land_polygons

or

CREATE TABLE test4 AS SELECT ST_Union(ST_SnapToGrid(geom,0.0001)) FROM land_polygons;

I do not know what I could do more...