shp2pgsql uses deprecated addGeometryColumn function

Question

It is very likely that I am misunderstanding the documentation as I'm new to PostGIS, but it says here regarding the "addGeometryColumn" function that since version 2.0.0 of PostGIS:

This function no longer updates geometry_columns [...] So for example building a wgs84 POINT column with this function is now equivalent to: ALTER TABLE some_table ADD COLUMN geom geometry(Point,4326);

However, the command line tool "shp2pgsql" which was shipped with the "postgis" package (I am on Ubuntu 16.04 LTS xenial) generates SQL containing the addGeometryColumn() function, for example:

SELECT AddGeometryColumn('','mytestgeo','geom','26918','POINT',2);

I am definitely using PostGIS 2.5:

SELECT postgis_version();
2.5 USE_GEOS=1 USE_PROJ=1 USE_STATS=1

Since the shp2pgsql tool was shipped with the postgis package, I would have expected it to conform to PostGIS 2.5 specification. Why then does it still use this method? Is it for backwards compatibility? Have I misunderstood the docs?

Answer 1

The AddGeometryColumn is not deprecated. It is a function that validates and transform your input, and wraps them in a valid ALTER TABLE statement.

First, let's note that you can create the geometry column at the same time as the table, or later by altering it:

CREATE TABLE testgeo (id integer, geom geometry(point,3857));
ALTER TABLE testgeo ADD COLUMN geom2 geometry(point,3857);

AddGeometryColumn allows specifying the geometry base name and the desired number of dimension, while a manual creation requires the exact name:

 SELECT addgeometrycolumn('testgeo','geom3','3857','point',3);
 ALTER TABLE testgeo ADD COLUMN geom4 geometry(pointZ,3857);

AddGeometryColumn will also validate the input SRID.

Let's find the smallest valid SRID:

select min(srid) from spatial_ref_sys;
 min
------
 2000
(1 row)


-- create a point column with SRID 1999 (invalid)

--OK 
ALTER TABLE testgeo ADD COLUMN geom5 geometry(point,1999);

--FAIL 
select addgeometrycolumn('testgeo ','geom6','1999','point',2);

ERROR:  AddGeometryColumn() - invalid SRID

And we finally get:

\d testgeo
                     Table "public.testgeo"
 Column |         Type          | Collation | Nullable | Default
--------+-----------------------+-----------+----------+---------
 id     | integer               |           |          |
 geom   | geometry(Point,3857)  |           |          |
 geom2  | geometry(Point,3857)  |           |          |
 geom3  | geometry(PointZ,3857) |           |          |
 geom4  | geometry(PointZ,3857) |           |          |
 geom5  | geometry(Point,1999)  |           |          |

So AddGeometryColumn is doing more than a simple ALTER TABLE, but it is not required either.