What is the reason not to use select *?

Question

I\'ve seen a number of people claim that you should specifically name each column you want in your select query.

Assuming I\'m going to use all of the columns anyway

Answer 1

SELECT * is not always evil. In my opinion, at least. I use it quite often for dynamic queries returning a whole table, plus some computed fields.

For instance, I want to compute geographical geometries from a "normal" table, that is a table without any geometry field, but with fields containing coordinates. I use postgresql, and its spatial extension postgis. But the principle applies for many other cases.

An example:

• a table of places, with coordinates stored in fields labeled x, y, z:

  CREATE TABLE places (place_id integer, x numeric(10, 3), y numeric(10, 3), z numeric(10, 3), description varchar);

• let's feed it with a few example values:

  INSERT INTO places (place_id, x, y, z, description) VALUES
  (1, 2.295, 48.863, 64, 'Paris, Place de l\'Étoile'),
  (2, 2.945, 48.858, 40, 'Paris, Tour Eiffel'),
  (3, 0.373, 43.958, 90, 'Condom, Cathédrale St-Pierre');

• I want to be able to map the contents of this table, using some GIS client. The normal way is to add a geometry field to the table, and build the geometry, based on the coordinates. But I would prefer to get a dynamic query: this way, when I change coordinates (corrections, more accuracy, etc.), the objects mapped actually move, dynamically. So here is the query with the SELECT *:

  CREATE OR REPLACE VIEW places_points AS
  SELECT *,
  GeomFromewkt('SRID=4326; POINT ('|| x || ' ' || y || ' ' || z || ')')
  FROM places;

  Refer to postgis, for GeomFromewkt() function use.

• Here is the result:

  SELECT * FROM places_points;

 place_id |   x   |   y    |   z    |         description          |                            geomfromewkt                            
----------+-------+--------+--------+------------------------------+--------------------------------------------------------------------  
        1 | 2.295 | 48.863 | 64.000 | Paris, Place de l'Étoile     | 01010000A0E61000005C8FC2F5285C02405839B4C8766E48400000000000005040  
        2 | 2.945 | 48.858 | 40.000 | Paris, Tour Eiffel           | 01010000A0E61000008FC2F5285C8F0740E7FBA9F1D26D48400000000000004440
        3 | 0.373 | 43.958 | 90.000 | Condom, Cathédrale St-Pierre | 01010000A0E6100000AC1C5A643BDFD73FB4C876BE9FFA45400000000000805640
(3 lignes)

The rightmost column can now be used by any GIS program to properly map the points.

• If, in the future, some fields get added to the table: no worries, I just have to run again the same VIEW definition.

I wish the definition of the VIEW could be kept "as is", with the *, but hélas it is not the case: this is how it is internally stored by postgresql:

SELECT places.place_id, places.x, places.y, places.z, places.description, geomfromewkt(((((('SRID=4326; POINT ('::text || places.x) || ' '::text) || places.y) || ' '::text) || places.z) || ')'::text) AS geomfromewkt FROM places;