title: ERROR: operator is not unique: unknown + unknown
body: <p>I have a PostGIS database and I have to compute new values for rows in a new column. These values should be the average of the values of several columns. So I do the query:</p>
  
  <pre><code>INSERT INTO bdps (da_m) 
  VALUES (avg('da_1'+'da_2'+'da_3'+'da_4'+'da_5'+'da_6'+'da_7'));
  </code></pre>
  
  <p>In this query bdps is my database, da_m is the new column and da_1 to da_7 are existing columns that have double precision type.</p>
  
  <p>da_m was created using </p>
  
  <p><code>ALTER TABLE bdps ADD COLUMN da_m double precision;</code></p>
  
  <p>I get the following error:</p>
  
  <pre><code>    ERROR:  operator is not unique: unknown + unknown
  LINE 2:  VALUES (avg('da_1'+'da_2'+'da_3'+'da_4'+'da_5'+'da_6'+'da_7...
                             ^
  HINT:  Could not choose a best candidate operator. You might need to add explicit type casts.
  
  ********** Error **********
  
  ERROR: operator is not unique: unknown + unknown
  SQL state: 42725
  Hint: Could not choose a best candidate operator. You might need to add explicit type casts.
  Character: 45
  </code></pre>
  
  <p>I google it and It has something to do with casts.
  I run PostGreSQL 8.3 and the database is under PostGIS 1.5, on Windows 7.</p>