alter table add column implementation undesirable? - Mailing list pgsql-sql

A coworker told me that the postgres implementation of ALTER TABLE ADD
COLUMN creates an inefficient database.  He said it results in a table
whose new column is stored someplace other than the rest of the columns.
(A hidden auxiliary table?)  Is this true in postgres 6.5.3?  7.x?  Was it
ever true?

Forest Wilkinson