This catalog stores column default values.  The main information
   about columns is stored in pg_attribute
   (see below).  Only columns that explicitly specify a default value
   (when the table is created or the column is added) will have an
   entry here.
  
Table 3-6. pg_attrdef Columns
| Name | Type | References | Description | 
|---|
| adrelid | oid | pg_class.oid | The table this column belongs to | 
| adnum | int2 | pg_attribute.attnum | The number of the column | 
| adbin | text |  | An internal representation of the column default value | 
| adsrc | text |  | A human-readable representation of the default value |