pg_attrdef   The catalog pg_attrdef 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 51.6. pg_attrdef Columns
| Name | Type | References | Description | 
|---|---|---|---|
| oid | oid | Row identifier (hidden attribute; must be explicitly selected) | |
| adrelid | oid |  | The table this column belongs to | 
| adnum | int2 |  | The number of the column | 
| adbin | pg_node_tree | The internal representation of the column default value | |
| adsrc | text | A human-readable representation of the default value | 
    The adsrc field is historical, and is best
    not used, because it does not track outside changes that might affect
    the representation of the default value.  Reverse-compiling the
    adbin field (with pg_get_expr for
    example) is a better way to display the default value.