pg_attrdef
   The catalog pg_attrdef stores column default
   values.  The main information about columns is stored in
   pg_attribute.
   Only columns for which a default value has been explicitly set will have
   an entry here.
  
Table 51.6. pg_attrdef Columns
| Name | Type | References | Description | 
|---|---|---|---|
| oid | oid | Row identifier | |
| adrelid | oid |  | The table this column belongs to | 
| adnum | int2 |  | The number of the column | 
| adbin | pg_node_tree | The column default value, in nodeToString()representation.  Usepg_get_expr(adbin, adrelid)to
      convert it to an SQL expression. |