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. Use pg_get_expr(adbin, adrelid) to
                convert it to an SQL expression. |