Comparing two postgres tables and updating one with differences.

I have two tables in a postgres database
  Table 1
        newacctnumber
        oldaccountnumber
        date
  Table 2
        newacctnumber
        oldaccountnumber
        done
Table 2 contains all the newacctnumber/oldaccountnumber entrees that exist in table 1.  Some other steps are take and then the done is set to Y.

Table 1 is automatically generated each week, resulting in additional entreens being added.

What I need to do is update table 2 with new entrees that are in Table 1 but not already in Table 2.

This will be done thru an online page (administrators page) with php code.  I have one method but it seems slow to me.

      $query="Select * from table1";
      $db = "poscoid";
      $result = db_query($query, $db);
      
      while ($r=pg_fetch_array($result))
               {
               $newacctnumber = $r["newacctnumber"];
               $oldacctnuber = $r["oldacctnumber"];
   
    $query="Insert into table2(newacctnumber, oldacctnumber) values($newacctnumber,$oldacctnumber) where $newacctnumber not in (select newacctnumber from table2 where done = 'N')";
      
      $db = "poscoid";
      $updateresult = db_query($query, $db);
      
          }