Postgresql – How to alter column from float(8) to numeric(64,8)

Yes, hacking into the catalog is bad. Reason #1 is that if you upgrade to new version and forget to move the hack, things start breaking. Just running pg_dump and loading to the same version on another instance will also lose the hack. There's also always the chance that a new version of Postgres will change so much that your hack is now not possible and force you to go back and re-engineer.

Overriding with your own function is the correct way to go.

This looks ugly:

  with uu(unit, coefficient, u_ord) as (
    select
     unit, 
     coefficient,
     case 
      when log(u.coefficient) < 0 
      then floor (log(u.coefficient)) 
      else ceil(log(u.coefficient)) 
     end u_ord
    from
     unit_conversion u 
  ),
  norm (label, norm_qty) as (
   select
    s.label,
    sum( uc.coefficient * s.quantity ) AS norm_qty
  from
    unit_conversion uc,
    substance s
  where
    uc.unit = s.unit
  group by
    s.label
  ),
  norm_ord (label, norm_qty, log, ord) as (
   select 
    label,
    norm_qty, 
    log(t.norm_qty) as log,
    case 
     when log(t.norm_qty) < 0 
     then floor(log(t.norm_qty)) 
     else ceil(log(t.norm_qty)) 
    end ord
   from norm t
  )
  select
   norm_ord.label,
   norm_ord.norm_qty,
   norm_ord.norm_qty / uu.coefficient val,
   uu.unit
  from 
   norm_ord,
   uu where uu.u_ord = 
     (select max(uu.u_ord) 
      from uu 
      where mod(norm_ord.norm_qty , uu.coefficient) = 0);

but seems to do the trick:

|   LABEL | NORM_QTY | VAL |       UNIT |
-----------------------------------------
| mercury |   1.5e-8 |  15 | microlitre |
|   water |  0.00112 | 112 | centilitre |

You don't really need the parent-child relationship in the unit_conversion table, because the units in the same family are naturally related to each other by the order of coefficient, as long as you have the family identified.