Re: Inheritance

Jan Wieck wrote:
  >Oliver Elphick wrote:
  >> Jan Wieck wrote:
  >>   >> Yes, but what about referential integrity? Can I have a table column
  >>   >> reference a column from Entities*? In my experimentation, this is not
      > the
  >>   >> case.
  >>   >
  >>   >    Referential integrity doesn't work with inheritance, and will
  >>   >    not in 7.1.
  >>   >
  >>   >    It isn't possible to put a unique constraint on a  column  of
  >>   >    Entities*  (AFAIK).  So that a SELECT pkey FROM Entities will
  >>   >    never  return  any  duplicates?  The  RI  implementation   of
  >>   >    PostgreSQL  doesn't  insist  on  such  a unique constraint to
  >>   >    exist up to now, but it is required by the SQL specs and thus
  >>   >    we'll do so someday.
  >>
  >> The corollary of using RI on an inheritance tree is that there should be a
  >> unique index on the primary keys of the whole tree.  If constraints could
      >be
  >> inherited, this would become available.
  >
  >    The  UNIQUE  constraint  is  implemented as a UNIQUE INDEX in
  >    PostgreSQL. And exactly what you  say:  "ONE"  index  on  the
  >    whole tree would be required to do it.
  >
  >    What  you're  asking  for  is index inheritance instead of RI
  >    trigger inheritance first - right?  So  that  an  index  will
  >    contain  tuples  for  all  the  keys  present  in it's tables
  >    subclasses, for beeing able to detect a dupkey.

Yes, at least as an option.  In many cases of inheritance the id should be
unique in the whole tree, and so a unique index on the whole tree is
needed.  If this index is present, the tree can be available for RI use.
There may be cases where a unique index is not wanted, but in that case the
cost is that RI becomes unavailable except on individual tables.

--
Oliver Elphick                                Oliver.Elphick@lfix.co.uk
Isle of Wight                              http://www.lfix.co.uk/oliver
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