Re: a request for some experienced input..... - Mailing list pgsql-general

On Tue, 14 Nov 2000, Beth K. Gatewood wrote:

> >
>
> Stephan-
>
> Thank you so much for taking the effort to answer this these questions.  You
> help is truly appreciated....
>
> I just have a few points for clarification.
>
> >
> > MATCH PARTIAL is a specific match type which describes which rows are
> > considered matching rows for purposes of meeting or failing the
> > constraint.  (In match partial, a fktable (NULL, 2) would match a pk
> > table (1,2) as well as a pk table (2,2).  It's different from match
> > full in which case (NULL,2) would be invalid or match unspecified
> > in which case it would match due to the existance of the NULL in any
> > case).  There are some bizarre implementation details involved with
> > it and it's different from the others in ways that make it difficult.
> > It's in my list of things to do, but I haven't come up with an acceptable
> > mechanism in my head yet.
>
> Does this mean, currently that I can not have foreign keys with null values?

Not exactly...

Match full = In FK row, all columns must be NULL or the value of each
    column must not be null and there is a row in the PK table where
    each referencing column equals the corresponding referenced
    column.

Unspecified = In FK row, at least one column must be NULL or each
    referencing column shall be equal to the corresponding referenced
    column in some row of the referenced table

Match partial is similar to match full except we ignore the null columns
 for purposes of the each referencing column equals bit.

For example:
           PK Table Key values: (1,2), (1,3), (3,3)
 Attempted FK Table Key values: (1,2), (1,NULL), (5,NULL), (NULL, NULL)
 (hopefully I get this right)...
 In match full, only the 1st and 4th fk values are valid.
 In match partial, the 1st, 2nd, and 4th fk values are valid.
 In match unspecified, all the fk values are valid.

The other note is that generally speaking, all three are basically the
same for the single column key.  If you're only doing references on one
column, the match type is mostly meaningless.

> > PENDANT adds that for each row of the referenced table the values of
> > the specified column(s) are the same as the values of the specified
> > column(s) in some row of the referencing tables.
>
> I am not sure I know what you mean here.....Are you saying that the value for
> the FK column must match the value for the PK column?

I haven't really looked at PENDANT, the above was just a small rewrite of
some descriptive text in the sql99 draft I have.  There's a whole bunch
of rules in the actual text of the referential constraint definition.

The base stuff seems to be: (Rf is the referencing columns, T is the
referenced table)

      3) If PENDANT is specified, then:
         a) For a given row in the referencing table, let pendant
           reference designate an instance in which all Rf are
           non-null.

         b) Let number of pendant paths be the number of pendant
           references to the same referenced row in a referenced table
           from all referencing rows in all base tables.

         c) For every row in T, the number of pendant paths is equal to
       or greater than 1.

So, I'd read it as every row in T must have at least one referencing row
in some base table.

There are some details about updates and that you can't mix PENDANT and
MATCH PARTIAL or SET DEFAULT actions.

> > The main issues in 7.0 are that older versions (might be fixed in
> > 7.0.3) would fail very badly if you used alter table to rename tables that
> > were referenced in a fk constraint and that you need to give update
> > permission to the referenced table.  For the former, 7.1 will (and 7.0.3
> > may) give an elog(ERROR) to you rather than crashing the backend and the
> > latter should be fixed for 7.1 (although you still need to have write
> > perms to the referencing table for referential actions to work properly)
>
> Are the steps to this outlined somewhere then?

The permissions stuff is just a matter of using GRANT and REVOKE to set
the permissions that a user has to a table.