While looking at Vik Reykja's pending patch to improve the FK triggers
by skipping processing when a NULL column didn't change, I started to
wonder whether that really had no user-visible semantic effect.
In particular, in ON UPDATE SET NULL/SET DEFAULT cases, it seemed like
this could change the set of referencing columns that get set to NULL
or to their defaults.  So the next question was which behavior actually
conforms to the SQL standard, and the answer to that is ... disturbing.

The code in ri_triggers.c was written against SQL92's definition of ON
UPDATE SET NULL/SET DEFAULT, which is (ignoring the MATCH PARTIAL case,
which we don't implement):

         6) If an <update rule> is specified and a non-null value of a ref-
            erenced column in the referenced table is updated to a value
            that is distinct from the current value of that column, then

            a) If <match type> is not specified or if FULL is specified,
              then

             ii) If the <update rule> specifies SET NULL, then

                 Case:

                 1) If <match type> is not specified, then in all matching
                   rows the referencing column that corresponds with the
                   referenced column is set to the null value.

                 2) If <match type> specifies FULL, then in all matching
                   rows each referencing column is set to the null value.

            iii) If the <update rule> specifies SET DEFAULT, then in all
                 matching rows the referencing column that corresponds with
                 the referenced column is set to the default value specified
                 in the General Rules of Subclause 11.5, "<default clause>".

Note that only in the MATCH FULL + SET NULL case does it say to set
*all* the referencing columns in each matching row.  Otherwise, you are
only supposed to change columns that correspond to referenced columns
that were changed.  It's notable that SET NULL and SET DEFAULT have
different behaviors here.

On the other hand, in SQL:2008 I find (some boilerplate text omitted):

        10) If a non-null value of a referenced column RC in the
        referenced table is updated to a value that is distinct from the
        current value of RC, then, for every member F of the subtable
        family of the referencing table:

        Case:

          a) If M specifies SIMPLE or FULL, then

            Case:

                ii) If UR specifies SET NULL, then

                  Case:

                        1) If M specifies SIMPLE, then each matching row
                        MR in F is paired with the candidate replacement
                        row NMR, formed by copying MR and setting each
                        referencing column in the copy to the null
                        value. MR is identified for replacement by NMR
                        in F.

                        2) If M specifies FULL, then each matching row
                        MR in F is paired with the candidate replacement
                        row NMR, formed by copying MR and setting each
                        referencing column in the copy to the null
                        value. MR is identified for replacement by NMR
                        in F.

                iii) If UR specifies SET DEFAULT, then each matching row
                MR in F is paired with the candidate replacement row
                NMR, formed by copying MR and setting each referencing
                column in the copy to the default value specified in the
                General Rules of Subclause 11.5, "<default clause>". MR
                is identified for replacement by NMR in F.

So far as I can see, this says to set *all* referencing columns to
nulls or their defaults, in all four cases, whether the corresponding
referenced column was one that changed or not.  This is very clearly
different from what SQL92 says.  It's also rather curious that they
distinguish two "cases" for SET NULL when the texts are exactly alike.

It looks to me like this change occurred in SQL:2003, although
SQL:1999's version of the text is such badly written pseudo-mathematical
gobbledygook that it's a bit hard to tell which behavior they meant.
However, neither of those specs list any change in referential
constraint behavior as being an acknowledged incompatibility with the
prior standard.  Have the SQL committee simply failed to notice that in
whacking this text around they changed the meaning?  Which behavior is
actually implemented by other RDBMSes?

                        regards, tom lane

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