Re: MERGE ... RETURNING - Mailing list pgsql-hackers

On Wed, 6 Mar 2024 at 08:51, Peter Eisentraut <peter@eisentraut.org> wrote:
>
> For comparison with standard SQL (see <data change delta table>):
>
> For an INSERT you could write
>
> SELECT whatever FROM NEW TABLE (INSERT statement here)
>
> or for an DELETE
>
> SELECT whatever FROM OLD TABLE (DELETE statement here)
>
> And for an UPDATE could can pick either OLD or NEW.
>

Thanks, that's very interesting. I hadn't seen that syntax before.

Over on [1], I have a patch in the works that extends RETURNING,
allowing it to return OLD.colname, NEW.colname, OLD.*, and NEW.*. It
looks like this new SQL standard syntax could be built on top of that
(perhaps by having the rewriter turn queries of the above form into
CTEs).

However, the RETURNING syntax is more powerful, because it allows OLD
and NEW to be used together in arbitrary expressions, for example:

    RETURNING ..., NEW.val - OLD.val AS delta, ...

> > The current implementation uses a special function MERGING (a
> > grammatical construct without an OID that parses into a new MergingFunc
> > expr), which takes keywords ACTION or CLAUSE_NUMBER in the argument
> > positions. That's not totally unprecedented in SQL -- the XML and JSON
> > functions are kind of similar. But it's different in the sense that
> > MERGING is also context-sensitive: grammatically, it fits pretty much
> > anywhere a function fits, but then gets rejected at parse analysis time
> > (or perhaps even execution time?) if it's not called from the right
> > place.
>
> An analogy here might be that MATCH_RECOGNIZE (row-pattern recognition)
> has a magic function MATCH_NUMBER() that can be used inside that clause.
>   So a similar zero-argument magic function might make sense.  I don't
> like the MERGING(ACTION) syntax, but something like MERGE_ACTION() might
> make sense.  (This is just in terms of what kind of syntax might be
> palatable.  Depending on where the syntax of the overall clause ends up,
> we might not need it (see above).)
>

It could be that having the ability to return OLD and NEW values, as
in [1], is sufficient for use in MERGE, to identify the action
performed. However, I still think that dedicated functions would be
useful, if we can agree on names/syntax.

I think that I prefer the names MERGE_ACTION() and
MERGE_CLAUSE_NUMBER() from an aesthetic point of view, but it requires
2 new COL_NAME_KEYWORD keywords. Maybe that's OK, I don't know.

Alternatively, we could avoid adding new keywords by going back to
making these regular functions, as they were in an earlier version of
this patch, and then use some special-case code during parse analysis
to turn them into MergeFunc nodes (not quite a complete revert back to
an earlier version of the patch, but not far off).

Regards,
Dean

[1] https://www.postgresql.org/message-id/flat/CAEZATCWx0J0-v=Qjc6gXzR=KtsdvAE7Ow=D=mu50AgOe+pvisQ@mail.gmail.com