Re: MERGE Specification

On Thu, Aug 5, 2010 at 7:25 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:

> On Thu, 2010-08-05 at 12:29 +0300, Heikki Linnakangas wrote:
> > On 05/08/10 10:46, Simon Riggs wrote:
> > > On Mon, 2008-04-21 at 21:08 +0100, Simon Riggs wrote:
> > >> The following two files specify the behaviour of the MERGE statement
> and
> > >> how it will work in the world of PostgreSQL.
> > >
> > >> The HTML file was generated from SGML source, though the latter is not
> > >> included here for clarity.
> > >
> > > Enclose merge.sgml docs for forthcoming MERGE command, as originally
> > > written.
> >
> > Oh, cool, I wasn't aware you had written that already. Boxuan, please
> > include this in your patch, after reviewing and removing/editing
> > anything that doesn't apply to your patch.
>
>
Thanks a lot for the instruction file of MERGE command. I have read through
it carefully. It is really a great work. I have to admit that I am not
familiar with the sgml language, and I cannot write the instruction by
myself.

All features of MERGE demonstrated in this file are consistent with my
implementation, EXCEPT the DO NOTHING option. In current edition, we don't
have the DO NOTHING action type. That is, during the execution of MERGE
commands, if one tuple is not caught by any of the merge actions, it will be
ignored. In another word, DO NOTING (although cannot be specified explicitly
by user) is the DEFAULT action for tuples.

In the contrary, Simon's instruction says that the DEFAULT action for the
tuple caught by no actions is
WHEN NOT MATCHED THEN INSERT DEFAULT VALUES

From the user's point of view, these two kinds of MERGE command may have
not much differences. But, as the coder, I prefer current setting, because
we can save the implementation for a new type of MERGE actions (DO
NOTHING is a special merge action type). And, thus, no checks and special
process for it. (For example, we need to make sure that DO NOTHING is the
last WHEN clause, and it has no additional qual. And we have to generate a
INSERT DEFAULT VALUES action for the MERGE command if we don't find the DO
NOTHING action)

Well, if people want the DO NOTHING action, I will add it in the system.

Now, I have changed the RULE strategy of MERGE to the better logic. And I
am working on triggers for MERGE, which is also mentioned in the instruction
file. I will build a new patch with no long comment and blank line around
functions, and possibly contain the regress test file and this sgml
instructions in it.

I wish we can reach a agreement on the DO NOTHING thing before my next
submission, so I can make necessary modification on my code for it. (the new
patch may be finished in one or two days, I think)

Thanks!

PS: I have an embarrassing question: how to view the sgml instructions of
postgres in web page form, rather than read the source code of them?

> Also had these fragments as well, if they're still useful. Probably just
> useful as pointers as to what else to change to include the docs.
>
>
> The tests and docs were written from SQL standard, so any deviations
> would need to be flagged. The idea of writing the tests first was that
> they provide an objective test of whether the implementation works
> according to spec.
>
> I'd quite like a commentary on anything that needs changing. Not saying
> I will necessarily object to differences, but knowing the differences
> sounds important for us.
>
> --
> Simon Riggs www.2ndQuadrant.com <http://www.2ndquadrant.com/>
> PostgreSQL Development, 24x7 Support, Training and Services
>