Re: factorial function/phase out postfix operators? - Mailing list pgsql-hackers

On Tue, May 19, 2020 at 2:30 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Might work.  My main concern would be if we have to forbid those keywords
> as column names --- for words like "year", in particular, that'd be a
> disaster.  If the net effect is only that they can't be AS-less col labels,
> it won't break any cases that worked before.

ISTM that all we have to do to avoid that is switch from a four-way
classification to a five-way classification: just split
unreserved_keyword into totally_unreserved_keyword and
very_slightly_reserved_keyword.

> Our existing four-way keyword classification is not something that was
> handed down on stone tablets.  I wonder whether postfix-ectomy changes
> the situation enough that a complete rethinking would be helpful.

I don't see that they do, but I might be missing something. I think
there's an excellent argument for adding one new category, but it's
not clear to me why it should reshape the landscape any more than
that.

> I also continue to think that more lookahead and token-merging would
> be interesting to pursue.  It'd hardly surprise anybody if the
> token pair "character varying" were always treated as a type name,
> for instance.

I think that line of attack will not buy very much. The ability to
avoid unexpected consequences is entirely contingent on the
unlikeliness of the keywords appearing adjacent to each other in some
other context, and the only argument for that here is that neither of
those words is a terribly likely column name. I think that when you
try to solve interesting problems with this, though, you very quickly
run into problems where that's not the case, and you'll need a
technique that has some knowledge of the parser state to actually do
something that works well. I read a paper some years ago that proposed
a solution to this problem: if the parser generator sees a
shift/reduce conflict, it checks whether the conflict can be resolve
by looking ahead one or more additional tokens. If so, it can build a
little DFA that gets run when you enter that state, with edges labeled
with lookahead tokens, and it runs that DFA whenever you reach the
problematic state. Since, hopefully, such states are relatively rarely
encountered, the overhead is low, yet it still gives you a way out of
conflicts in many practical cases. Unfortunately, the chances of bison
implementing such a thing do not seem very good.

> Anyway, the bottom-line conclusion remains the same: let's make sure
> we know what we'd do after getting rid of postfix ops, before we do
> that.

Well, I don't think we really need to get too conservative here. I've
studied this issue enough over the years to be pretty darn sure that
this is a necessary prerequisite to doing something about the "AS
unreserved_keyword" issue, and that it is by far the most significant
issue in doing something about that problem. Sure, there are other
issues, but I think they are basically matters of politics or policy.
For example, if some key people DID think that the four-way keyword
classification was handed down on stone tablets, that could be quite a
problem, but if we're willing to take the view that solving the "AS
unreserved_keyword" problem is pretty important and we need to find a
way to get it done, then I think we an do that. It seems to me that
the first thing that we need to do here is get a deprecation notice
out, so that people know that we're planning to break this. I think we
should go ahead and make that happen now, or at least pretty soon.

I'm still interested in hearing what people think about hard-coding !
as a postfix operator vs. removing postfix operators altogether. I
think Vik and Tom are against keeping just !, Kenneth Marshall are for
it, and I'm not sure I understand Pavel's position. I'm about +0.3 for
keeping just ! myself. Maybe we'll get some other votes. If you're
willing to be persuaded that keeping only ! is a sensible thing to
consider, I could probably draft a very rough patch showing that it
would still be sufficient to get us out from under the "AS
unreserved_keyword" problem, but if you and/or enough other people
hate that direction with a fiery passion, I won't bother. I'm pretty
sure it's technically possible, but the issue is more about what
people actually want.

-- 
Robert Haas
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The Enterprise PostgreSQL Company