Re: Window Functions: v07 APIs and buffering strateties

On Tue, Oct 28, 2008 at 01:50:26PM -0400, Tom Lane wrote:
> > Given that the only problematic case is if expr_list ends with a
> > postfix operator, wouldn't it be sufficient to simply decree that in
> > that case you need parentheses? Seems a lot less painful than adding
> > two reserved words.
>
> Hmm ... actually, it might be possible to fix it with a suitable
> precedence declaration? The trick is to make sure that in
> ... ORDER BY foo ! ROWS ...
> the operator is taken as postfix not infix, which is the exact opposite
> of what we did for AS-less column aliases (and, in fact, is the opposite
> of what bison will do by default, IIRC). So it might be possible to fix
> by attaching some new precedence level to the ROWS token.

Yes. Bison's default is to shift, which means that if you do nothing it
will treat ROWS as part of the expression if it makes any sense at all.
Given the requirement for a following UNBOUNDED or BETWEEN, the only
problem is that you'll get a syntax error if the expr_list ends in a
postfix operator, I don't see how you get hidden ambiguity.

Operator precedence is exactly the way to do this, since operator
precedence rules exist solely to resolve the shift/reduce conflicts.

You're right about the documentation though. I suppose you could put in
the documentation for the ROWS stuff something along the lines of: If
the last expression of your ORDER by ends in a postfix operator, you
must use parentheses. How many postfix operators are in common use in
ORDER BY expressions anyway?

Have a nice day,
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> Please line up in a tree and maintain the heap invariant while
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