Re: Add RESPECT/IGNORE NULLS and FROM FIRST/LAST options

>>>>> "Tom" == Tom Lane <tgl@sss.pgh.pa.us> writes:

 >> So I've tried to rough out a decision tree for the various options
 >> on how this might be implemented (discarding the "use precedence
 >> hacks" option). Opinions? Additions?

 Tom> I think it'd be worth at least drafting an implementation for the
 Tom> lexical-lookahead fix. I think it's likely that we'll need to
 Tom> extend base_yylex to do more lookahead in the future even if we
 Tom> don't do it for this, given the SQL committee's evident love for
 Tom> COBOL-ish syntax and lack of regard for what you can do in
 Tom> LALR(1).

That's not _quite_ a fair criticism of the SQL committee; they're not
ignoring the capabilities of parsers, they're just not making their
syntax robust in the presence of the kinds of extensions and
generalizations that we want to do.

Without exception that I know of, every time we've run into a problem
that needed ugly precedence rules or extra lookahead it's been caused by
us not reserving something that is reserved in the spec, or by us
allowing constructs that are not in the spec at all (postfix operators,
especially), or by us deliberately generalizing what the spec allows
(e.g. allowing full expressions where the spec only allows column
references, or allowing extra parens around subselects) or where we've
repurposed syntax from the spec in an incompatible way (as in
ANY(array)).

Anyway, for the time being I will mark this patch as "returned with
feedback".

 >> If the clauses are legal on all window functions, what to do about
 >> existing window functions for which the clauses do not make sense?

 Tom> Option 1: do nothing, document that nothing happens if w.f.
 Tom> doesn't implement it.

That was 1.1.1 on my list.

 Tom> Option 2: record whether the inquiry functions got called. At end
 Tom> of query, error out if they weren't and the options were used.

Erroring at the _end_ of the query seems a bit of a potential surprise.

 Tom> It's also worth wondering if we couldn't just implement the flags
 Tom> in some generic fashion and not need to involve the window
 Tom> functions at all.

That was what I meant by option 1.1.2 on my list.

 Tom> FROM LAST, for example, could and perhaps should be implemented by
 Tom> inverting the sort order.

Actually that can't work for reasons brought up in the recent discussion
of optimization of window function sorts: if you change the sort order
you potentially disturb the ordering of peer rows, and the spec requires
that an (nth_value(x,n) from last over w) and (otherfunc(x) over w) for
order-equivalent windows "w" must see the peer rows in the same order.

So FROM LAST really does have to keep the original sort order, and count
backwards from the end of the window.

 Tom> Possibly IGNORE NULLS could be implemented inside the
 Tom> WinGetFuncArgXXX functions? These behaviors might or might not
 Tom> make much sense with other window functions, but that doesn't seem
 Tom> like it's our problem.

That's about what I was thinking for option 1.1.2, yes.

-- 
Andrew (irc:RhodiumToad)