From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andrew Gierth <andrew@tao11.riddles.org.uk>
cc: pgsql-hackers@postgresql.org, david@fetter.org,
        Oliver Ford <ojford@gmail.com>, Krasiyan Andreev <krasiyan@gmail.com>
Subject: Re: Add RESPECT/IGNORE NULLS and FROM FIRST/LAST options
In-reply-to: <8736tygr5e.fsf@news-spur.riddles.org.uk>
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 <878t3vj279.fsf@news-spur.riddles.org.uk> <2897.1537737232@sss.pgh.pa.us>
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Comments: In-reply-to Andrew Gierth <andrew@tao11.riddles.org.uk>
	message dated "Tue, 25 Sep 2018 05:16:27 +0100"
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Date: Tue, 25 Sep 2018 10:07:19 -0400
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Andrew Gierth <andrew@tao11.riddles.org.uk> 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?

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

The questions of how we interface to the individual window functions
are really independent of how we handle the parsing problem.  My
first inclination is to just pass the flags down to the window functions
(store them in WindowObject and provide some additional inquiry functions
in windowapi.h) and let them deal with it.

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

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

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

It's also worth wondering if we couldn't just implement the flags in
some generic fashion and not need to involve the window functions at
all.  FROM LAST, for example, could and perhaps should be implemented
by inverting the sort order.  Possibly IGNORE NULLS could be implemented
inside the WinGetFuncArgXXX functions?  These behaviors might or might
not make much sense with other window functions, but that doesn't seem
like it's our problem.

			regards, tom lane