Re: Compatible defaults for LEAD/LAG

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
Cc: Daniel Gustafsson <daniel(at)yesql(dot)se>, Vik Fearing <vik(at)postgresfriends(dot)org>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Compatible defaults for LEAD/LAG
Date: 2020-09-22 00:33:24
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Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
> I see few possibilities how to finish and close this issue:
> 1. use anycompatible type and add note to documentation so expression of
> optional argument can change a result type, and so this is Postgres
> specific - other databases and ANSI SQL disallow this.
> It is the most simple solution, and it is good enough for this case, that
> is not extra important.
> 2. choose the correct type somewhere inside the parser - for these two
> functions.
> 3. introduce and implement some generic solution for this case - it can be
> implemented on C level via some function helper or on syntax level
> CREATE OR REPLACE FUNCTION lag(a anyelement, offset int, default defval
> a%type) ...
> "defval argname%type" means for caller's expression "CAST(defval AS
> typeof(argname))"

I continue to feel that the spec's definition of this is not so
obviously right that we should jump through hoops to duplicate it.
In fact, I don't even agree that we need a disclaimer in the docs
saying that it's not quite the same. Only the most nitpicky
language lawyers would ever even notice.

In hopes of moving this along a bit, I experimented with converting
the other functions I listed to use anycompatible. I soon found that
touching any functions/operators that are listed in operator classes
would open a can of worms far larger than I'd previously supposed.
To maintain consistency, we'd have to propagate the datatype changes
to *every* function/operator associated with those opfamilies --- many
of which only have one any-foo input and thus aren't on my original
list. (An example here is that extending btree array_ops will require
changing the max(anyarray) and min(anyarray) aggregates too.) We'd
then end up with a situation that would be rather confusing from a
user's standpoint, in that it'd be quite un-obvious why some array
functions use anyarray while other ones use anycompatiblearray.

So I backed off to just changing the functions/operators that have
no opclass connections, such as array_cat. Even that has some
downsides --- for example, in the attached patch, it's necessary
to change some polymorphism.sql examples that explicitly reference
array_cat(anyarray). I wonder whether this change would break a
significant number of user-defined aggregates or operators.

(Note that I think we'd have to resist the temptation to fix that
by letting CREATE AGGREGATE et al accept support functions that
take anyarray/anycompatiblearray (etc) interchangeably. A lot of
the security analysis that went into CVE-2020-14350 depended on
the assumption that utility commands only do exact lookups of
support functions. If we tried to be lax about this, we'd
re-introduce the possibility for hostile capture of function
references in extension scripts.)

Another interesting issue, not seen in the attached but which
came up while I was experimenting with the more aggressive patch,
was this failure in the polymorphism test:

select max(histogram_bounds) from pg_stats where tablename = 'pg_am';
-ERROR: cannot compare arrays of different element types
+ERROR: function max(anyarray) does not exist

That's because we don't accept pg_stats.histogram_bounds (of
declared type anyarray) as a valid input for anycompatiblearray.
I wonder if that isn't a bug we need to fix in the anycompatible
patch, independently of anything else. We'd not be able to deduce
an actual element type from such an input, but we already cannot
do so when we match it to an anyarray parameter.

Anyway, attached find

0001 - updates Vik's original patch to HEAD and tweaks the
grammar in the docs a bit.

0002 - add-on patch to convert array_append, array_prepend,
array_cat, array_position, array_positions, array_remove,
array_replace, and width_bucket to use anycompatiblearray.

I think 0001 is committable, but 0002 is just WIP since
I didn't touch the docs. I'm slightly discouraged about
whether 0002 is worth proceeding with. Any thoughts?

regards, tom lane

Attachment Content-Type Size
0001-lead_lag_compatible_default-2.patch text/x-diff 6.7 KB
0002-make-some-array-functions-use-anycompatible.patch text/x-diff 8.2 KB

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