Re: fix for BUG #3720: wrong results at using ltree

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Filip Rembiałkowski <filip(dot)rembialkowski(at)gmail(dot)com>
Cc: Nikita Glukhov <n(dot)gluhov(at)postgrespro(dot)ru>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org>, Oleg Bartunov <obartunov(at)postgrespro(dot)ru>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Teodor Sigaev <teodor(at)sigaev(dot)ru>
Subject: Re: fix for BUG #3720: wrong results at using ltree
Date: 2020-03-30 00:53:46
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I wrote:
> My main complaint about it remains the same, that it changes a
> disturbingly large number of existing regression-test results,
> suggesting that there's not a meeting of the minds about what
> this logic is supposed to do. Maybe it's okay or maybe it's
> not, but who's going to decide?

Well ... *somebody's* got to decide, and since Oleg and Teodor aren't
stepping up, I took it on myself to study this more closely.

It seems to me that indeed the existing behavior is broken: given
what the documentation has to say, it's really hard to argue that
an lquery like "*.!foo.*" means something other than "match any
ltree that has at least one label that is not 'foo'". But the
existing code produces

regression=# select ''::ltree ~ '*.!foo.*'::lquery;
(1 row)

I agree that's just plain wrong, and so are all the regression
test cases that this patch is changing the results of.

However, I think there is a valid use-case that the existing
code is trying to solve: how can you say "match any ltree in
which no label is 'foo'"? That is the effective behavior right
now of a pattern like this, and it seems useful. So if we change
this, we ought to provide some other way to get that result.

What I propose we do about that is allow lquery quantifiers to
be attached to regular items as well as star items, so that the
need is met by saying this:

regression=# select ''::ltree ~ '!foo{,}'::lquery;
(1 row)

regression=# select 'bar.fool.baz'::ltree ~ '!foo{,}'::lquery;
(1 row)

Also, once we do that, it's possible to treat star and non-star items
basically alike in checkCond, with checkLevel being the place that
accounts for them being different. This results in logic that's far
simpler and much more nearly like the way that LIKE patterns are
implemented, which seems like a good thing to me.

Hence, attached are two revised patches that attack the problem
this way. The first one is somewhat unrelated to the original
point --- it's trying to clean up the error messages in ltree_in
and lquery_in so that they are more consistent and agree with
the terminology used in the documentation. (Notably, the term
"level" is used nowhere in the ltree docs, except in the legacy
function name nlevel().) However its movement of the check for
high < low is needed to make that cover the case of a bogus non-star
quantifier in patch 0002. These also depend on the cosmetic
patches I just committed, so you need HEAD as of today to get
them to apply.

regards, tom lane

Attachment Content-Type Size
0001-rationalize-ltree-input-errors-1.patch text/x-diff 10.3 KB
0002-ltree-not-fixes-and-better-quantifiers-1.patch text/x-diff 23.4 KB

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