pg.ddx.io  pgsql-hackers@postgresql.org mailing list archive  

help / color / mirror / Atom feed
From: Tom Lane <tgl@sss.pgh.pa.us>
To: Nikita Glukhov <n.gluhov@postgrespro.ru>
Cc: Pgsql Hackers <pgsql-hackers@postgresql.org>
Cc: Filip Rembiałkowski <filip.rembialkowski@gmail.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Alexander Korotkov <a.korotkov@postgrespro.ru>
Cc: Oleg Bartunov <obartunov@postgrespro.ru>
Cc: Thomas Munro <thomas.munro@gmail.com>
Cc: Teodor Sigaev <teodor@sigaev.ru>
Subject: Re: fix for BUG #3720: wrong results at using ltree
Date: Tue, 31 Mar 2020 11:47:30 -0400
Message-ID: <4902.1585669650@sss.pgh.pa.us> (raw)
In-Reply-To: <6352.1585609100@sss.pgh.pa.us>
References: <CAP_rww=waX2Oo6q+MbMSiZ9ktdj6eaJj0cQzNu=Ry2cCDij5fw@mail.gmail.com>
	<28639.1554565562@sss.pgh.pa.us>
	<CA+hUKGKVbTGHGpHBX6EGew13CrfYcagThJqBAmeATCMKnVqisQ@mail.gmail.com>
	<CAF4Au4yV8vsgA_7XCce4kFUa2BjseqWj=KVuOwcE+DGLj6AW7g@mail.gmail.com>
	<5eedfd9e-2f14-a2df-3bd9-4c92eb0f08a5@postgrespro.ru>
	<CAPpHfdsvEaqXDsLWebQ4Q4GszRZ_P6h-1_cidmiqPMtWzXnmhA@mail.gmail.com>
	<20200124182915.zz7hzdkijzsb4ic4@development>
	<a315c67d-5b3b-8005-99ef-daa9ea7036a0@postgrespro.ru>
	<19375.1585438012@sss.pgh.pa.us>
	<15582.1585529626@sss.pgh.pa.us>
	<23304.1585591242@sss.pgh.pa.us>
	<943dc224-eb82-8abd-15c3-f4b516f89767@postgrespro.ru>
	<3007.1585605566@sss.pgh.pa.us>
	<3843.1585606362@sss.pgh.pa.us>
	<9247ddc2-d1e0-7d62-55a6-62a089e99a8a@postgrespro.ru>
	<5210.1585607748@sss.pgh.pa.us>
	<4c00219b-7094-e568-cc2c-6a52cf621701@postgrespro.ru>
	<6352.1585609100@sss.pgh.pa.us>

I wrote:
> I've marked this RFC, and will push tomorrow unless somebody wants
> to object to the loss of backwards compatibility.

And done.  I noticed in some final testing that it's possible to
make this code take a long time by forcing it to backtrack a lot:

regression=# SELECT (('1' || repeat('.1', 65534))::ltree) ~ '*.*.x';
 ?column? 
----------
 f
(1 row)

Time: 54015.421 ms (00:54.015)

so I threw in a CHECK_FOR_INTERRUPTS().  Maybe it'd be worth trying
to optimize such cases, but I'm not sure that it'd ever matter for
real-world cases with reasonable-size label strings.

The old implementation seems to handle that particular case well,
evidently because it more-or-less folds adjacent stars together.
However, before anyone starts complaining about regressions, they
should note that it's really easy to get the old code to fail
via stack overflow:

regression=# SELECT (('1' || repeat('.1', 65534))::ltree) ~ '*.!1.*';
ERROR:  stack depth limit exceeded

(That's as of five minutes ago, before that it dumped core.)
So I don't feel bad about the tradeoff.  At least now we have
simple, visibly correct code that could serve as a starting
point for optimization if anyone feels the need to.

			regards, tom lane





view thread (25+ messages)

Message-ID: <4902.1585669650@sss.pgh.pa.us>
Permalink:  ../4902.1585669650@sss.pgh.pa.us/
Also on:    postgresql.org/message-id/4902.1585669650@sss.pgh.pa.us

 · 

reply
Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: pgsql-hackers@postgresql.org
  Cc: tgl@sss.pgh.pa.us, n.gluhov@postgrespro.ru, filip.rembialkowski@gmail.com, tomas.vondra@2ndquadrant.com, a.korotkov@postgrespro.ru, obartunov@postgrespro.ru, thomas.munro@gmail.com, teodor@sigaev.ru
  Subject: Re: fix for BUG #3720: wrong results at using ltree
  In-Reply-To: <4902.1585669650@sss.pgh.pa.us>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox