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

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Nikita Glukhov <n(dot)gluhov(at)postgrespro(dot)ru>
Cc: Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org>, Filip Rembiałkowski <filip(dot)rembialkowski(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, 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: 2020-03-31 15:47:30
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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';
(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

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