MIME-Version: 1.0
References: 
 <CAHg+QDfgK0xddH8f3eAb+UVn7sBDOnv8RvM6OkP4HtHAt6aD7w@mail.gmail.com>
In-Reply-To: 
 <CAHg+QDfgK0xddH8f3eAb+UVn7sBDOnv8RvM6OkP4HtHAt6aD7w@mail.gmail.com>
From: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Date: Wed, 15 Apr 2026 20:37:40 +0530
Message-ID: 
 <CAExHW5sEQOK3F+pkZcFy=FX-dOhdittBS6A7Hf88HT841bunLg@mail.gmail.com>
Subject: Re: Bug: Missing check_stack_depth() in GRAPH_TABLE rewriter
To: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
	Peter Eisentraut <peter@eisentraut.org>
Content-Type: multipart/mixed; boundary="00000000000084964c064f811462"
Archived-At: 
 <https://www.postgresql.org/message-id/CAExHW5sEQOK3F%2BpkZcFy%3DFX-dOhdittBS6A7Hf88HT841bunLg%40mail.gmail.com>
Precedence: bulk

--00000000000084964c064f811462
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Sat, Apr 11, 2026 at 9:28=E2=80=AFAM SATYANARAYANA NARLAPURAM
<satyanarlapuram@gmail.com> wrote:
>
> Hi Hackers,
>
> Two recursive functions, generate_setop_from_pathqueries() and
> generate_queries_for_path_pattern_recurse() in GRAPH_TABLE rewriter are m=
issing check_stack_depth() calls. Consider a property graph with  400 edge =
tables with a 2-hop pattern produce 160,000 path queries and therefore 160,=
000 recursion frames in
> generate_setop_from_pathqueries(). This can easily exceed the OS stack li=
mit for some systems and crashes the backend.
>
> I am proposing a stop gap fix of checking stack depth by calling check_st=
ack_depth for now but in future we may want to reduce the recursion depth (=
for example use a balanced tree to reduce depth from O(N) to O(log N).
>
> Attached a patch for adding the check_stack_depth() check.
>
> Repro:
>
> CREATE TABLE sv (id int PRIMARY KEY);
>   INSERT INTO sv VALUES (1);
>
>   DO $$
>   BEGIN
>     FOR i IN 1..400 LOOP
>       EXECUTE format(
>         'CREATE TABLE se_%s (id int PRIMARY KEY, src int, dst int)', i);
>     END LOOP;
>   END$$;
>
>   DO $$
>   DECLARE
>     sql text;
>     edges text :=3D '';
>   BEGIN
>     FOR i IN 1..400 LOOP
>       IF i > 1 THEN edges :=3D edges || ', '; END IF;
>       edges :=3D edges || format(
>         'se_%s KEY (id) SOURCE KEY (src) REFERENCES sv (id) '
>         'DESTINATION KEY (dst) REFERENCES sv (id)', i);
>     END LOOP;
>     EXECUTE 'CREATE PROPERTY GRAPH g VERTEX TABLES (sv KEY (id)) '
>          || 'EDGE TABLES (' || edges || ')';
>   END$$;
>
>   SELECT * FROM GRAPH_TABLE(g
>     MATCH (a)-[e1]->(b)-[e2]->(c)
>     COLUMNS(a.id AS a_id))
>   LIMIT 1;
>

Thanks for the report. I could reproduce the segfault on my laptop.
The attached patch fixes it and gives ERROR:  stack depth limit
exceeded.

generate_queries_for_path_pattern_recurse() - has to work in a linear
fashion since the elements need to be processed in an order. Each
permutation of elements produces one query. These queries can be
arranged in a balanced tree as you  suggest OR when constructing the
setop tree we could generate it in divide-and-conquer manner. However,
the tree will be flattened in the planner anyway (See
flatten_simple_union_all() and pull_up_simple_union_all()). Thus the
final planning will require a deeper stack anyway. The code complexity
doesn't seem to be worth it.

I also looked at a few commits that add check_stack_depth() to see if
we add tests for these scenarios. But I didn't find any. So no tests
added with this commit.


--
Best Wishes,
Ashutosh Bapat

--00000000000084964c064f811462
Content-Type: text/x-patch; charset="US-ASCII";
	name="v20260415-0001-Check-for-stack-overflow-when-rewriting-gr.patch"
Content-Disposition: attachment;
	filename="v20260415-0001-Check-for-stack-overflow-when-rewriting-gr.patch"
Content-Transfer-Encoding: base64
Content-ID: <f_mo06pkyz0>
X-Attachment-Id: f_mo06pkyz0
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--00000000000084964c064f811462--