Re: unrecognized node type while displaying a Path due to dangling pointer

Hi Tom,

On Tue, Jul 11, 2023 at 4:30 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Jeevan Chalke <jeevan(dot)chalke(at)enterprisedb(dot)com> writes:
> > Attached patch.
>
> I would be astonished if this fixes anything. The code still doesn't
> know which paths are referenced by which other ones, and so the place
> where we free a previously-added path can't know what to do.
>
> I've speculated about adding some form of reference counting to paths
> (maybe just a "pin" flag rather than a full refcount) so that we could
> be smarter about this. The existing kluge for "don't free IndexPaths"
> could be replaced by setting the pin mark on any IndexPath that we
> make a bitmap path from. Up to now it hasn't seemed necessary to
> generalize that hack, but maybe it's time. Can you show a concrete
> case where we are freeing a still-referenced path?
>

As mentioned earlier, while debugging some issues, we have put an elog
displaying the foreignrel contents using nodeToString(). Like below:

@@ -1238,6 +1238,8 @@ postgresGetForeignPlan(PlannerInfo *root,
bool has_limit = false;
ListCell *lc;

+ elog(INFO, "foreignrel: %s", nodeToString(foreignrel));
+

And ran the postgres_fdw regression and observed many warnings saying "could
not dump unrecognized node type". Here are the queries retrieved and
adjusted from postgres_fdw.sql

CREATE EXTENSION postgres_fdw;
CREATE SERVER loopback FOREIGN DATA WRAPPER postgres_fdw OPTIONS (dbname
'postgres', port '5432');
CREATE USER MAPPING FOR CURRENT_USER SERVER loopback;
CREATE TABLE t1 (c1 int NOT NULL, c2 int NOT NULL, CONSTRAINT t1_pkey
PRIMARY KEY (c1));
INSERT INTO t1 SELECT id, id % 10 FROM generate_series(1, 1000) id;
ANALYZE t1;
CREATE FOREIGN TABLE ft2 (c1 int NOT NULL, c2 int NOT NULL) SERVER loopback
OPTIONS (schema_name 'public', table_name 't1');

explain (verbose, costs off)
select c2, sum(c1) from ft2 group by c2 having avg(c1) < 500 and sum(c1) <
49800 order by c2;

With the above elog() in place, we can see the warning. And the pathlist
has a second path as empty ({}). Which got freed but has a reference in
this foreignrel.

Thanks

>
> regards, tom lane
>

--
Jeevan Chalke

*Senior Staff SDE, Database Architect, and ManagerProduct Development*

edbpostgres.com