Re: SQL Property Graph Queries (SQL/PGQ)

Hi all,

I hit a Segmentation fault due to what might be an non-standard coding approach I used. Here are the test details and error logs:
```
postgres(at)zxm-VMware-Virtual-Platform:~/code/postgres$ psql
psql (19devel)
Type "help" for help.

postgres=# CREATE TABLE users (
id INT PRIMARY KEY,
name VARCHAR(50)
);

CREATE TABLE follows (
id INT PRIMARY KEY,
follower INT REFERENCES users(id),
following INT REFERENCES users(id)
);

INSERT INTO users VALUES
(1, 'A'), (2, 'B'), (3, 'C'), (4, 'D');

INSERT INTO follows VALUES
(1, 1, 2),
(2, 1, 3),
(3, 2, 3),
(4, 2, 4),
(5, 3, 4);

CREATE PROPERTY GRAPH social_graph
VERTEX TABLES (users)
EDGE TABLES (
follows
SOURCE KEY (follower) REFERENCES users(id)
DESTINATION KEY (following) REFERENCES users(id)
);

-- ok
SELECT *
FROM GRAPH_TABLE (
social_graph
MATCH (a IS users)-[]->(x IS users)<-[]-(b IS users)
WHERE b.name != a.name
COLUMNS (a.name AS usera, x.name AS common_name, b.name AS userb)
);

-- error
SELECT *
FROM GRAPH_TABLE (
social_graph
MATCH (a IS users)-[]->(x IS users)<-[]-(b IS users WHERE b.name != a.name)
COLUMNS (a.name AS usera, x.name AS common_name, b.name AS userb)
);
CREATE TABLE
CREATE TABLE
INSERT 0 4
INSERT 0 5
CREATE PROPERTY GRAPH
usera | common_name | userb
-------+-------------+-------
A | C | B
B | C | A
B | D | C
C | D | B
(4 rows)

server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: 2026-03-18 14:09:29.522 CST [103556] LOG: client backend (PID 103851) was terminated by signal 11: Segmentation fault
2026-03-18 14:09:29.522 CST [103556] DETAIL: Failed process was running: SELECT *
FROM GRAPH_TABLE (
social_graph
MATCH (a IS users)-[]->(x IS users)<-[]-(b IS users WHERE b.name != a.name)
COLUMNS (a.name AS usera, x.name AS common_name, b.name AS userb)
);
2026-03-18 14:09:29.522 CST [103556] LOG: terminating any other active server processes
Failed.
The connection to the server was lost. Attempting reset: Failed.
2026-03-18 14:09:29.524 CST [103556] LOG: all server processes terminated; reinitializing
!?> 2026-03-18 14:09:29.546 CST [103857] LOG: database system was interrupted; last known up at 2026-03-18 14:08:06 CST
2026-03-18 14:09:29.570 CST [103857] LOG: database system was not properly shut down; automatic recovery in progress
2026-03-18 14:09:29.572 CST [103857] LOG: redo starts at 0/02487AE8
2026-03-18 14:09:29.578 CST [103857] LOG: invalid record length at 0/024E0AB0: expected at least 24, got 0
2026-03-18 14:09:29.578 CST [103857] LOG: redo done at 0/024E07A8 system usage: CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s
2026-03-18 14:09:29.596 CST [103556] LOG: database system is ready to accept connections
```

I checked the code and found that `found_mapping` was a null pointer because I didn’t enable assertions. The code in question is in `src/backend/rewrite/rewriteGraphTable.c`:

```
/*
* transformGraphTablePropertyRef() would not create a
* GraphPropertyRef for a variable which is not present in the graph
* path pattern.
*/
Assert(found_mapping);

mapping_factor = found_mapping->path_factor;
```

Should we remove this assertion and throw an error message instead to handle this case?

--
regards,
Man Zeng