A `WHERE EXISTS { ... }` subquery may crash Apache AGE when it contains both:

[Silence6666668]

Describe the bug
A WHERE EXISTS { ... } subquery may crash Apache AGE when it contains both:

1. a variable-length relationship match
2. a UNION branch

In the minimized repro below, the server connection is terminated while evaluating the query.

How are you accessing AGE (Command line, driver, etc.)?

• PostgreSQL cypher(...) wrapper through the local Python differential-testing harness
• Reproducible directly in psql inside the Docker container

What data setup do we need to do?

SELECT * FROM cypher('fuzz_graph', $$
  CREATE (:Country {name: 'Germany'}),
         (:Country {name: 'United Kingdom'}),
         (:Person {name: 'Anna'})
$$) AS (v agtype);

SELECT * FROM cypher('fuzz_graph', $$
  MATCH (a:Person {name:'Anna'}), (g:Country {name:'Germany'})
  CREATE (a)-[:LIVING_IN]->(g)
$$) AS (v agtype);

What is the necessary configuration info needed?

• Plain Apache AGE Docker image was enough
• Docker image in local repro: apache/age
• AGE extension version: 1.7.0
• PostgreSQL version: 18.1
• Graph name used in repro: fuzz_graph
• No extra extensions or special configuration were required

What is the command that caused the error?

SELECT * FROM cypher('fuzz_graph', $$
  MATCH (person:Person)
  WHERE EXISTS {
    MATCH (person)-[:LIVING_IN*1..2]->(:Country)
    UNION
    MATCH (person)-[:WORKING_IN]->(:Country)
  }
  RETURN person.name AS name
$$) AS (name agtype);

Returned result on AGE:

server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.

Expected behavior
The query should succeed and return:

Anna

Neo4j returns exactly that result for the equivalent Cypher query.

Environment (please complete the following information):

• Version: Apache AGE 1.7.0
• PostgreSQL: 18.1
• Host OS: Windows
• Architecture: x86_64
• Deployment: Docker

Additional context
Two nearby control cases behave correctly on the same AGE instance:

1. The variable-length EXISTS branch alone works:

SELECT * FROM cypher('fuzz_graph', $$
  MATCH (person:Person)
  WHERE EXISTS { MATCH (person)-[:LIVING_IN*1..2]->(:Country) }
  RETURN person.name AS name
$$) AS (name agtype);

Observed result:

Anna

2. Splitting the logic into two separate EXISTS predicates joined by OR also works:

SELECT * FROM cypher('fuzz_graph', $$
  MATCH (person:Person)
  WHERE EXISTS { MATCH (person)-[:LIVING_IN*1..2]->(:Country) }
     OR EXISTS { MATCH (person)-[:WORKING_IN]->(:Country) }
  RETURN person.name AS name
$$) AS (name agtype);

Observed result:

Anna

So the crash appears to be specifically tied to combining a variable-length pattern and UNION inside the same EXISTS subquery.

This issue was first found during automated Neo4j-vs-AGE differential testing in a larger query:

MATCH (person:Person)
WHERE EXISTS {
  MATCH (person)-[:LIVING_IN*1..2]->(:Country {name: 'Germany'})
  UNION
  MATCH (person)-[:WORKING_IN]->(:Country {name: 'United Kingdom'})
}
OPTIONAL MATCH (person)-[r:LIVING_IN]->(c:Country)
WITH person, collect({rel: r, country: c}) AS connections
RETURN person.name AS name, size(connections) AS connection_count
ORDER BY name DESC
LIMIT 10

After minimization, the extra OPTIONAL MATCH, collect(...), and ORDER BY/LIMIT clauses were no longer needed to reproduce the server crash.