YGY-001 opened a new issue, #2594:
URL: https://github.com/apache/age/issues/2594
## Environment
- Apache AGE `1.8.0`, PostgreSQL `18.6`
- Docker image: `apache/age:release_PG18_1.8.0`
## Reproduction
Run setup and target queries as separate statements. The two AGE graphs start
with the same single relationship.
```sql
LOAD 'age';
SET search_path = ag_catalog, public;
SELECT ag_catalog.create_graph('age_repro22_left');
SELECT *
FROM ag_catalog.cypher('age_repro22_left', $$
CREATE ()-[:R]->()
$$) AS (v agtype);
SELECT ag_catalog.create_graph('age_repro22_right');
SELECT *
FROM ag_catalog.cypher('age_repro22_right', $$
CREATE ()-[:R]->()
$$) AS (v agtype);
```
### Baseline query — `age_repro22_left`
```sql
SELECT *
FROM ag_catalog.cypher('age_repro22_left', $$
UNWIND [1, 2] AS i
MATCH (a)-[:R]->(b)
SET b.x = 1
MERGE (a)-[:R]->(b)<-[:R]-({id: 1})
RETURN i
$$) AS (i agtype);
```
### Row-preserving barrier query — `age_repro22_right`
```sql
SELECT *
FROM ag_catalog.cypher('age_repro22_right', $$
UNWIND [1, 2] AS i
MATCH (a)-[:R]->(b)
SET b.x = 1
WITH {i: i, a: a, b: b} AS row
WITH collect(row) AS rows
UNWIND rows AS row
WITH row.i AS i, row.a AS a, row.b AS b
MERGE (a)-[:R]->(b)<-[:R]-({id: 1})
RETURN i
$$) AS (i agtype);
```
## Expected and actual
Both target queries should return three rows: `1`, `2`, `2`. The two input
rows
are distinct (`i` is `1` and `2`), and the map/`collect`/`UNWIND` barrier
retains both rows and their multiplicity.
On AGE `1.8.0` / PostgreSQL `18.6`:
```text
baseline: 1, 2, 2 (3 rows)
barrier: 1, 2 (2 rows)
```
The three expected rows follow from the full-pattern `MERGE`: the first input
row creates the missing pattern, and the next input row can then match two
complete paths. Cypher `MERGE` matches the full pattern or creates it; see
the
[Cypher `MERGE`
documentation](https://neo4j.com/docs/cypher-manual/current/clauses/merge/).
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