shulei5831sl opened a new issue, #2473:
URL: https://github.com/apache/age/issues/2473
### Apache AGE version
Observed with the official Docker image:
```text
Image: apache/age:latest
Apache AGE: 1.7.0
PostgreSQL: 18.1
```
### Environment
- Deployment: Docker
- Query interface: PostgreSQL SQL client
- Query language: Cypher through `cypher()`
- Differential comparison targets:
- Neo4j `2026.05`
- Neo4j `5.26`
- Memgraph `3.11.0`
- FalkorDB `4.18.11`
### Description
A standalone `OPTIONAL MATCH` returns zero rows when its pattern does not
match.
`OPTIONAL MATCH` should preserve its input row and bind variables introduced
by the unmatched optional pattern to `null`. At the beginning of a query,
there is an implicit initial input row. Therefore, a standalone unmatched
`OPTIONAL MATCH` should return one null-filled row.
Apache AGE instead drops the initial row and returns an empty result.
When the same `OPTIONAL MATCH` follows a successful `MATCH`, AGE correctly
preserves the preceding row and binds the unmatched variable to `null`.
The issue therefore appears specific to the initial input row of a standalone
`OPTIONAL MATCH`.
### Steps to reproduce
Use an existing empty graph named `test_graph`.
#### Test 1: standalone `OPTIONAL MATCH`
```sql
SELECT *
FROM cypher('test_graph', $$
OPTIONAL MATCH (n:DefinitelyMissing)
RETURN n AS result
$$) AS (result agtype);
```
### Expected behavior
The query should return one row:
```text
result
------
null
```
Equivalent logical result:
```text
[{result: null}]
```
### Actual behavior
Apache AGE returns zero rows:
```text
[]
```
The initial input row is lost when the optional pattern does not match.
### Control: `OPTIONAL MATCH` after a successful `MATCH`
Create one seed node:
```sql
SELECT *
FROM cypher('test_graph', $$
CREATE (:V1 {val: 1})
$$) AS (result agtype);
```
Run an unmatched `OPTIONAL MATCH` after matching the seed node:
```sql
SELECT *
FROM cypher('test_graph', $$
MATCH (v:V1 {val: 1})
OPTIONAL MATCH (n:DefinitelyMissing)
RETURN v.val AS seed, n AS result
$$) AS (seed agtype, result agtype);
```
Apache AGE correctly returns:
```text
seed | result
-----+-------
1 | null
```
This shows that AGE implements null-preserving behavior correctly when an
input row is supplied by a preceding `MATCH`. The failure occurs when
`OPTIONAL MATCH` is the first clause and must operate on the query's initial
input row.
### Cross-engine comparison
| Engine | Standalone unmatched `OPTIONAL MATCH` |
|---|---|
| Neo4j `2026.05` | One row: `{result: null}` |
| Neo4j `5.26` | One row: `{result: null}` |
| Memgraph `3.11.0` | One row: `{result: null}` |
| FalkorDB `4.18.11` | One row: `{result: null}` |
| Apache AGE `1.7.0` | Zero rows |
### Why this looks like a bug
The following two query shapes should apply the same null-preserving
`OPTIONAL MATCH` semantics:
```cypher
OPTIONAL MATCH (n:DefinitelyMissing)
RETURN n
```
and:
```cypher
MATCH (v:V1)
OPTIONAL MATCH (n:DefinitelyMissing)
RETURN v, n
```
In the second query, AGE correctly preserves each incoming row and binds
`n` to `null`.
In the first query, the query begins with one implicit input row. AGE should
preserve that row in the same way, but instead removes it entirely.
This suggests that the standalone `OPTIONAL MATCH` execution path starts
with an empty input relation rather than the required single initial row.
### Related but distinct issue
Apache AGE issue #2378 reports a different `OPTIONAL MATCH` null-preservation
problem involving a preceding `MATCH` and correlated subquery predicates.
This report is distinct because:
- there is no preceding clause;
- there is no correlated subquery;
- no graph data is required for the failing query;
- the failure is specifically the loss of the initial input row for a
standalone `OPTIONAL MATCH`.
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