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Fix VLE [*0..N] zero-hop self-binding when edge label is missing (#2382) (#2419)
A variable-length relationship pattern with a zero lower bound, e.g. `(p)-[:LABEL*0..N]-(f)`, must produce the zero-hop self-binding row (`f` = `p`) regardless of whether any edge of `LABEL` exists in the graph. This matches Neo4j/openCypher semantics. Previously, when the edge label did not exist in the label cache, AGE short-circuited the entire MATCH to zero rows (or NULL-extended rows for OPTIONAL MATCH). The fix has three parts: 1. parser/cypher_clause.c: A new helper `is_zero_lower_bound_vle()` inspects the FuncCall produced by `build_VLE_relation()` and reports whether the relationship is a zero-bound VLE. It is intentionally defensive about the FuncCall shape so that any future parser changes fall back to the existing short-circuit safely. `match_check_valid_label()` and `path_check_valid_label()` now treat a missing edge label as fatal only when the relationship requires at least one edge of that label. Patterns mixing a zero-bound segment with another impossible segment (e.g. `(a)-[:NOEXIST*0..1]-(b)-[:STILL_MISSING]-(c)`) still correctly resolve to zero rows because the second segment independently fails the label check. 2. utils/adt/age_vle.c: `is_an_edge_match()` now returns false early when the user requested a specific label that does not exist (`edge_label_name != NULL && edge_label_name_oid == InvalidOid`). This prevents a zero-bound traversal of `[:NOEXIST*0..N]` from incorrectly walking arbitrary other-label edges via the existing "no constraints -> match all" fast path. The zero-hop case itself is unaffected because it is generated by `build_VLE_zero_container()` without ever consulting `is_an_edge_match()`. 3. regress/sql/cypher_vle.sql: Adds seven regression cases that lock in the new behaviour, including the rubber-duck scenarios where another label exists in the graph (must NOT be matched by the missing-label VLE), where another segment is unsatisfiable (must still produce zero rows), and where the label exists (sanity check, unchanged behaviour).
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Lines changed: 269 additions & 2 deletions

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‎regress/expected/cypher_vle.out‎

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Original file line numberDiff line numberDiff line change
@@ -1219,6 +1219,121 @@ NOTICE: graph "cypher_vle" has been dropped
12191219

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(1 row)
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--
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-- Issue #2382: variable-length relationships with a zero lower bound must
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-- still produce the zero-hop self-binding even when the edge label does not
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-- exist in the graph (Neo4j/openCypher semantics).
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--
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SELECT create_graph('issue_2382');
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NOTICE: graph "issue_2382" has been created
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create_graph
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--------------
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(1 row)
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SELECT * FROM cypher('issue_2382', $$
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CREATE (:Person {name: 'Alice'})-[:KNOWS]->(:Person {name: 'Bob'})
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$$) AS (v agtype);
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v
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---
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(0 rows)
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-- Plain MATCH on a non-existent edge label with [*0..N] must return the
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-- zero-hop self-binding row (Alice -> Alice). It must NOT match arbitrary
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-- edges of other labels (e.g. KNOWS).
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SELECT * FROM cypher('issue_2382', $$
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MATCH (p:Person {name: 'Alice'})
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MATCH (p)-[:NOEXIST*0..1]-(f:Person)
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RETURN p.name AS person, f.name AS friend
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$$) AS (person agtype, friend agtype);
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person | friend
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---------+---------
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"Alice" | "Alice"
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(1 row)
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-- OPTIONAL MATCH form (the exact shape from the issue report).
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SELECT * FROM cypher('issue_2382', $$
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MATCH (p:Person {name: 'Alice'})
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OPTIONAL MATCH (p)-[:NOEXIST*0..1]-(f:Person)
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RETURN p.name AS person, f.name AS friend
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$$) AS (person agtype, friend agtype);
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person | friend
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---------+---------
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"Alice" | "Alice"
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(1 row)
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-- [*0..0] still emits exactly the zero-hop self-binding.
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SELECT * FROM cypher('issue_2382', $$
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MATCH (p:Person {name: 'Alice'})
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MATCH (p)-[:NOEXIST*0..0]-(f:Person)
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RETURN p.name AS person, f.name AS friend
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$$) AS (person agtype, friend agtype);
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person | friend
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---------+---------
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"Alice" | "Alice"
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(1 row)
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-- Fixed-length (lower bound > 0) on a missing label must still return zero
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-- rows: there is no edge of that label, so the pattern is unsatisfiable.
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SELECT * FROM cypher('issue_2382', $$
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MATCH (p:Person {name: 'Alice'})
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MATCH (p)-[:NOEXIST*1..1]-(f:Person)
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RETURN p.name AS person, f.name AS friend
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$$) AS (person agtype, friend agtype);
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person | friend
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--------+--------
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(0 rows)
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-- OPTIONAL MATCH on the unsatisfiable fixed-length pattern still preserves
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-- the outer row with NULL bindings.
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SELECT * FROM cypher('issue_2382', $$
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MATCH (p:Person {name: 'Alice'})
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OPTIONAL MATCH (p)-[:NOEXIST*1..1]-(f:Person)
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RETURN p.name AS person, f.name AS friend
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$$) AS (person agtype, friend agtype);
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person | friend
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---------+--------
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"Alice" |
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(1 row)
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-- Mixed pattern: a zero-bound VLE on a missing label combined with another
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-- fixed-length missing label segment must still yield zero rows. The other
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-- segment is impossible regardless of the zero-hop case.
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SELECT * FROM cypher('issue_2382', $$
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MATCH (a:Person {name: 'Alice'})
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MATCH (a)-[:NOEXIST*0..1]-(b:Person)-[:STILL_MISSING]-(c:Person)
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RETURN a.name, b.name, c.name
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$$) AS (a agtype, b agtype, c agtype);
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a | b | c
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---+---+---
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(0 rows)
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-- Sanity: zero-bound VLE on an EXISTING label still works the way it did
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-- before (Alice via zero-hop, Bob via 1-hop KNOWS).
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SELECT * FROM cypher('issue_2382', $$
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MATCH (p:Person {name: 'Alice'})
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MATCH (p)-[:KNOWS*0..1]-(f:Person)
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RETURN p.name AS person, f.name AS friend
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ORDER BY f.name
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$$) AS (person agtype, friend agtype);
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person | friend
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---------+---------
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"Alice" | "Alice"
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"Alice" | "Bob"
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(2 rows)
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SELECT drop_graph('issue_2382', true);
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NOTICE: drop cascades to 4 other objects
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DETAIL: drop cascades to table issue_2382._ag_label_vertex
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drop cascades to table issue_2382._ag_label_edge
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drop cascades to table issue_2382."Person"
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drop cascades to table issue_2382."KNOWS"
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NOTICE: graph "issue_2382" has been dropped
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drop_graph
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------------
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(1 row)
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12221337
--
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-- End
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--

‎regress/sql/cypher_vle.sql‎

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Original file line numberDiff line numberDiff line change
@@ -417,6 +417,75 @@ SELECT drop_graph('issue_2092', true);
417417
DROP TABLE start_and_end_points;
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SELECT drop_graph('cypher_vle', true);
420+
--
421+
-- Issue #2382: variable-length relationships with a zero lower bound must
422+
-- still produce the zero-hop self-binding even when the edge label does not
423+
-- exist in the graph (Neo4j/openCypher semantics).
424+
--
425+
SELECT create_graph('issue_2382');
426+
427+
SELECT * FROM cypher('issue_2382', $$
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CREATE (:Person {name: 'Alice'})-[:KNOWS]->(:Person {name: 'Bob'})
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$$) AS (v agtype);
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-- Plain MATCH on a non-existent edge label with [*0..N] must return the
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-- zero-hop self-binding row (Alice -> Alice). It must NOT match arbitrary
433+
-- edges of other labels (e.g. KNOWS).
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SELECT * FROM cypher('issue_2382', $$
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MATCH (p:Person {name: 'Alice'})
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MATCH (p)-[:NOEXIST*0..1]-(f:Person)
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RETURN p.name AS person, f.name AS friend
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$$) AS (person agtype, friend agtype);
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-- OPTIONAL MATCH form (the exact shape from the issue report).
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SELECT * FROM cypher('issue_2382', $$
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MATCH (p:Person {name: 'Alice'})
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OPTIONAL MATCH (p)-[:NOEXIST*0..1]-(f:Person)
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RETURN p.name AS person, f.name AS friend
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$$) AS (person agtype, friend agtype);
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-- [*0..0] still emits exactly the zero-hop self-binding.
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SELECT * FROM cypher('issue_2382', $$
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MATCH (p:Person {name: 'Alice'})
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MATCH (p)-[:NOEXIST*0..0]-(f:Person)
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RETURN p.name AS person, f.name AS friend
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$$) AS (person agtype, friend agtype);
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-- Fixed-length (lower bound > 0) on a missing label must still return zero
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-- rows: there is no edge of that label, so the pattern is unsatisfiable.
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SELECT * FROM cypher('issue_2382', $$
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MATCH (p:Person {name: 'Alice'})
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MATCH (p)-[:NOEXIST*1..1]-(f:Person)
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RETURN p.name AS person, f.name AS friend
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$$) AS (person agtype, friend agtype);
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-- OPTIONAL MATCH on the unsatisfiable fixed-length pattern still preserves
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-- the outer row with NULL bindings.
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SELECT * FROM cypher('issue_2382', $$
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MATCH (p:Person {name: 'Alice'})
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OPTIONAL MATCH (p)-[:NOEXIST*1..1]-(f:Person)
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RETURN p.name AS person, f.name AS friend
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$$) AS (person agtype, friend agtype);
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-- Mixed pattern: a zero-bound VLE on a missing label combined with another
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-- fixed-length missing label segment must still yield zero rows. The other
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-- segment is impossible regardless of the zero-hop case.
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SELECT * FROM cypher('issue_2382', $$
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MATCH (a:Person {name: 'Alice'})
475+
MATCH (a)-[:NOEXIST*0..1]-(b:Person)-[:STILL_MISSING]-(c:Person)
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RETURN a.name, b.name, c.name
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$$) AS (a agtype, b agtype, c agtype);
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-- Sanity: zero-bound VLE on an EXISTING label still works the way it did
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-- before (Alice via zero-hop, Bob via 1-hop KNOWS).
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SELECT * FROM cypher('issue_2382', $$
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MATCH (p:Person {name: 'Alice'})
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MATCH (p)-[:KNOWS*0..1]-(f:Person)
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RETURN p.name AS person, f.name AS friend
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ORDER BY f.name
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$$) AS (person agtype, friend agtype);
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SELECT drop_graph('issue_2382', true);
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--
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-- End

‎src/backend/parser/cypher_clause.c‎

Lines changed: 71 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -132,6 +132,60 @@ static Expr *transform_cypher_edge(cypher_parsestate *cpstate,
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static Expr *transform_cypher_node(cypher_parsestate *cpstate,
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cypher_node *node, List **target_list,
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bool output_node, bool valid_label);
135+
/*
136+
* Issue #2382: For variable-length relationships with a lower bound of 0
137+
* (e.g., [:LABEL*0..N]), the zero-hop self-binding case must succeed even
138+
* when LABEL is missing from the cache, because Neo4j/openCypher semantics
139+
* say a zero-hop pattern matches the same node regardless of any edges.
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*
141+
* By the time match_check_valid_label() runs, build_VLE_relation() (in
142+
* cypher_gram.y) has rewritten cypher_relationship.varlen from A_Indices
143+
* into a FuncCall named "vle" whose argument list is:
144+
* (start_id, end_id, edge_match_proto, lidx, uidx, dir, unique_id)
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* so the lower-bound is the 4th argument (1-based).
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*
147+
* This helper is intentionally defensive: every assumption about the shape
148+
* of the FuncCall is guarded so any parser refactor that changes it will
149+
* fall back to "not zero-bound", which is the safe behaviour (the existing
150+
* false-where short-circuit will still kick in for impossible patterns).
151+
*/
152+
static bool is_zero_lower_bound_vle(Node *varlen)
153+
{
154+
FuncCall *fc;
155+
String *fname;
156+
Node *lidx_node;
157+
A_Const *lidx;
158+
159+
if (varlen == NULL || !IsA(varlen, FuncCall))
160+
return false;
161+
162+
fc = (FuncCall *) varlen;
163+
164+
if (list_length(fc->funcname) != 1)
165+
return false;
166+
fname = (String *) linitial(fc->funcname);
167+
if (fname == NULL || !IsA(fname, String))
168+
return false;
169+
if (strcmp(strVal(fname), "vle") != 0)
170+
return false;
171+
172+
/* args = {start, end, edge_match, lidx, uidx, dir, uniq} */
173+
if (list_length(fc->args) < 5)
174+
return false;
175+
176+
lidx_node = (Node *) list_nth(fc->args, 3);
177+
if (lidx_node == NULL || !IsA(lidx_node, A_Const))
178+
return false;
179+
180+
lidx = (A_Const *) lidx_node;
181+
if (lidx->isnull)
182+
return false;
183+
if (lidx->val.ival.type != T_Integer)
184+
return false;
185+
186+
return lidx->val.ival.ival == 0;
187+
}
188+
135189
static bool match_check_valid_label(cypher_match *match,
136190
cypher_parsestate *cpstate);
137191
static Node *make_vertex_expr(cypher_parsestate *cpstate,
@@ -2927,7 +2981,14 @@ static bool match_check_valid_label(cypher_match *match,
29272981

29282982
if (lcd == NULL || lcd->kind != LABEL_KIND_EDGE)
29292983
{
2930-
return false;
2984+
/*
2985+
* Issue #2382: a missing edge label is fatal only if
2986+
* the pattern actually requires an edge of that label.
2987+
* For VLE with lower bound 0, the zero-hop self-bind
2988+
* case must still produce rows.
2989+
*/
2990+
if (!is_zero_lower_bound_vle(rel->varlen))
2991+
return false;
29312992
}
29322993
}
29332994
}
@@ -5047,7 +5108,15 @@ static bool path_check_valid_label(cypher_path *path,
50475108

50485109
if (lcd == NULL || lcd->kind != LABEL_KIND_EDGE)
50495110
{
5050-
return false;
5111+
/*
5112+
* Issue #2382: Don't invalidate the whole path just
5113+
* because a VLE edge with lower bound 0 references a
5114+
* missing label. The zero-hop self-binding semantics
5115+
* still allow the surrounding nodes to bind, so the
5116+
* other vertex labels in this path must be honoured.
5117+
*/
5118+
if (!is_zero_lower_bound_vle(rel->varlen))
5119+
return false;
50515120
}
50525121
}
50535122
}

‎src/backend/utils/adt/age_vle.c‎

Lines changed: 14 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -417,6 +417,20 @@ static bool is_an_edge_match(VLE_local_context *vlelctx, edge_entry *ee)
417417
/* get the number of conditions from the prototype edge */
418418
num_edge_property_constraints = AGT_ROOT_COUNT(vlelctx->edge_property_constraint);
419419

420+
/*
421+
* Issue #2382: If the user asked for a specific edge label but that label
422+
* does not exist in the graph (edge_label_name_oid == InvalidOid while
423+
* edge_label_name is non-NULL), no real edge can match. Returning false
424+
* here ensures that for VLE patterns like [:NOEXIST*0..N] we do not
425+
* traverse arbitrary other-label edges. Zero-hop self-binding is handled
426+
* separately via build_VLE_zero_container() so this does not break it.
427+
*/
428+
if (vlelctx->edge_label_name != NULL &&
429+
vlelctx->edge_label_name_oid == InvalidOid)
430+
{
431+
return false;
432+
}
433+
420434
/*
421435
* We only care about verifying that we have all of the property conditions.
422436
* We don't care about extra unmatched properties. If there aren't any edge

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