From 8f8778b416f9bb1b3796b8ee4d557acd5ca9e62a Mon Sep 17 00:00:00 2001
From: SAY-5 <SAY-5@users.noreply.github.com>
Date: Sun, 19 Apr 2026 22:52:35 -0700
Subject: [PATCH] agtype: return an empty list (not null) from tail() for empty
 and singleton lists

age_tail() in src/backend/utils/adt/agtype.c had an explicit early return
that mapped both the empty-list and the singleton-list cases to SQL NULL:

    count = AGT_ROOT_COUNT(agt_arg);
    /* if we have an empty list or only one element in the list, return null */
    if (count <= 1)
    {
        PG_RETURN_NULL();
    }

The Cypher specification (and Neo4j / Memgraph, as shown in #2384) both
define tail(list) as "the list without its first element". For a
zero-or-one-element input that is an empty list [], not null. Differential
tests against Neo4j surface this divergence immediately.

Remove the early return. The loop below already iterates from i=1 so it
naturally emits [] for count <= 1 and emits the correct tail for the
general case. No behaviour change for count >= 2. Tests and expected
output are updated to match the new, spec-correct result.

Fixes #2384
---
 regress/expected/expr.out      |  6 +++---
 regress/sql/expr.sql           |  2 +-
 src/backend/utils/adt/agtype.c | 10 +++++-----
 3 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/regress/expected/expr.out b/regress/expected/expr.out
index 6d9341451..e95fe14e3 100644
--- a/regress/expected/expr.out
+++ b/regress/expected/expr.out
@@ -8039,17 +8039,17 @@ SELECT * FROM cypher('list', $$ RETURN tail(["a","b","c","d","e"]) $$) AS (tail
  ["b", "c", "d", "e"]
 (1 row)
 
--- should return null
+-- should return an empty list
 SELECT * FROM cypher('list', $$ RETURN tail([1]) $$) AS (tail agtype);
  tail 
 ------
- 
+ []
 (1 row)
 
 SELECT * FROM cypher('list', $$ RETURN tail([]) $$) AS (tail agtype);
  tail 
 ------
- 
+ []
 (1 row)
 
 -- should throw errors
diff --git a/regress/sql/expr.sql b/regress/sql/expr.sql
index 445e2d237..7304d4d65 100644
--- a/regress/sql/expr.sql
+++ b/regress/sql/expr.sql
@@ -3269,7 +3269,7 @@ SELECT * from cypher('list', $$RETURN range(0, -10.0, -3.0)$$) as (range agtype)
 -- should return the last elements of the list
 SELECT * FROM cypher('list', $$ RETURN tail([1,2,3,4,5]) $$) AS (tail agtype);
 SELECT * FROM cypher('list', $$ RETURN tail(["a","b","c","d","e"]) $$) AS (tail agtype);
--- should return null
+-- should return an empty list
 SELECT * FROM cypher('list', $$ RETURN tail([1]) $$) AS (tail agtype);
 SELECT * FROM cypher('list', $$ RETURN tail([]) $$) AS (tail agtype);
 -- should throw errors
diff --git a/src/backend/utils/adt/agtype.c b/src/backend/utils/adt/agtype.c
index 386219556..35443d8fc 100644
--- a/src/backend/utils/adt/agtype.c
+++ b/src/backend/utils/adt/agtype.c
@@ -6056,11 +6056,11 @@ Datum age_tail(PG_FUNCTION_ARGS)
 
     count = AGT_ROOT_COUNT(agt_arg);
 
-    /* if we have an empty list or only one element in the list, return null */
-    if (count <= 1)
-    {
-        PG_RETURN_NULL();
-    }
+    /*
+     * For an empty or singleton list, tail() returns an empty list. The loop
+     * below already produces that result (i starts at 1 so nothing is pushed
+     * when count <= 1), so we do not special-case the count here.
+     */
 
     /* clear the result structure */
     MemSet(&agis_result, 0, sizeof(agtype_in_state));