SAY-5 opened a new pull request, #2399:
URL: https://github.com/apache/age/pull/2399
Fixes #2384.
`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`:
```c
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 the issue —
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.
Removing the early return is enough: 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`.
Regression tests and expected output are updated accordingly — the two
"should return null" cases at the end of the `tail()` section now expect `[]`
and the comment is corrected.
```sql
SELECT * FROM cypher('list', $$ RETURN tail([1]) $$) AS (tail agtype); --
[]
SELECT * FROM cypher('list', $$ RETURN tail([]) $$) AS (tail agtype); --
[]
```
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