[I] `tail([1])` and `tail([])` return `null` instead of `[]` [age]

["shulei5831sl (via GitHub)" <gi...@apache.org>]

shulei5831sl opened a new issue, #2472:
URL: https://github.com/apache/age/issues/2472

   ### Apache AGE version
   
   Observed on Docker image:
   
   ```text
   docker pull apache/age:latest
   Pulled: 2026-07-09
   
   Apache AGE: PG18
   
   Endpoint:
   postgres://127.0.0.1:5432
   ```
   
   ### How are you accessing AGE
   
   psql and Python `psycopg2` driver
   
   ### Environment
   
   * Host OS: Linux `5.4.0-216-generic`
   * Deployment: Docker `apache/age:latest`
   * Query interface: psql and psycopg2
   * Query language: Cypher via `ag_catalog.cypher`
   * Configuration: default AGE on PostgreSQL 18
   * Differential comparison targets:
   
     * Neo4j `2026.05.0`
   
   ### Description
   
   AGE's `tail()` function returns `null` when the result should be an empty list.
   
   The issue occurs for:
   
   ```cypher
   tail([1])
   tail([])
   ```
   
   Both should return `[]`.
   
   `tail()` is a list-returning function: it returns all elements of the input list except the first one. Therefore, dropping the only element from `[1]` should produce an empty list, and applying `tail()` to an already empty list should also produce an empty list.
   
   Neo4j returns `[]` for both cases.
   
   ### Setup
   
   ```pgsql
   LOAD 'age';
   SET search_path = ag_catalog, '$user', public;
   SELECT * FROM ag_catalog.create_graph('test_graph');
   ```
   
   ### Failing queries
   
   ```pgsql
   SELECT * FROM cypher('test_graph', $$
     RETURN tail([1]) AS r
   $$) AS (r agtype);
   ```
   
   AGE returns:
   
   ```text
   r = null
   ```
   
   Expected:
   
   ```text
   r = []
   ```
   
   ```pgsql
   SELECT * FROM cypher('test_graph', $$
     RETURN tail([]) AS r
   $$) AS (r agtype);
   ```
   
   AGE returns:
   
   ```text
   r = null
   ```
   
   Expected:
   
   ```text
   r = []
   ```
   
   ### Control queries
   
   #### Normal `tail()` works
   
   ```pgsql
   SELECT * FROM cypher('test_graph', $$
     RETURN tail([1,2,3]) AS r
   $$) AS (r agtype);
   ```
   
   AGE returns:
   
   ```text
   r = [2, 3]
   ```
   
   This is correct.
   
   #### `tail(null)` should return `null`
   
   ```pgsql
   SELECT * FROM cypher('test_graph', $$
     RETURN tail(null) AS r
   $$) AS (r agtype);
   ```
   
   Expected:
   
   ```text
   r = null
   ```
   
   This distinguishes null input from empty-list output.
   
   #### Other list functions show the expected boundary behavior
   
   ```pgsql
   SELECT * FROM cypher('test_graph', $$
     RETURN reverse([]) AS r
   $$) AS (r agtype);
   ```
   
   AGE returns:
   
   ```text
   r = []
   ```
   
   This is correct: a list-returning function applied to an empty list returns an empty list.
   
   ```pgsql
   SELECT * FROM cypher('test_graph', $$
     RETURN head([]) AS r
   $$) AS (r agtype);
   ```
   
   AGE returns:
   
   ```text
   r = null
   ```
   
   This is correct: `head()` returns an element, and there is no element in an empty list.
   
   ```pgsql
   SELECT * FROM cypher('test_graph', $$
     RETURN last([]) AS r
   $$) AS (r agtype);
   ```
   
   AGE returns:
   
   ```text
   r = null
   ```
   
   This is correct: `last()` returns an element, and there is no element in an empty list.
   
   ### Cross-engine comparison
   
   | Expression      | Apache AGE | Neo4j `2026.05.0` |
   | --------------- | ---------: | ----------------: |
   | `tail([1])`     |     `null` |              `[]` |
   | `tail([])`      |     `null` |              `[]` |
   | `tail([1,2,3])` |    `[2,3]` |           `[2,3]` |
   | `tail(null)`    |     `null` |            `null` |
   | `reverse([])`   |       `[]` |              `[]` |
   | `head([])`      |     `null` |            `null` |
   | `last([])`      |     `null` |            `null` |
   
   ### Impact
   
   This can silently change query logic when `tail()` is used in list pipelines.
   
   For example:
   
   ```pgsql
   SELECT * FROM cypher('test_graph', $$
     RETURN size(tail([1])) AS r
   $$) AS (r agtype);
   ```
   
   Expected:
   
   ```text
   r = 0
   ```
   
   AGE may return `null` because `tail([1])` returns `null`.
   
   This also affects predicates and CASE expressions that distinguish an empty list from `null`.
   
   ### Summary
   
   `tail()` should return an empty list when all elements are removed from the input list. AGE currently returns `null` for `tail([1])` and `tail([])`, even though it correctly returns lists for non-empty outputs such as `tail([1,2,3])`.
   


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