TIMESTAMP WITH TIME ZONE can resolve to a timezone-naive type, and casting to it discards an existing timezone #25166

Describe the bug

Two related type-level problems with TIMESTAMP WITH TIME ZONE. They share a root cause and I think they should be decided together.

1. TIMESTAMP WITH TIME ZONE can resolve to a timezone-naive Arrow type.

With the default configuration (datafusion.execution.time_zone unset):

SELECT arrow_typeof('2024-01-01T12:00:00Z'::timestamptz) AS type,
       '2024-01-01T12:00:00Z'::timestamptz AS value;
+---------------+---------------------+
| type          | value               |
+---------------+---------------------+
| Timestamp(ns) | 2024-01-01T12:00:00 |
+---------------+---------------------+

The Z is accepted and then discarded. A SQL type whose name is "with time zone" produced an Arrow type with no time zone.

2. Casting an already timezone-aware value to timestamptz overwrites its timezone.

SET datafusion.execution.time_zone = 'UTC';
SELECT arrow_typeof((t AT TIME ZONE 'Europe/Brussels')::timestamptz) FROM …;
-- Timestamp(ns, "UTC")   -- the Brussels annotation is gone

This is the root cause of #13962: date_trunc('day', (t AT TIME ZONE 'Europe/Brussels')::timestamptz) groups by UTC day while naming Brussels. Remove the ::timestamptz and the same query is correct. It is also the same defect family as #23841 (to_timestamp discarding an aware input's timezone).

To Reproduce

Both snippets above, on DataFusion 55.0.0 (da89c7c85b).

Expected behavior

Two invariants, which I think are worth stating explicitly whatever the fix:

1. TIMESTAMP WITH TIME ZONE never resolves to Timestamp(_, None). When the session time zone is unset it should fall back to a fixed zone rather than to naive.
2. CAST(x AS TIMESTAMPTZ) where x is already aware preserves x's own timezone.

Additional context

This is a traceable side effect, not a deliberate design. The default changed from "+00:00" to None in #18359 (DataFusion 51.0.0). That change was deliberate and well reasoned — it followed #18017 making now()/current_date()/current_time() read the session timezone, and #18204 / #18081 proposed making the config an Option.

But the whole of its effect on the SQL type system was this edit in datafusion/sql/src/planner.rs:

-  Some(self.context_provider.options().execution.time_zone.clone())
+  self.context_provider.options().execution.time_zone.clone()

a mechanical unwrap because the field's type changed. The comment two lines above it is still in the tree today and still says:

// OUTPUT: [ArrowDataType] Timestamp<TimeUnit, Some(Time Zone)>

Neither the PR body, nor any of the three driver issues, nor the upgrade-guide note mentions timestamptz or TIMESTAMP WITH TIME ZONE — the note frames the change purely as "to better support using the default timezone in scalar UDF functions such as now, current_date, current_time, and to_timestamp". The type-system consequence was never discussed.

PostgreSQL and DuckDB (measured, PG 17.11 / DuckDB 1.5.2) both default TimeZone to the machine's zone, never unset, so timestamptz is always aware in both. Their type is also invariant to the session zone — only rendering changes:

PG      SET TimeZone='UTC';              pg_typeof -> timestamp with time zone,  12:00:00+00
        SET TimeZone='Europe/Brussels';  pg_typeof -> timestamp with time zone,  13:00:00+01
DuckDB  same in both

So invariant 1 has two reference implementations. Invariant 2 does not, and this is worth being explicit about: neither system can represent a per-value timezone at all, so neither can arbitrate. Worse, copying PostgreSQL literally gives you the current behaviour — in PG, tstz::timestamptz renders in the session zone because the value never carried one. Invariant 2 has to be argued on DataFusion's own terms: Arrow puts the zone in the value, so an operation not asked to change it shouldn't. AT TIME ZONE 'Z' already means "convert to Z" and arrow_cast already means "give me exactly this Arrow type", so ::timestamptz does not need to be a third way to change a timezone.

Note that simply changing the default would not fix #13962. With the default set to UTC, invariant 1 holds but the cast still relabels Brussels to UTC and date_trunc still groups by the wrong day. Invariant 2 is the one that fixes it. Changing the default is a separate question, and would flip now()'s type a second time in five releases (aware ≤50 → naive 51+ → aware again), so it deserves its own discussion.

Related: #13962 (user-visible symptom), #23841 (same defect in to_timestamp), #13212 (which timezone an implicit coercion should pick), #21116 (TimestampWithOffset, the direction the ecosystem is moving).