[Bug] AVG(decimal) over a window always falls back to Spark on Spark 4.x (AvgDecimal window branch is dead)

[andygrove]

Describe the bug

AVG(<decimal>) over a window falls back to Spark on Spark 4.x, even though CometWindowExec reports AVG as a natively supported window aggregate and process_agg_func has an AvgDecimal window branch.

On Spark 4.x, Average of a decimal is represented in the physical plan as a Cast(avg(UnscaledValue(child)) / pow10 AS decimal(p, s)) wrapping the WindowExpression, for example:

Window [cast((avg(UnscaledValue(v#39)) windowspecdefinition(...) / 100.0) as decimal(14,6)) AS run_avg#58]

CometWindowExec.convert only unwraps two shapes:

case Alias(w: WindowExpression, _) => w
case Alias(MakeDecimal(w: WindowExpression, _, _, _), _) => w
case other => withFallbackReason(...); return None

The Cast(Divide(...)) form matches neither, so the whole WindowExec falls back to Spark. Results stay correct (Spark computes them), but:

• The AvgDecimal native window branch in process_agg_func (native/core/src/execution/planner.rs) is effectively dead on Spark 4.x.
• AVG decimal window aggregates never run natively on Spark 4.x, contrary to the coverage implied by feat: extend native windows support #4209.

Steps to reproduce

statement
CREATE TABLE dec_avg(g int, v decimal(10,2)) USING parquet

statement
INSERT INTO dec_avg VALUES (1, 10.10), (1, 20.25), (1, 30.33), (1, 41.00)

-- runs natively on integer/double, falls back on decimal under Spark 4.x
SELECT v, AVG(v) OVER (PARTITION BY g ORDER BY v
  ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS run_avg
FROM dec_avg

Comet falls back: Unsupported window expression: cast((avg(UnscaledValue(v)) ... / 100.0) as decimal(14,6)).

Expected behavior

AVG(<decimal>) over an ever-expanding window frame runs natively via the AvgDecimal UDAF and matches Spark, the same way SUM(<decimal>) already does. This needs CometWindowExec.convert to recognize the Spark 4.x Cast(Divide(WindowExpression, ...)) average shape (in addition to the existing MakeDecimal shape).

Additional context

• Surfaced by an audit of the native window support added in feat: extend native windows support #4209.
• The MakeDecimal unwrap suggests decimal AVG is reachable on some Spark versions (likely 3.x); the divergent shape is the 4.x Cast(Divide(...)) form. Behavior should be confirmed across 3.4 / 3.5 / 4.0 / 4.1 when fixing.
• Once native decimal AVG is reachable, the sliding-frame overflow guard added for [Bug] SUM(decimal) over a sliding window frame returns wrapped out-of-range value on overflow instead of NULL #4729 (which also covers AVG) becomes the relevant safeguard for sliding frames.
• This is a coverage gap, not a correctness divergence: fallback produces correct results.