kosiew opened a new issue, #17206:
URL: https://github.com/apache/datafusion/issues/17206
Summary
-------
When the build side of a hash join contains only NULL join keys and the join
is configured with
`datafusion_common::NullEquality::NullEqualsNothing`, the dynamic filter
generated for the probe side
is produced as range comparisons against `NULL` (e.g. `a >= NULL AND a <=
NULL`). In the current
optimizer/test code this is treated as a no-op (a tautology) instead of
being treated as either
unsatisfiable or as an explicit "no matches" condition.
Why this matters
----------------
- This is a surprising corner case and may hide regressions if semantics or
simplification rules change.
- If NullEquality semantics change (or the filter simplifier is updated),
behavior and test expectations
could silently diverge.
- We should monitor and decide whether this should be:
- left as a tautology (no-op),
- treated as unsatisfiable (prune everything),
- or canonicalized/annotated to make intent explicit.
Where to look / repro
---------------------
Test: `datafusion/core/tests/physical_optimizer/filter_pushdown/mod.rs`
Specifically the test `test_hashjoin_dynamic_filter_pushdown_null_keys` in
#17090
Reproduction steps (test harness)
1. Run that single async test (or the optimizer tests):
- cargo test --test <appropriate test binary> --filter
test_hashjoin_dynamic_filter_pushdown_null_keys
2. Observe plan printed by `format_plan_for_test(&plan)` contains:
`DynamicFilterPhysicalExpr [ a@0 >= NULL AND a@0 <= NULL AND b@1 >= NULL
AND b@1 <= NULL ]`
Observed behavior
-----------------
The optimizer generates a dynamic filter with min/max bounds set to NULL;
the code interprets this as
a tautology (no filtering). The current test documents this and asserts the
presence of the `>= NULL` / `<= NULL` pattern.
Expected / options
------------------
We need a decision on intended semantics. Options:
- Keep current behavior (tautology/no-op). Document it clearly in code/tests.
- Treat NULL-only build-side as unsatisfiable filter (drop all probe rows).
- Change filter generation to avoid producing `>= NULL` / `<= NULL` and
instead produce an explicit marker (e.g., `unsatisfiable`) so optimizer
simplification can handle it deterministically.
References
----------
File: `datafusion/core/tests/physical_optimizer/filter_pushdown/mod.rs`
Test: `test_hashjoin_dynamic_filter_pushdown_null_keys`
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