Rename scalar function strictness contract

[connortsui20]

Changes is_null_sensitive to is_strict and flips all current implementations (as is_strict implies !is_null_sensitive).

A function is strict only when any null input forces a null output and its return dtype propagates that nullability.

Also fixes Between and Dynamic scalar functions as they were both set to strict when they should have been non-strict, so now they are is_strict = false.

Rationale for this change

Dictionary pushdown rewrites f(dict(codes, values), constant) as dict(codes, f(values, constant)). At a null code, the original evaluates f(null, constant), while the rewritten dictionary is always null. This is only sound when nulling any one argument forces a null result while the other arguments remain unchanged.

is_null_sensitive documented the unary case but left this multi-argument contract ambiguous. Kleene AND/OR show the difference: masking both inputs produces null, but false AND null = false and true OR null = true. That ambiguity caused the bug fixed in #8929.

Additionally, we have noticed that a large majority of our scalar functions have strict semantics, and so in the future (hopefully soon) we can lift out a lot of the shared null logic.

What changes are included in this PR?

Functions that are strict, previously treated as non-strict

These functions are strict, so the old null-sensitivity classification was overly conservative:

• ArrayExpr and RowIdx inherited the old conservative default. These functions take no arguments, so the strictness condition does not apply.
• Mask inherited the old conservative default, but a null input remains null and its mask argument is required to be non-nullable.
• VariantGet inherited the old conservative default, but a null Variant input produces a null output.
• GetItem and Select preserve the parent struct validity and propagate its nullability.
• Merge only accepts non-nullable inputs, so the strictness law is vacuously satisfied for well-typed arguments.

Functions that are non-strict, previously treated as strict

These functions are non-strict, so the old classification allowed unsound dictionary pushdown:

• Between: its comparisons are combined with Kleene AND; for example, 10 BETWEEN null AND 5 is false, not null.
• DynamicComparison: when the dynamic RHS is absent, it returns its configured default even when the LHS is null.

Remaining non-strict functions

• Binary for Kleene AND/OR
• CaseWhen and Zip
• Cast
• FillNull, IsNull, and IsNotNull
• ForeignScalarFnVTable
• ListContains
• Pack
• RowEncode and RowSize
• StatFn

Remaining strict functions

• Binary for all operators except AND/OR
• ByteLength, ExtStorage, Like, ListLength, ListSum, and Not
• Literal, Root, and RowCount (nullary)
• JsonToVariant
• GeoContains, GeoDistance, GeoEnvelope, and GeoIntersects
• CosineSimilarity, InnerProduct, L2Denorm, and L2Norm

[joseph-isaacs]

good direction. just want to ensure it's all sound

[joseph-isaacs]

Can we rewrite the description to just mention strict and non-strict results. mentioning is_null_sensitive = true here is confusing!

[joseph-isaacs]

this is not the default, is it?

[connortsui20]

I changed the default to false on the vtable?

[joseph-isaacs]

Add this file is the wrong vtable lol

[codecov]

Codecov Report

❌ Patch coverage is 60.13072% with 61 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.27%. Comparing base (caedc63) to head (a3494a5).
⚠️ Report is 7 commits behind head on develop.

Files with missing lines	Patch %	Lines
vortex-array/src/arrays/scalar_fn/vtable/mod.rs	0.00%	3 Missing ⚠️
vortex-array/src/scalar_fn/fns/mask/mod.rs	0.00%	3 Missing ⚠️
vortex-array/src/scalar_fn/fns/stat.rs	0.00%	3 Missing ⚠️
vortex-array/src/scalar_fn/fns/variant_get/mod.rs	0.00%	3 Missing ⚠️
vortex-array/src/scalar_fn/foreign.rs	0.00%	3 Missing ⚠️
vortex-layout/src/layouts/row_idx/expr.rs	0.00%	3 Missing ⚠️
vortex-array/src/scalar_fn/fns/case_when.rs	0.00%	2 Missing ⚠️
vortex-array/src/scalar_fn/fns/cast/mod.rs	0.00%	2 Missing ⚠️
vortex-array/src/scalar_fn/fns/ext_storage.rs	0.00%	2 Missing ⚠️
...ortex-array/src/scalar_fn/fns/list_contains/mod.rs	0.00%	2 Missing ⚠️
... and 19 more

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