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fix(ddx-ad): to_sql writes a NULL whose type has no kind as a bare NULL - #244
On DuckDB, grad refused any loss with a bare NULL in it:
invalid Substrait plan: a type's kind is missing
DuckDB's Substrait producer gives a bare NULL literal a Type with no kind. It also folds CAST(NULL AS BIGINT) into one, so a cast didn't help. On DuckDB every step runs as the SQL to_sql writes, and to_sql refused a type with no kind. DataFusion gave the right gradients.
How
to_sql already writes DataFusion's untyped NULL (its user-defined type null) as a bare NULL. A literal NULL whose type has no kind is the same thing, so it is now read as one too (Reader::ty_or_none). The engine types a bare NULL from its context, as it typed the user's.
Edge cases
SELECT NULL AS k is still refused, but by DuckDB, before ddx sees a plan: its producer can't write a column of type NULL ("Logical Type NULL not implemented as Substrait Schema Result").
A virtual table column whose type has no kind is read the same way, as a column of no type.
Tests
reader.rs: a projection of a kindless NULL is written as a bare NULL.
test_duckdb.py: the issue's three losses (a NULL key, a NULL value, a cast NULL key joined), each against DataFusion's gradient for the same SQL. They fail on main.
Improved handling of untyped NULL values in DuckDB plans. These values are now emitted as bare NULL rather than being wrapped in an unnecessary cast, helping queries with nullable values and join keys produce results consistent with DataFusion.
DuckDB's Substrait producer gives a bare NULL literal a type with no
kind, and folds a CAST(NULL AS BIGINT) into one. On DuckDB every step
runs as the SQL to_sql writes, which refused such a literal ("a type's
kind is missing"), so grad refused any loss holding one (#235). It is a
NULL of no type, as DataFusion's `null` type is, and is written as one.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CvszJhH8pn8H9G69wEPMU2
The SQL reader now treats Substrait types without a kind as untyped NULLs. Tests cover generated SQL and DuckDB gradient results for queries involving NULL values or keys.
Changes: Kindless NULL handling
Layer / File(s)
Summary
Read kindless types as untyped NULLs crates/ddx-ad/src/to_sql/reader.rs
Reader::ty_or_none returns no type when a Substrait type has no kind. A regression test checks that generated SQL uses bare NULL, not CAST(NULL ...).
Check DuckDB gradients with NULLs python/ddxdb/tests/test_duckdb.py
Parameterized tests compare DuckDB gradients with DataFusion results for three queries involving NULL values or keys.
xqlsystems/ddx#232: Handles the related untyped-NULL failure for DataFusion’s user-defined null type.
xqlsystems/ddx#191: Introduces the to_sql reader that this change updates.
xqlsystems/ddx#192: Adds the DuckDB Substrait planning path tested by these NULL gradient cases.
Suggested labels:testing
Merge Risk: Minimal · up to 96e35
The change handles kindless NULLs with SQL-generation and gradient tests. Additional virtual-table coverage would improve confidence, but no merge-blocking issue remains.
Pre-merge checks: 4 · 1
Failed checks: 1 warning
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Docstring Coverage
Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…
Write docstrings for the functions missing them to satisfy the coverage threshold.
Passed checks: 4 passed
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Title check
The title names the change: to_sql now writes a NULL whose Substrait type has no kind as a bare NULL. It also names the crate (ddx-ad) and uses a conventional fix() prefix. The raw summary confirms th…
Linked Issues check
Issue [#235] requires DuckDB grad to handle losses that contain kindless NULL literals. Reader::ty_or_none now returns no type for a Substrait type with no kind, and Reader::literal converts t…
Out of Scope Changes check
The changes are limited to kindless-NULL handling in crates/ddx-ad/src/to_sql/reader.rs and regression coverage in python/ddxdb/tests/test_duckdb.py. The implementation and tests directly support …
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The title names the change: to_sql now writes a NULL whose Substrait type has no kind as a bare NULL. It also names the crate (ddx-ad) and uses a conventional fix() prefix. The raw summary confirms the change in Reader::ty_or_none and the regression test, and the PR objectives confirm the same goal of fixing the DuckDB grad failure described in issue #235. The title is specific and on-topic. It is 71 characters, which is within a reasonable length for a single-sentence title.
Full details: Linked Issues check
Explanation
Issue [#235] requires DuckDB grad to handle losses that contain kindless NULL literals. Reader::ty_or_none now returns no type for a Substrait type with no kind, and Reader::literal converts the NULL literal to Literal::UntypedNull. The existing SQL path writes that literal as bare NULL. The reported regression tests cover a nullable key, a nullable value, and a cast NULL used as a join key, with results compared with DataFusion. DuckDB producer failure for SELECT NULL AS k occurs before ddx and is outside this requirement.
Full details: Out of Scope Changes check
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The changes are limited to kindless-NULL handling in crates/ddx-ad/src/to_sql/reader.rs and regression coverage in python/ddxdb/tests/test_duckdb.py. The implementation and tests directly support issue [#235]. No unrelated change is established.
Full details: Docstring Coverage
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Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 2 functions across 2 files.
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Write docstrings for the functions missing them to satisfy the coverage threshold.
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Add a test for a virtual-table column with no kind.
ty_or_none now maps a kindless type to None. The virtual-table branch at reader.rs:598 uses that mapping for column types, and its typed closure writes VALUES entries without a cast when the type is None. The new test only projects a kindless NULL over read_step, so it never reaches that branch. If the branch later called ty directly, kindless columns can be rejected, and this test can still pass.
Add a test in the same tests module that reads a virtual table whose column schema has a type with no kind. Assert that the generated SQL for that read has no CAST on the affected column values.
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Review comment at @crates/ddx-ad/src/to_sql/reader.rs at line 1720:
In the tests module, add a test that reads a virtual table with a column whose
type has no kind, then assert the generated SQL has no CAST on that column’s
values. Exercise the virtual-table branch and its typed closure rather than only
projecting a kindless NULL over read_step.
Reviewing files that changed from the base of the PR and between af71683 and 96e35ed.
Files selected for processing: 2
crates/ddx-ad/src/to_sql/reader.rs
python/ddxdb/tests/test_duckdb.py
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Closes #235.
Why
On DuckDB,
gradrefused any loss with a bareNULLin it:DuckDB's Substrait producer gives a bare
NULLliteral aTypewith no kind. It also foldsCAST(NULL AS BIGINT)into one, so a cast didn't help. On DuckDB every step runs as the SQLto_sqlwrites, andto_sqlrefused a type with no kind. DataFusion gave the right gradients.How
to_sqlalready writes DataFusion's untyped NULL (its user-defined typenull) as a bareNULL. A literal NULL whose type has no kind is the same thing, so it is now read as one too (Reader::ty_or_none). The engine types a bareNULLfrom its context, as it typed the user's.Edge cases
SELECT NULL AS kis still refused, but by DuckDB, before ddx sees a plan: its producer can't write a column of type NULL ("Logical Type NULL not implemented as Substrait Schema Result").Tests
reader.rs: a projection of a kindless NULL is written as a bareNULL.test_duckdb.py: the issue's three losses (a NULL key, a NULL value, a cast NULL key joined), each against DataFusion's gradient for the same SQL. They fail on main.to_sql.rs, and ddxdb's tests.🤖 Generated with Claude Code
https://claude.ai/code/session_01CvszJhH8pn8H9G69wEPMU2
Summary by CodeRabbit
NULLvalues in DuckDB plans. These values are now emitted as bareNULLrather than being wrapped in an unnecessary cast, helping queries with nullable values and join keys produce results consistent with DataFusion.