[SQL] Rework the implementation of SAFE_CAST #5428
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Signed-off-by: Mihai Budiu <mbudiu@feldera.com>
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Pull request overview
This PR reworks the SAFE_CAST implementation to properly handle complex nested types (MAP, ARRAY, ROW). The key innovation is distinguishing between SQL-level casts (which may fail) and Rust-level casts (which return SqlResult), then using error handlers to convert failures into NULL values for safe casts or panics for unsafe casts.
Changes:
- Introduced
DBSPTypeSqlResulttype andCastTypeenum to distinguish SQL vs Rust casts - Split cast expansion into
ExpandSafeCastsandExpandUnsafeCastsclasses - Added safe conversion functions to Rust sqllib (
array_map_safe,map_map_safe) - Restricted SQL dialect to disallow MAP keys of type MAP and SAFE_CAST on ROW types
- Updated error handling to use
RuntimeBehaviorenum inDBSPHandleErrorExpression
Reviewed changes
Copilot reviewed 120 out of 122 changed files in this pull request and generated no comments.
Show a summary per file
| File | Description |
|---|---|
| DBSPCastExpression.java | Added CastType enum (SqlSafe/SqlUnsafe/RustCast/UnwrapResultSafe/UnwrapResultUnsafe) |
| DBSPTypeSqlResult.java | New type representing SqlResult[T] from sqllib |
| ExpandSafeCasts.java | New visitor for expanding safe casts into recursive safe conversions |
| ExpandUnsafeCasts.java | New visitor for expanding unsafe casts with panic behavior |
| DBSPHandleErrorExpression.java | Updated to use RuntimeBehavior enum and handle SqlResult unwrapping |
| CreateRuntimeErrorWrappers.java | Refactored to call expandCasts and wrap errors |
| ExpressionCompiler.java | Updated cast validation and all cast call sites to use CastType enum |
| Test files | Added comprehensive tests for safe cast on arrays, maps, and edge cases |
| sqllib (Rust) | Added array_map_safe and map_map_safe functions for safe conversions |
| Documentation | Updated casts.md and map.md with new restrictions |
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Fixes #5378
Fixes #5389
Fixes #5390
Fixes #5391
Most changes in this PR are due to automatic refactoring. I split the code to implement complex casts into two classes, handling safe and unsafe casts separately (they were unified, which made everything unreadable).
I have also cut the Gordian knot and removed from our SQL dialect (in the documentation and validation) several kinds of operations, including creating MAP values with MAP keys or using SAFE_CAST on ROW values. This bounded the implementation effort; we can bring these back later if anyone asks.
SAFE_CASTs are supposed to attempt a conversion and return NULL if the conversion fails. This is easy for scalar values, but gets trickier for nested objects like MAP, ARRAY, and ROW, or, if you like, MAP<ARRAY, ROW>. This is now implemented by essentially using the
SqlResulttype (which is just an instance of Rust'sResulttype) to signal errors in cast expression evaluation, and handling these errors at the appropriate place (the outermost scope).The compiler now distinguishes between two kinds of casts: high-level SQL casts, and low-level Rust casts. (They are represented by the same Java class, with different flags.) They used to be conflated previously, which made things REALLY confusing. (Note to self: don't do this again.)