fix: don't commit to case when binary format

vortex-array/public-api.lock

@@ -13328,7 +13328,7 @@ pub fn vortex_array::scalar_fn::fns::case_when::CaseWhen::reduce(&self, options:
 
 pub fn vortex_array::scalar_fn::fns::case_when::CaseWhen::return_dtype(&self, options: &Self::Options, arg_dtypes: &[vortex_array::dtype::DType]) -> vortex_error::VortexResult<vortex_array::dtype::DType>
 
-pub fn vortex_array::scalar_fn::fns::case_when::CaseWhen::serialize(&self, options: &Self::Options) -> vortex_error::VortexResult<core::option::Option<alloc::vec::Vec<u8>>>
+pub fn vortex_array::scalar_fn::fns::case_when::CaseWhen::serialize(&self, _options: &Self::Options) -> vortex_error::VortexResult<core::option::Option<alloc::vec::Vec<u8>>>
 
 pub fn vortex_array::scalar_fn::fns::case_when::CaseWhen::simplify(&self, options: &Self::Options, expr: &vortex_array::expr::Expression, ctx: &dyn vortex_array::scalar_fn::SimplifyCtx) -> vortex_error::VortexResult<core::option::Option<vortex_array::expr::Expression>>
 
@@ -15232,7 +15232,7 @@ pub fn vortex_array::scalar_fn::fns::case_when::CaseWhen::reduce(&self, options:
 
 pub fn vortex_array::scalar_fn::fns::case_when::CaseWhen::return_dtype(&self, options: &Self::Options, arg_dtypes: &[vortex_array::dtype::DType]) -> vortex_error::VortexResult<vortex_array::dtype::DType>
 
-pub fn vortex_array::scalar_fn::fns::case_when::CaseWhen::serialize(&self, options: &Self::Options) -> vortex_error::VortexResult<core::option::Option<alloc::vec::Vec<u8>>>
+pub fn vortex_array::scalar_fn::fns::case_when::CaseWhen::serialize(&self, _options: &Self::Options) -> vortex_error::VortexResult<core::option::Option<alloc::vec::Vec<u8>>>
 
 pub fn vortex_array::scalar_fn::fns::case_when::CaseWhen::simplify(&self, options: &Self::Options, expr: &vortex_array::expr::Expression, ctx: &dyn vortex_array::scalar_fn::SimplifyCtx) -> vortex_error::VortexResult<core::option::Option<vortex_array::expr::Expression>>
 

vortex-array/src/scalar_fn/fns/case_when.rs

@@ -1,7 +1,14 @@
 // SPDX-License-Identifier: Apache-2.0
 // SPDX-FileCopyrightText: Copyright the Vortex contributors
 
-//! N-ary CASE WHEN expression for conditional value selection.
+//! SQL-style CASE WHEN: evaluates `(condition, value)` pairs in order and returns
+//! the value from the first matching condition (first-match-wins). NULL conditions
+//! are treated as false. If no ELSE clause is provided, unmatched rows produce NULL;
+//! otherwise they get the ELSE value.
+//!
+//! Unlike SQL which coerces all branches to a common supertype, all THEN/ELSE
+//! branches must share the same base dtype (ignoring nullability). The result
+//! nullability is the union of all branches (forced nullable if no ELSE).
 
 use std::fmt;
 use std::fmt::Formatter;
@@ -69,9 +76,11 @@ impl ScalarFnVTable for CaseWhen {
         ScalarFnId::from("vortex.case_when")
     }
 
-    fn serialize(&self, options: &Self::Options) -> VortexResult<Option<Vec<u8>>> {
-        let num_children = options.num_when_then_pairs * 2 + u32::from(options.has_else);
-        Ok(Some(pb::CaseWhenOpts { num_children }.encode_to_vec()))
+    fn serialize(&self, _options: &Self::Options) -> VortexResult<Option<Vec<u8>>> {
+        // let num_children = options.num_when_then_pairs * 2 + u32::from(options.has_else);
+        // Ok(Some(pb::CaseWhenOpts { num_children }.encode_to_vec()))
+        // stabilize the expr
+        vortex_bail!("cannot serialize")
     }
 
     fn deserialize(
@@ -147,8 +156,9 @@ impl ScalarFnVTable for CaseWhen {
             );
         }
 
-        // The return dtype is based on the first THEN expression (index 1).
-        // Validate all other THEN branches match and union their nullability.
+        // Unlike SQL which coerces all branches to a common supertype, we require
+        // all THEN/ELSE branches to have the same base dtype (ignoring nullability).
+        // The result nullability is the union of all branches.
         let first_then = &arg_dtypes[1];
         let mut result_dtype = first_then.clone();
 
@@ -166,7 +176,7 @@ impl ScalarFnVTable for CaseWhen {
 
         if options.has_else {
             let else_dtype = &arg_dtypes[options.num_when_then_pairs as usize * 2];
-            if !first_then.eq_ignore_nullability(else_dtype) {
+            if !result_dtype.eq_ignore_nullability(else_dtype) {
                 vortex_bail!(
                     "CaseWhen THEN and ELSE dtypes must match (ignoring nullability), got {} and {}",
                     first_then,
@@ -198,6 +208,9 @@ impl ScalarFnVTable for CaseWhen {
             ConstantArray::new(Scalar::null(then_dtype), row_count).into_array()
         };
 
+        // TODO(perf): this reverse-zip approach touches every row for every condition.
+        // A left-to-right filter approach could maintain an "unmatched" mask, narrow it
+        // as conditions match, and exit early once all rows are resolved.
         for i in (0..num_pairs).rev() {
             let condition = args.get(i * 2)?;
             let then_value = args.get(i * 2 + 1)?;
@@ -279,6 +292,7 @@ mod tests {
     // ==================== Serialization Tests ====================
 
     #[test]
+    #[should_panic(expected = "cannot serialize")]
     fn test_serialization_roundtrip() {
         let options = CaseWhenOptions {
             num_when_then_pairs: 1,
@@ -292,6 +306,7 @@ mod tests {
     }
 
     #[test]
+    #[should_panic(expected = "cannot serialize")]
     fn test_serialization_no_else() {
         let options = CaseWhenOptions {
             num_when_then_pairs: 1,
@@ -448,6 +463,7 @@ mod tests {
     // ==================== N-ary Serialization Tests ====================
 
     #[test]
+    #[should_panic(expected = "cannot serialize")]
     fn test_serialization_roundtrip_nary() {
         let options = CaseWhenOptions {
             num_when_then_pairs: 3,
@@ -461,6 +477,7 @@ mod tests {
     }
 
     #[test]
+    #[should_panic(expected = "cannot serialize")]
     fn test_serialization_roundtrip_nary_no_else() {
         let options = CaseWhenOptions {
             num_when_then_pairs: 4,