add rewrite rule

src/frontend/src/binder/expr/mod.rs

@@ -602,7 +602,7 @@ impl Binder {
         );
 
         if constant_case_when_flag {
-            return Ok(FunctionCall::new(ExprType::ConstantCaseWhenEval, constant_case_when_eval_inputs)?.into());
+            return Ok(FunctionCall::new(ExprType::ConstantLookup, constant_case_when_eval_inputs)?.into());
         }
 
         // See if the case-when expression can be optimized

src/frontend/src/optimizer/plan_expr_rewriter/const_case_when_rewriter.rs

@@ -12,13 +12,23 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use crate::expr::{ExprImpl, ExprRewriter, FunctionCall};
+use risingwave_common::error::RwError;
 
-pub struct ConstCaseWhenRewriter {}
+use crate::expr::{ExprImpl, ExprRewriter, FunctionCall, ExprType};
+
+pub struct ConstCaseWhenRewriter {
+    pub error: Option<RwError>,
+}
 
 impl ExprRewriter for ConstCaseWhenRewriter {
     fn rewrite_function_call(&mut self, func_call: FunctionCall) -> ExprImpl {
-        println!("Current func_call: {:#?}", func_call);
-        todo!()
+        if func_call.func_type() != ExprType::ConstantLookup {
+            return func_call.into();
+        }
+        if func_call.inputs().len() != 1 {
+            // Normal constant lookup pass
+            return func_call.into();
+        }
+        func_call.inputs()[0].clone().into()
     }
 }

src/frontend/src/optimizer/rule/const_case_when_eval_rule.rs

@@ -19,11 +19,9 @@ use crate::optimizer::plan_expr_rewriter::ConstCaseWhenRewriter;
 pub struct ConstCaseWhenEvalRule {}
 impl Rule for ConstCaseWhenEvalRule {
     fn apply(&self, plan: PlanRef) -> Option<PlanRef> {
-        println!("Current plan: {:#?}", plan);
         let values: &LogicalValues = plan.as_logical_values()?;
-        println!("Current values: {:#?}", values);
-        let _const_case_when_rewriter = ConstCaseWhenRewriter {};
-        todo!()
+        let mut const_case_when_rewriter = ConstCaseWhenRewriter { error: None };
+        Some(values.rewrite_exprs(&mut const_case_when_rewriter))
     }
 }