From 2d2f4121b52e75e9cb4662b34450807e6266149a Mon Sep 17 00:00:00 2001
From: Michael Xu <xzhseh@gmail.com>
Date: Sat, 3 Feb 2024 17:26:07 -0500
Subject: [PATCH] add rewrite rule

---
 src/frontend/src/binder/expr/mod.rs            |  2 +-
 .../const_case_when_rewriter.rs                | 18 ++++++++++++++----
 .../rule/const_case_when_eval_rule.rs          |  6 ++----
 3 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/src/frontend/src/binder/expr/mod.rs b/src/frontend/src/binder/expr/mod.rs
index 2de6db034eec4..2a517771db62a 100644
--- a/src/frontend/src/binder/expr/mod.rs
+++ b/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
diff --git a/src/frontend/src/optimizer/plan_expr_rewriter/const_case_when_rewriter.rs b/src/frontend/src/optimizer/plan_expr_rewriter/const_case_when_rewriter.rs
index 10c2cc3e1188d..0489a4e41af02 100644
--- a/src/frontend/src/optimizer/plan_expr_rewriter/const_case_when_rewriter.rs
+++ b/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()
     }
 }
\ No newline at end of file
diff --git a/src/frontend/src/optimizer/rule/const_case_when_eval_rule.rs b/src/frontend/src/optimizer/rule/const_case_when_eval_rule.rs
index 60ec95b746d84..73d712505269e 100644
--- a/src/frontend/src/optimizer/rule/const_case_when_eval_rule.rs
+++ b/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))
     }
 }