feat(binder): add const case-when evaluation optimization during binding

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

@@ -471,6 +471,60 @@ impl Binder {
         Ok(func_call.into())
     }
 
+    /// The optimization check for the following case-when expression pattern
+    /// e.g., select case 1 when (...) then (...) else (...) end;
+    fn check_constant_case_when_optimization(
+        &mut self,
+        conditions: Vec<Expr>,
+        results_expr: Vec<ExprImpl>,
+        operand: Option<Box<Expr>>,
+        fallback: Option<ExprImpl>,
+        constant_case_when_eval_inputs: &mut Vec<ExprImpl>,
+    ) -> bool {
+        // The operand value to be compared later
+        let operand_value;
+
+        if let Some(operand) = operand {
+            let Ok(operand) = self.bind_expr_inner(*operand) else {
+                return false;
+            };
+            if !operand.is_const() {
+                return false;
+            }
+            operand_value = operand;
+        } else {
+            return false;
+        }
+
+        for (condition, result) in zip_eq_fast(conditions, results_expr) {
+            if let Expr::Value(_) = condition.clone() {
+                let Ok(res) = self.bind_expr_inner(condition.clone()) else {
+                    return false;
+                };
+                // Found a match
+                if res == operand_value {
+                    constant_case_when_eval_inputs.push(result);
+                    return true;
+                }
+            } else {
+                return false;
+            }
+        }
+
+        // Otherwise this will eventually go through fallback arm
+        debug_assert!(
+            constant_case_when_eval_inputs.is_empty(),
+            "expect `inputs` to be empty"
+        );
+
+        let Some(fallback) = fallback else {
+            return false;
+        };
+
+        constant_case_when_eval_inputs.push(fallback);
+        true
+    }
+
     /// The helper function to check if the current case-when
     /// expression in `bind_case` could be optimized
     /// into `ConstantLookupExpression`
@@ -493,6 +547,12 @@ impl Binder {
             let Ok(operand) = self.bind_expr_inner(*operand) else {
                 return false;
             };
+            // This optimization should be done in subsequent optimization phase
+            // if the operand is const
+            // e.g., select case 1 when 1 then 114514 else 1919810 end;
+            if operand.is_const() {
+                return false;
+            }
             constant_lookup_inputs.push(operand);
         } else {
             return false;
@@ -506,7 +566,7 @@ impl Binder {
                 constant_lookup_inputs.push(input);
             } else {
                 // If at least one condition is not in the simple form / not constant,
-                // we can NOT do the subsequent optimization then
+                // we can NOT do the subsequent optimization pass
                 return false;
             }
 
@@ -538,6 +598,27 @@ impl Binder {
             .transpose()?;
 
         let mut constant_lookup_inputs = Vec::new();
+        let mut constant_case_when_eval_inputs = Vec::new();
+
+        let constant_case_when_flag = self.check_constant_case_when_optimization(
+            conditions.clone(),
+            results_expr.clone(),
+            operand.clone(),
+            else_result_expr.clone(),
+            &mut constant_case_when_eval_inputs,
+        );
+
+        if constant_case_when_flag {
+            // Sanity check
+            if constant_case_when_eval_inputs.len() != 1 {
+                return Err(ErrorCode::BindError(
+                    "expect `constant_case_when_eval_inputs` only contains a single bound expression".to_string()
+                )
+                .into());
+            }
+            // Directly return the first element of the vector
+            return Ok(constant_case_when_eval_inputs[0].take());
+        }
 
         // See if the case-when expression can be optimized
         let optimize_flag = self.check_bind_case_optimization(