fix(binder): fix explain with parameters; simplify IN expr without consts

src/frontend/planner_test/tests/testdata/input/parameters.yaml

@@ -0,0 +1,17 @@
+- id: explain parameters
+  sql: |
+    explain select $1
+  expected_outputs:
+    - explain_output
+- id: explain parameters in in-expr
+  sql: |
+    create table t (id int primary key, a int);
+    explain select * from t where id in($1)
+  expected_outputs:
+    - explain_output
+- id: explain parameters in in-expr
+  sql: |
+    create table t (id int primary key, a int);
+    explain select * from t where id in($1, 123)
+  expected_outputs:
+    - explain_output

src/frontend/planner_test/tests/testdata/output/parameters.yaml

@@ -0,0 +1,23 @@
+# This file is automatically generated. See `src/frontend/planner_test/README.md` for more information.
+- id: explain parameters
+  sql: |
+    explain select $1
+  explain_output: |
+    BatchProject { exprs: [Parameter(index: 1, type: None) as $expr1] }
+    └─BatchValues { rows: [[]] }
+- id: explain parameters in in-expr
+  sql: |
+    create table t (id int primary key, a int);
+    explain select * from t where id in($1)
+  explain_output: |
+    BatchExchange { order: [], dist: Single }
+    └─BatchFilter { predicate: (t.id = Parameter(index: 1, type: Some(Int32))) }
+      └─BatchScan { table: t, columns: [id, a] }
+- id: explain parameters in in-expr
+  sql: |
+    create table t (id int primary key, a int);
+    explain select * from t where id in($1, 123)
+  explain_output: |
+    BatchExchange { order: [], dist: Single }
+    └─BatchFilter { predicate: (In(t.id, 123:Int32) OR (t.id = Parameter(index: 1, type: Some(Int32)))) }
+      └─BatchScan { table: t, columns: [id, a] }

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

@@ -278,22 +278,36 @@ impl Binder {
                 false => non_const_exprs.push(expr),
             }
         }
-        let mut ret = FunctionCall::new(ExprType::In, bound_expr_list)?.into();
+
+        let mut ret = if bound_expr_list.len() == 1 {
+            None
+        } else {
+            Some(FunctionCall::new(ExprType::In, bound_expr_list)?.into())
+        };
         // Non-const exprs are not part of IN-expr in backend and rewritten into OR-Equal-exprs.
         for expr in non_const_exprs {
-            ret = FunctionCall::new(
-                ExprType::Or,
-                vec![
-                    ret,
-                    FunctionCall::new(ExprType::Equal, vec![left.clone(), expr])?.into(),
-                ],
-            )?
-            .into();
+            if let Some(inner_ret) = ret {
+                ret = Some(
+                    FunctionCall::new(
+                        ExprType::Or,
+                        vec![
+                            inner_ret,
+                            FunctionCall::new(ExprType::Equal, vec![left.clone(), expr])?.into(),
+                        ],
+                    )?
+                    .into(),
+                );
+            } else {
+                ret = Some(FunctionCall::new(ExprType::Equal, vec![left.clone(), expr])?.into());
+            }
         }
         if negated {
-            Ok(FunctionCall::new_unchecked(ExprType::Not, vec![ret], DataType::Boolean).into())
+            Ok(
+                FunctionCall::new_unchecked(ExprType::Not, vec![ret.unwrap()], DataType::Boolean)
+                    .into(),
+            )
         } else {
-            Ok(ret)
+            Ok(ret.unwrap())
         }
     }
 

src/frontend/src/optimizer/plan_expr_rewriter/const_eval_rewriter.rs

@@ -31,10 +31,6 @@ impl ExprRewriter for ConstEvalRewriter {
                     expr
                 }
             }
-        } else if let ExprImpl::Parameter(_) = expr {
-            unreachable!(
-                "Parameter should not appear here. It will be replaced by a literal before this step."
-            )
         } else {
             default_rewrite_expr(self, expr)
         }