add comments

src/frontend/src/binder/expr/function.rs

@@ -42,6 +42,16 @@ use crate::expr::{
 };
 use crate::utils::Condition;
 
+// Defines system functions that without args, ref: https://www.postgresql.org/docs/current/functions-info.html
+pub const SYS_FUNCTION_WITHOUT_ARGS: &[&str] = &[
+    "session_user",
+    "user",
+    "current_user",
+    "current_role",
+    "current_schema",
+    "current_timestamp",
+];
+
 impl Binder {
     pub(in crate::binder) fn bind_function(&mut self, f: Function) -> Result<ExprImpl> {
         let function_name = match f.name.0.as_slice() {
@@ -477,6 +487,32 @@ impl Binder {
             })
         }
 
+        // `SESSION_USER` is the user name of the user that is connected to the database.
+        fn session_user() -> Handle {
+            guard_by_len(
+                0,
+                raw(|binder, _inputs| {
+                    Ok(ExprImpl::literal_varchar(
+                        binder.auth_context.user_name.clone(),
+                    ))
+                }),
+            )
+        }
+
+        // `CURRENT_USER` is the user name of the user that is executing the command,
+        // `CURRENT_ROLE`, `USER` are synonyms for `CURRENT_USER`. Since we don't support
+        // `SET ROLE xxx` for now, they will all returns session user name.
+        fn current_user() -> Handle {
+            guard_by_len(
+                0,
+                raw(|binder, _inputs| {
+                    Ok(ExprImpl::literal_varchar(
+                        binder.auth_context.user_name.clone(),
+                    ))
+                }),
+            )
+        }
+
         static HANDLES: LazyLock<HashMap<&'static str, Handle>> = LazyLock::new(|| {
             [
                 (
@@ -723,16 +759,10 @@ impl Binder {
                         DataType::Varchar,
                     ))
                 })),
-                ("session_user", guard_by_len(0, raw(|binder, _inputs| {
-                    Ok(ExprImpl::literal_varchar(
-                        binder.auth_context.user_name.clone(),
-                    ))
-                }))),
-                ("user", guard_by_len(0, raw(|binder, _inputs| {
-                    Ok(ExprImpl::literal_varchar(
-                        binder.auth_context.user_name.clone(),
-                    ))
-                }))),
+                ("session_user", session_user()),
+                ("current_role", current_user()),
+                ("current_user", current_user()),
+                ("user", current_user()),
                 ("pg_get_userbyid", guard_by_len(1, raw(|binder, inputs|{
                         let input = &inputs[0];
                         let bound_query = binder.bind_get_user_by_id_select(input)?;

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

@@ -22,6 +22,7 @@ use risingwave_sqlparser::ast::{
     TrimWhereField, UnaryOperator,
 };
 
+use crate::binder::expr::function::SYS_FUNCTION_WITHOUT_ARGS;
 use crate::binder::Binder;
 use crate::expr::{Expr as _, ExprImpl, ExprType, FunctionCall, Parameter, SubqueryKind};
 
@@ -87,14 +88,9 @@ impl Binder {
             Expr::Row(exprs) => self.bind_row(exprs),
             // input ref
             Expr::Identifier(ident) => {
-                if [
-                    "session_user",
-                    "user",
-                    "current_schema",
-                    "current_timestamp",
-                ]
-                .iter()
-                .any(|e| ident.real_value().as_str() == *e)
+                if SYS_FUNCTION_WITHOUT_ARGS
+                    .iter()
+                    .any(|e| ident.real_value().as_str() == *e)
                 {
                     // Rewrite a system variable to a function call, e.g. `SELECT current_schema;`
                     // will be rewritten to `SELECT current_schema();`.