diff --git a/proto/expr.proto b/proto/expr.proto
index a0e655aaee0f7..7d24241850918 100644
--- a/proto/expr.proto
+++ b/proto/expr.proto
@@ -224,6 +224,9 @@ message ExprNode {
     PG_SLEEP = 2024;
     PG_SLEEP_FOR = 2025;
     PG_SLEEP_UNTIL = 2026;
+
+    // Adminitration functions
+    COL_DESCRIPTION = 2100;
   }
   Type function_type = 1;
   data.DataType return_type = 3;
diff --git a/src/batch/src/executor/sort_over_window.rs b/src/batch/src/executor/sort_over_window.rs
index f12ebc2452384..c8b6c7ef9388c 100644
--- a/src/batch/src/executor/sort_over_window.rs
+++ b/src/batch/src/executor/sort_over_window.rs
@@ -19,8 +19,9 @@ use risingwave_common::error::{Result, RwError};
 use risingwave_common::row::{OwnedRow, Row, RowExt};
 use risingwave_common::util::chunk_coalesce::DataChunkBuilder;
 use risingwave_common::util::iter_util::ZipEqFast;
-use risingwave_expr::function::window::WindowFuncCall;
-use risingwave_expr::window_function::{create_window_state, StateKey, WindowStates};
+use risingwave_expr::window_function::{
+    create_window_state, StateKey, WindowFuncCall, WindowStates,
+};
 use risingwave_pb::batch_plan::plan_node::NodeBody;
 
 use super::{BoxedDataChunkStream, BoxedExecutor, BoxedExecutorBuilder, Executor, ExecutorBuilder};
diff --git a/src/expr/macro/src/gen.rs b/src/expr/macro/src/gen.rs
index 56464f2bf2809..77026581492c2 100644
--- a/src/expr/macro/src/gen.rs
+++ b/src/expr/macro/src/gen.rs
@@ -71,7 +71,7 @@ impl FunctionAttr {
 
         let pb_type = format_ident!("{}", utils::to_camel_case(&name));
         let ctor_name = format_ident!("{}", self.ident_name());
-        let descriptor_type = quote! { crate::sig::func::FuncSign };
+        let descriptor_type = quote! { risingwave_expr::sig::func::FuncSign };
         let build_fn = if build_fn {
             let name = format_ident!("{}", user_fn.name);
             quote! { #name }
@@ -80,10 +80,10 @@ impl FunctionAttr {
         };
         let deprecated = self.deprecated;
         Ok(quote! {
-            #[ctor::ctor]
+            #[risingwave_expr::ctor]
             fn #ctor_name() {
                 use risingwave_common::types::{DataType, DataTypeName};
-                unsafe { crate::sig::func::_register(#descriptor_type {
+                unsafe { risingwave_expr::sig::func::_register(#descriptor_type {
                     func: risingwave_pb::expr::expr_node::Type::#pb_type,
                     inputs_type: &[#(#args),*],
                     variadic: #variadic,
@@ -203,7 +203,7 @@ impl FunctionAttr {
                     let #prebuilt_inputs = match &#prebuilt_inputs {
                         Some(s) => s.as_scalar_ref_impl().try_into()?,
                         // the function should always return null if any const argument is null
-                        None => return Ok(Box::new(crate::expr::LiteralExpression::new(
+                        None => return Ok(Box::new(risingwave_expr::expr::LiteralExpression::new(
                             return_type,
                             None,
                         ))),
@@ -217,8 +217,8 @@ impl FunctionAttr {
 
         // ensure the number of children matches the number of arguments
         let check_children = match variadic {
-            true => quote! { crate::ensure!(children.len() >= #num_args); },
-            false => quote! { crate::ensure!(children.len() == #num_args); },
+            true => quote! { risingwave_expr::ensure!(children.len() >= #num_args); },
+            false => quote! { risingwave_expr::ensure!(children.len() == #num_args); },
         };
 
         // evaluate variadic arguments in `eval`
@@ -407,8 +407,8 @@ impl FunctionAttr {
         };
 
         Ok(quote! {
-            |return_type: DataType, children: Vec<crate::expr::BoxedExpression>|
-                -> crate::Result<crate::expr::BoxedExpression>
+            |return_type: DataType, children: Vec<risingwave_expr::expr::BoxedExpression>|
+                -> risingwave_expr::Result<risingwave_expr::expr::BoxedExpression>
             {
                 use std::sync::Arc;
                 use risingwave_common::array::*;
@@ -418,8 +418,8 @@ impl FunctionAttr {
                 use risingwave_common::util::iter_util::ZipEqFast;
                 use itertools::multizip;
 
-                use crate::expr::{Context, BoxedExpression};
-                use crate::Result;
+                use risingwave_expr::expr::{Context, BoxedExpression};
+                use risingwave_expr::Result;
 
                 #check_children
                 let prebuilt_arg = #prebuild_const;
@@ -435,7 +435,7 @@ impl FunctionAttr {
                     prebuilt_arg: #prebuilt_arg_type,
                 }
                 #[async_trait::async_trait]
-                impl crate::expr::Expression for #struct_name {
+                impl risingwave_expr::expr::Expression for #struct_name {
                     fn return_type(&self) -> DataType {
                         self.context.return_type.clone()
                     }
@@ -497,7 +497,7 @@ impl FunctionAttr {
             false => format_ident!("{}", self.ident_name()),
             true => format_ident!("{}_append_only", self.ident_name()),
         };
-        let descriptor_type = quote! { crate::sig::agg::AggFuncSig };
+        let descriptor_type = quote! { risingwave_expr::sig::agg::AggFuncSig };
         let build_fn = if build_fn {
             let name = format_ident!("{}", user_fn.as_fn().name);
             quote! { #name }
@@ -505,11 +505,11 @@ impl FunctionAttr {
             self.generate_agg_build_fn(user_fn)?
         };
         Ok(quote! {
-            #[ctor::ctor]
+            #[risingwave_expr::ctor]
             fn #ctor_name() {
                 use risingwave_common::types::{DataType, DataTypeName};
-                unsafe { crate::sig::agg::_register(#descriptor_type {
-                    func: crate::agg::AggKind::#pb_type,
+                unsafe { risingwave_expr::sig::agg::_register(#descriptor_type {
+                    func: risingwave_expr::agg::AggKind::#pb_type,
                     inputs_type: &[#(#args),*],
                     state_type: #state_type,
                     ret_type: #ret,
@@ -678,8 +678,8 @@ impl FunctionAttr {
                 use risingwave_common::buffer::Bitmap;
                 use risingwave_common::estimate_size::EstimateSize;
 
-                use crate::Result;
-                use crate::agg::AggregateState;
+                use risingwave_expr::Result;
+                use risingwave_expr::agg::AggregateState;
 
                 #[derive(Clone)]
                 struct Agg {
@@ -688,7 +688,7 @@ impl FunctionAttr {
                 }
 
                 #[async_trait::async_trait]
-                impl crate::agg::AggregateFunction for Agg {
+                impl risingwave_expr::agg::AggregateFunction for Agg {
                     fn return_type(&self) -> DataType {
                         self.return_type.clone()
                     }
@@ -780,7 +780,7 @@ impl FunctionAttr {
 
         let pb_type = format_ident!("{}", utils::to_camel_case(&name));
         let ctor_name = format_ident!("{}", self.ident_name());
-        let descriptor_type = quote! { crate::sig::table_function::FuncSign };
+        let descriptor_type = quote! { risingwave_expr::sig::table_function::FuncSign };
         let build_fn = if build_fn {
             let name = format_ident!("{}", user_fn.name);
             quote! { #name }
@@ -800,10 +800,10 @@ impl FunctionAttr {
             quote! { |_| Ok(#ty) }
         };
         Ok(quote! {
-            #[ctor::ctor]
+            #[risingwave_expr::ctor]
             fn #ctor_name() {
                 use risingwave_common::types::{DataType, DataTypeName};
-                unsafe { crate::sig::table_function::_register(#descriptor_type {
+                unsafe { risingwave_expr::sig::table_function::_register(#descriptor_type {
                     func: risingwave_pb::expr::table_function::Type::#pb_type,
                     inputs_type: &[#(#args),*],
                     ret_type: #ret,
@@ -910,7 +910,7 @@ impl FunctionAttr {
                 use risingwave_common::util::iter_util::ZipEqFast;
                 use itertools::multizip;
 
-                crate::ensure!(children.len() == #num_args);
+                risingwave_expr::ensure!(children.len() == #num_args);
                 let mut iter = children.into_iter();
                 #(let #all_child = iter.next().unwrap();)*
                 #(
@@ -918,7 +918,7 @@ impl FunctionAttr {
                     let #const_child = match &#const_child {
                         Some(s) => s.as_scalar_ref_impl().try_into()?,
                         // the function should always return empty if any const argument is null
-                        None => return Ok(crate::table_function::empty(return_type)),
+                        None => return Ok(risingwave_expr::table_function::empty(return_type)),
                     };
                 )*
 
@@ -930,7 +930,7 @@ impl FunctionAttr {
                     prebuilt_arg: #prebuilt_arg_type,
                 }
                 #[async_trait::async_trait]
-                impl crate::table_function::TableFunction for #struct_name {
+                impl risingwave_expr::table_function::TableFunction for #struct_name {
                     fn return_type(&self) -> DataType {
                         self.return_type.clone()
                     }
diff --git a/src/expr/src/lib.rs b/src/expr/src/lib.rs
index ee4cea38e4bb5..c8f2e432f79af 100644
--- a/src/expr/src/lib.rs
+++ b/src/expr/src/lib.rs
@@ -24,15 +24,17 @@
 #![feature(test)]
 #![feature(arc_unwrap_or_clone)]
 
+extern crate self as risingwave_expr;
+
 pub mod agg;
 mod error;
 pub mod expr;
-pub mod function;
 pub mod sig;
 pub mod table_function;
 pub mod vector_op;
 pub mod window_function;
 
+pub use ctor::ctor;
 pub use error::{ExprError, Result};
-use risingwave_common::{bail, ensure};
+pub use risingwave_common::{bail, ensure};
 pub use risingwave_expr_macro::*;
diff --git a/src/expr/src/window_function/state/aggregate.rs b/src/expr/src/window_function/state/aggregate.rs
index b67d42042bc91..749baf7784343 100644
--- a/src/expr/src/window_function/state/aggregate.rs
+++ b/src/expr/src/window_function/state/aggregate.rs
@@ -25,7 +25,7 @@ use smallvec::SmallVec;
 use super::buffer::WindowBuffer;
 use super::{StateEvictHint, StateKey, StatePos, WindowState};
 use crate::agg::{build_append_only, AggArgs, AggCall, BoxedAggregateFunction};
-use crate::function::window::{WindowFuncCall, WindowFuncKind};
+use crate::window_function::{WindowFuncCall, WindowFuncKind};
 use crate::Result;
 
 pub struct AggregateState {
diff --git a/src/expr/src/window_function/state/buffer.rs b/src/expr/src/window_function/state/buffer.rs
index 97f68b18375b2..a375c7bfec225 100644
--- a/src/expr/src/window_function/state/buffer.rs
+++ b/src/expr/src/window_function/state/buffer.rs
@@ -17,7 +17,7 @@ use std::ops::Range;
 
 use either::Either;
 
-use crate::function::window::{Frame, FrameBounds, FrameExclusion};
+use crate::window_function::{Frame, FrameBounds, FrameExclusion};
 
 struct Entry<K: Ord, V> {
     key: K,
@@ -238,7 +238,7 @@ mod tests {
     use itertools::Itertools;
 
     use super::*;
-    use crate::function::window::{Frame, FrameBound};
+    use crate::window_function::{Frame, FrameBound};
 
     #[test]
     fn test_rows_frame_unbounded_preceding_to_current_row() {
diff --git a/src/expr/src/window_function/state/mod.rs b/src/expr/src/window_function/state/mod.rs
index fe6e47b016d40..977a04b2a7a70 100644
--- a/src/expr/src/window_function/state/mod.rs
+++ b/src/expr/src/window_function/state/mod.rs
@@ -20,8 +20,7 @@ use risingwave_common::types::{Datum, DefaultOrdered};
 use risingwave_common::util::memcmp_encoding::MemcmpEncoded;
 use smallvec::SmallVec;
 
-use super::WindowFuncCall;
-use crate::function::window::WindowFuncKind;
+use super::{WindowFuncCall, WindowFuncKind};
 use crate::sig::FuncSigDebug;
 use crate::{ExprError, Result};
 
diff --git a/src/expr/src/window_function/state/row_number.rs b/src/expr/src/window_function/state/row_number.rs
index 01b713cbc1196..7abf30fc5f5c7 100644
--- a/src/expr/src/window_function/state/row_number.rs
+++ b/src/expr/src/window_function/state/row_number.rs
@@ -18,7 +18,7 @@ use risingwave_common::types::Datum;
 use smallvec::SmallVec;
 
 use super::{StateEvictHint, StateKey, StatePos, WindowState};
-use crate::function::window::WindowFuncCall;
+use crate::window_function::WindowFuncCall;
 use crate::Result;
 
 #[derive(EstimateSize)]
@@ -83,7 +83,7 @@ mod tests {
 
     use super::*;
     use crate::agg::AggArgs;
-    use crate::function::window::{Frame, FrameBound, WindowFuncKind};
+    use crate::window_function::{Frame, FrameBound, WindowFuncKind};
 
     fn create_state_key(pk: i64) -> StateKey {
         StateKey {
diff --git a/src/frontend/src/binder/expr/function.rs b/src/frontend/src/binder/expr/function.rs
index 44c79a1620c79..1146050e8dde4 100644
--- a/src/frontend/src/binder/expr/function.rs
+++ b/src/frontend/src/binder/expr/function.rs
@@ -26,7 +26,7 @@ use risingwave_common::session_config::USER_NAME_WILD_CARD;
 use risingwave_common::types::{DataType, ScalarImpl, Timestamptz};
 use risingwave_common::{GIT_SHA, RW_VERSION};
 use risingwave_expr::agg::{agg_kinds, AggKind};
-use risingwave_expr::function::window::{
+use risingwave_expr::window_function::{
     Frame, FrameBound, FrameBounds, FrameExclusion, WindowFuncKind,
 };
 use risingwave_sqlparser::ast::{
@@ -1127,7 +1127,7 @@ impl Binder {
                 // TODO: really implement them.
                 // https://www.postgresql.org/docs/9.5/functions-info.html#FUNCTIONS-INFO-COMMENT-TABLE
                 // WARN: Hacked in [`Binder::bind_function`]!!!
-                ("col_description", raw_literal(ExprImpl::literal_varchar("".to_string()))),
+                ("col_description", raw_call(ExprType::ColDescription)),
                 ("obj_description", raw_literal(ExprImpl::literal_varchar("".to_string()))),
                 ("shobj_description", raw_literal(ExprImpl::literal_varchar("".to_string()))),
                 ("pg_is_in_recovery", raw_literal(ExprImpl::literal_bool(false))),
diff --git a/src/expr/src/function/mod.rs b/src/frontend/src/expr/function_impl/col_description.rs
similarity index 62%
rename from src/expr/src/function/mod.rs
rename to src/frontend/src/expr/function_impl/col_description.rs
index ed628e99398ca..8b6458eba5d30 100644
--- a/src/expr/src/function/mod.rs
+++ b/src/frontend/src/expr/function_impl/col_description.rs
@@ -12,6 +12,14 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-pub mod window;
+use std::fmt::Write;
 
-// TODO(rc): this module is to be removed
+use risingwave_expr::{function, ExprError};
+
+#[function("col_description(varchar, int32) -> varchar")]
+fn col_description(_name: &str, _col: i32, writer: &mut impl Write) -> Result<(), ExprError> {
+    // TODO: Currently we don't support `COMMENT` statement, so we just return empty string.
+    writer.write_str("").unwrap();
+
+    Ok(())
+}
diff --git a/src/expr/src/function/window/mod.rs b/src/frontend/src/expr/function_impl/mod.rs
similarity index 94%
rename from src/expr/src/function/window/mod.rs
rename to src/frontend/src/expr/function_impl/mod.rs
index add145718c948..33d402b4bb6af 100644
--- a/src/expr/src/function/window/mod.rs
+++ b/src/frontend/src/expr/function_impl/mod.rs
@@ -12,4 +12,4 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-pub use crate::window_function::*;
+mod col_description;
diff --git a/src/frontend/src/expr/mod.rs b/src/frontend/src/expr/mod.rs
index d999fcbe4c1e8..11fa86d682b7b 100644
--- a/src/frontend/src/expr/mod.rs
+++ b/src/frontend/src/expr/mod.rs
@@ -46,6 +46,7 @@ pub use order_by_expr::{OrderBy, OrderByExpr};
 mod expr_mutator;
 mod expr_rewriter;
 mod expr_visitor;
+mod function_impl;
 mod session_timezone;
 mod type_inference;
 mod utils;
diff --git a/src/frontend/src/expr/pure.rs b/src/frontend/src/expr/pure.rs
index cea3b9eb46bc2..91e92ee651205 100644
--- a/src/frontend/src/expr/pure.rs
+++ b/src/frontend/src/expr/pure.rs
@@ -212,7 +212,8 @@ impl ExprVisitor<bool> for ImpureAnalyzer {
             | expr_node::Type::Proctime
             | expr_node::Type::PgSleep
             | expr_node::Type::PgSleepFor
-            | expr_node::Type::PgSleepUntil => true,
+            | expr_node::Type::PgSleepUntil
+            | expr_node::Type::ColDescription => true,
         }
     }
 }
diff --git a/src/frontend/src/expr/window_function.rs b/src/frontend/src/expr/window_function.rs
index 62f961515cdd0..371a00dc6b62a 100644
--- a/src/frontend/src/expr/window_function.rs
+++ b/src/frontend/src/expr/window_function.rs
@@ -15,7 +15,7 @@
 use itertools::Itertools;
 use risingwave_common::error::{ErrorCode, RwError};
 use risingwave_common::types::DataType;
-use risingwave_expr::function::window::{Frame, WindowFuncKind};
+use risingwave_expr::window_function::{Frame, WindowFuncKind};
 
 use super::{AggCall, Expr, ExprImpl, OrderBy, RwResult};
 
diff --git a/src/frontend/src/optimizer/plan_node/generic/over_window.rs b/src/frontend/src/optimizer/plan_node/generic/over_window.rs
index 5f7b0705fba26..c148711698a24 100644
--- a/src/frontend/src/optimizer/plan_node/generic/over_window.rs
+++ b/src/frontend/src/optimizer/plan_node/generic/over_window.rs
@@ -18,7 +18,7 @@ use risingwave_common::catalog::{Field, Schema};
 use risingwave_common::types::DataType;
 use risingwave_common::util::column_index_mapping::ColIndexMapping;
 use risingwave_common::util::sort_util::{ColumnOrder, ColumnOrderDisplay};
-use risingwave_expr::function::window::{Frame, WindowFuncKind};
+use risingwave_expr::window_function::{Frame, WindowFuncKind};
 use risingwave_pb::expr::PbWindowFunction;
 
 use super::{DistillUnit, GenericPlanNode, GenericPlanRef};
diff --git a/src/frontend/src/optimizer/plan_node/logical_over_window.rs b/src/frontend/src/optimizer/plan_node/logical_over_window.rs
index b2057f28e05fc..ed74f379cf4ba 100644
--- a/src/frontend/src/optimizer/plan_node/logical_over_window.rs
+++ b/src/frontend/src/optimizer/plan_node/logical_over_window.rs
@@ -18,7 +18,7 @@ use risingwave_common::error::{ErrorCode, Result, RwError};
 use risingwave_common::types::{DataType, Datum, ScalarImpl};
 use risingwave_common::util::sort_util::{ColumnOrder, OrderType};
 use risingwave_expr::agg::AggKind;
-use risingwave_expr::function::window::{Frame, FrameBound, WindowFuncKind};
+use risingwave_expr::window_function::{Frame, FrameBound, WindowFuncKind};
 
 use super::generic::{GenericPlanRef, OverWindow, PlanWindowFunction, ProjectBuilder};
 use super::utils::impl_distill_by_unit;
diff --git a/src/frontend/src/optimizer/rule/over_window_to_agg_and_join_rule.rs b/src/frontend/src/optimizer/rule/over_window_to_agg_and_join_rule.rs
index b9587650f8726..dbf3e9809675c 100644
--- a/src/frontend/src/optimizer/rule/over_window_to_agg_and_join_rule.rs
+++ b/src/frontend/src/optimizer/rule/over_window_to_agg_and_join_rule.rs
@@ -13,7 +13,7 @@
 // limitations under the License.
 
 use itertools::Itertools;
-use risingwave_expr::function::window::WindowFuncKind;
+use risingwave_expr::window_function::WindowFuncKind;
 use risingwave_pb::expr::expr_node::Type;
 use risingwave_pb::plan_common::JoinType;
 
diff --git a/src/frontend/src/optimizer/rule/over_window_to_topn_rule.rs b/src/frontend/src/optimizer/rule/over_window_to_topn_rule.rs
index 297522a41c8c9..dfb6963c7fb4f 100644
--- a/src/frontend/src/optimizer/rule/over_window_to_topn_rule.rs
+++ b/src/frontend/src/optimizer/rule/over_window_to_topn_rule.rs
@@ -14,7 +14,7 @@
 
 use fixedbitset::FixedBitSet;
 use risingwave_common::types::DataType;
-use risingwave_expr::function::window::WindowFuncKind;
+use risingwave_expr::window_function::WindowFuncKind;
 
 use super::Rule;
 use crate::expr::{collect_input_refs, ExprImpl, ExprType};
diff --git a/src/stream/src/from_proto/eowc_over_window.rs b/src/stream/src/from_proto/eowc_over_window.rs
index 0cd0060a40b68..bcee0736ae30f 100644
--- a/src/stream/src/from_proto/eowc_over_window.rs
+++ b/src/stream/src/from_proto/eowc_over_window.rs
@@ -14,7 +14,7 @@
 
 use std::sync::Arc;
 
-use risingwave_expr::function::window::WindowFuncCall;
+use risingwave_expr::window_function::WindowFuncCall;
 use risingwave_pb::stream_plan::PbEowcOverWindowNode;
 use risingwave_storage::StateStore;
 
diff --git a/src/stream/src/from_proto/over_window.rs b/src/stream/src/from_proto/over_window.rs
index 7d139ca3f74db..e18e753caf126 100644
--- a/src/stream/src/from_proto/over_window.rs
+++ b/src/stream/src/from_proto/over_window.rs
@@ -16,7 +16,7 @@ use std::sync::Arc;
 
 use risingwave_common::session_config::OverWindowCachePolicy;
 use risingwave_common::util::sort_util::ColumnOrder;
-use risingwave_expr::function::window::WindowFuncCall;
+use risingwave_expr::window_function::WindowFuncCall;
 use risingwave_pb::stream_plan::PbOverWindowNode;
 use risingwave_storage::StateStore;
 
diff --git a/src/stream/tests/integration_tests/eowc_over_window.rs b/src/stream/tests/integration_tests/eowc_over_window.rs
index d7d788680af55..35cc4954aff45 100644
--- a/src/stream/tests/integration_tests/eowc_over_window.rs
+++ b/src/stream/tests/integration_tests/eowc_over_window.rs
@@ -13,7 +13,7 @@
 // limitations under the License.
 
 use risingwave_expr::agg::{AggArgs, AggKind};
-use risingwave_expr::function::window::{Frame, FrameBound, WindowFuncCall, WindowFuncKind};
+use risingwave_expr::window_function::{Frame, FrameBound, WindowFuncCall, WindowFuncKind};
 use risingwave_stream::executor::{EowcOverWindowExecutor, EowcOverWindowExecutorArgs};
 
 use crate::prelude::*;
diff --git a/src/stream/tests/integration_tests/over_window.rs b/src/stream/tests/integration_tests/over_window.rs
index 4b7b53aaae31b..2377ce12e9147 100644
--- a/src/stream/tests/integration_tests/over_window.rs
+++ b/src/stream/tests/integration_tests/over_window.rs
@@ -14,7 +14,7 @@
 
 use risingwave_common::session_config::OverWindowCachePolicy;
 use risingwave_expr::agg::{AggArgs, AggKind};
-use risingwave_expr::function::window::{
+use risingwave_expr::window_function::{
     Frame, FrameBound, FrameExclusion, WindowFuncCall, WindowFuncKind,
 };
 use risingwave_stream::executor::monitor::StreamingMetrics;