diff --git a/src/coprocessor/dag/aggr_fn/impl_sum.rs b/src/coprocessor/dag/aggr_fn/impl_sum.rs
new file mode 100644
index 000000000000..6c22de319a4b
--- /dev/null
+++ b/src/coprocessor/dag/aggr_fn/impl_sum.rs
@@ -0,0 +1,190 @@
+// Copyright 2019 TiKV Project Authors. Licensed under Apache-2.0.
+
+use cop_codegen::AggrFunction;
+use cop_datatype::EvalType;
+use tipb::expression::{Expr, ExprType, FieldType};
+
+use super::summable::Summable;
+use crate::coprocessor::codec::data_type::*;
+use crate::coprocessor::codec::mysql::Tz;
+use crate::coprocessor::dag::expr::EvalContext;
+use crate::coprocessor::dag::rpn_expr::{RpnExpression, RpnExpressionBuilder};
+use crate::coprocessor::Result;
+
+/// The parser for SUM aggregate function.
+pub struct AggrFnDefinitionParserSum;
+
+impl super::parser::AggrDefinitionParser for AggrFnDefinitionParserSum {
+    fn check_supported(&self, aggr_def: &Expr) -> Result<()> {
+        assert_eq!(aggr_def.get_tp(), ExprType::Sum);
+        super::util::check_aggr_exp_supported_one_child(aggr_def)
+    }
+
+    fn parse(
+        &self,
+        mut aggr_def: Expr,
+        time_zone: &Tz,
+        src_schema: &[FieldType],
+        out_schema: &mut Vec<FieldType>,
+        out_exp: &mut Vec<RpnExpression>,
+    ) -> Result<Box<dyn super::AggrFunction>> {
+        use cop_datatype::FieldTypeAccessor;
+        use std::convert::TryFrom;
+
+        assert_eq!(aggr_def.get_tp(), ExprType::Sum);
+
+        // SUM outputs one column.
+        out_schema.push(aggr_def.take_field_type());
+
+        // Rewrite expression, inserting CAST if necessary. See `typeInfer4Sum` in TiDB.
+        let child = aggr_def.take_children().into_iter().next().unwrap();
+        let mut exp =
+            RpnExpressionBuilder::build_from_expr_tree(child, time_zone, src_schema.len())?;
+        // The rewrite should always success.
+        super::util::rewrite_exp_for_sum_avg(src_schema, &mut exp).unwrap();
+
+        let rewritten_eval_type = EvalType::try_from(exp.ret_field_type(src_schema).tp()).unwrap();
+        out_exp.push(exp);
+
+        // Choose a type-aware SUM implementation based on the eval type after rewriting exp.
+        Ok(match rewritten_eval_type {
+            EvalType::Decimal => Box::new(AggrFnSum::<Decimal>::new()),
+            EvalType::Real => Box::new(AggrFnSum::<Real>::new()),
+            // If we meet unexpected types after rewriting, it is an implementation fault.
+            _ => unreachable!(),
+        })
+    }
+}
+
+/// The SUM aggregate function.
+///
+/// Note that there are `SUM(Decimal) -> Decimal` and `SUM(Double) -> Double`.
+#[derive(Debug, AggrFunction)]
+#[aggr_function(state = AggrFnStateSum::<T>::new())]
+pub struct AggrFnSum<T>
+where
+    T: Summable,
+    VectorValue: VectorValueExt<T>,
+{
+    _phantom: std::marker::PhantomData<T>,
+}
+
+impl<T> AggrFnSum<T>
+where
+    T: Summable,
+    VectorValue: VectorValueExt<T>,
+{
+    pub fn new() -> Self {
+        Self {
+            _phantom: std::marker::PhantomData,
+        }
+    }
+}
+
+/// The state of the SUM aggregate function.
+#[derive(Debug)]
+pub struct AggrFnStateSum<T>
+where
+    T: Summable,
+    VectorValue: VectorValueExt<T>,
+{
+    sum: T,
+    has_value: bool,
+}
+
+impl<T> AggrFnStateSum<T>
+where
+    T: Summable,
+    VectorValue: VectorValueExt<T>,
+{
+    pub fn new() -> Self {
+        Self {
+            sum: T::zero(),
+            has_value: false,
+        }
+    }
+}
+
+impl<T> super::ConcreteAggrFunctionState for AggrFnStateSum<T>
+where
+    T: Summable,
+    VectorValue: VectorValueExt<T>,
+{
+    type ParameterType = T;
+
+    #[inline]
+    fn update_concrete(&mut self, ctx: &mut EvalContext, value: &Option<T>) -> Result<()> {
+        match value {
+            None => Ok(()),
+            Some(value) => {
+                self.sum.add_assign(ctx, value)?;
+                self.has_value = true;
+                Ok(())
+            }
+        }
+    }
+
+    #[inline]
+    fn push_result(&self, _ctx: &mut EvalContext, target: &mut [VectorValue]) -> Result<()> {
+        if !self.has_value {
+            target[0].push(None);
+        } else {
+            target[0].push(Some(self.sum.clone()));
+        }
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use cop_datatype::{FieldTypeAccessor, FieldTypeTp};
+    use tipb_helper::ExprDefBuilder;
+
+    use crate::coprocessor::codec::batch::{LazyBatchColumn, LazyBatchColumnVec};
+    use crate::coprocessor::dag::aggr_fn::parser::AggrDefinitionParser;
+
+    /// SUM(Bytes) should produce (Real).
+    #[test]
+    fn test_integration() {
+        let expr = ExprDefBuilder::aggr_func(ExprType::Sum, FieldTypeTp::Double)
+            .push_child(ExprDefBuilder::column_ref(0, FieldTypeTp::VarString))
+            .build();
+        AggrFnDefinitionParserSum.check_supported(&expr).unwrap();
+
+        let src_schema = [FieldTypeTp::VarString.into()];
+        let mut columns = LazyBatchColumnVec::from(vec![{
+            let mut col = LazyBatchColumn::decoded_with_capacity_and_tp(0, EvalType::Bytes);
+            col.mut_decoded().push_bytes(Some(b"12.5".to_vec()));
+            col.mut_decoded().push_bytes(None);
+            col.mut_decoded().push_bytes(Some(b"42.0".to_vec()));
+            col.mut_decoded().push_bytes(None);
+            col
+        }]);
+
+        let mut schema = vec![];
+        let mut exp = vec![];
+
+        let aggr_fn = AggrFnDefinitionParserSum
+            .parse(expr, &Tz::utc(), &src_schema, &mut schema, &mut exp)
+            .unwrap();
+        assert_eq!(schema.len(), 1);
+        assert_eq!(schema[0].tp(), FieldTypeTp::Double);
+
+        assert_eq!(exp.len(), 1);
+
+        let mut state = aggr_fn.create_state();
+        let mut ctx = EvalContext::default();
+
+        let exp_result = exp[0].eval(&mut ctx, 4, &src_schema, &mut columns).unwrap();
+        assert!(exp_result.is_vector());
+        let slice: &[Option<Real>] = exp_result.vector_value().unwrap().as_ref();
+        state.update_vector(&mut ctx, slice).unwrap();
+
+        let mut aggr_result = [VectorValue::with_capacity(0, EvalType::Real)];
+        state.push_result(&mut ctx, &mut aggr_result).unwrap();
+
+        assert_eq!(aggr_result[0].as_real_slice(), &[Real::new(54.5).ok()]);
+    }
+}
diff --git a/src/coprocessor/dag/aggr_fn/mod.rs b/src/coprocessor/dag/aggr_fn/mod.rs
index cf5326a9513b..afe1b73db5ed 100644
--- a/src/coprocessor/dag/aggr_fn/mod.rs
+++ b/src/coprocessor/dag/aggr_fn/mod.rs
@@ -5,6 +5,7 @@
 mod impl_avg;
 mod impl_count;
 mod impl_first;
+mod impl_sum;
 mod parser;
 mod summable;
 mod util;
diff --git a/src/coprocessor/dag/aggr_fn/parser.rs b/src/coprocessor/dag/aggr_fn/parser.rs
index ec6f25583b53..0a7fcd1dcada 100644
--- a/src/coprocessor/dag/aggr_fn/parser.rs
+++ b/src/coprocessor/dag/aggr_fn/parser.rs
@@ -43,6 +43,7 @@ pub trait AggrDefinitionParser {
 fn map_pb_sig_to_aggr_func_parser(value: ExprType) -> Result<Box<dyn AggrDefinitionParser>> {
     match value {
         ExprType::Count => Ok(Box::new(super::impl_count::AggrFnDefinitionParserCount)),
+        ExprType::Sum => Ok(Box::new(super::impl_sum::AggrFnDefinitionParserSum)),
         ExprType::Avg => Ok(Box::new(super::impl_avg::AggrFnDefinitionParserAvg)),
         ExprType::First => Ok(Box::new(super::impl_first::AggrFnDefinitionParserFirst)),
         v => Err(box_err!(