*: Address comment

ast/functions.go

@@ -557,17 +557,14 @@ func (n *AggregateFuncExpr) updateGroupConcat() error {
 			return nil
 		}
 	}
-	var buf bytes.Buffer
-	if ctx.Value != nil {
+	if ctx.Buffer.Len() > 0 {
 		// now use comma separator
-		buf.WriteString(ctx.Value.(string))
-		buf.WriteString(",")
+		ctx.Buffer.WriteString(",")
 	}
 	for _, val := range vals {
-		buf.WriteString(fmt.Sprintf("%v", val))
+		ctx.Buffer.WriteString(fmt.Sprintf("%v", val))
 	}
 	// TODO: if total length is greater than global var group_concat_max_len, truncate it.
-	ctx.Value = buf.String()
 	return nil
 }
 
@@ -624,5 +621,6 @@ type AggEvaluateContext struct {
 	distinctChecker *distinct.Checker
 	Count           int64
 	Value           interface{}
+	Buffer          bytes.Buffer // Buffer is used for group_concat.
 	evaluated       bool
 }

optimizer/evaluator/evaluator.go

@@ -960,8 +960,10 @@ func (e *Evaluator) aggregateFunc(v *ast.AggregateFuncExpr) bool {
 		e.evalAggAvg(v)
 	case ast.AggFuncCount:
 		e.evalAggCount(v)
-	case ast.AggFuncFirstRow, ast.AggFuncMax, ast.AggFuncMin, ast.AggFuncSum, ast.AggFuncGroupConcat:
+	case ast.AggFuncFirstRow, ast.AggFuncMax, ast.AggFuncMin, ast.AggFuncSum:
 		e.evalAggSetValue(v)
+	case ast.AggFuncGroupConcat:
+		e.evalAggGroupConcat(v)
 	}
 	return e.err == nil
 }
@@ -986,3 +988,12 @@ func (e *Evaluator) evalAggAvg(v *ast.AggregateFuncExpr) {
 	}
 	v.SetValue(ctx.Value)
 }
+
+func (e *Evaluator) evalAggGroupConcat(v *ast.AggregateFuncExpr) {
+	ctx := v.GetContext()
+	if ctx.Buffer.Len() > 0 {
+		v.SetValue(ctx.Buffer.String())
+	} else {
+		v.SetValue(nil)
+	}
+}