expression: implement vectorized evaluation for 'builtinCastDecimalAs…

…JSONSig' (#12803)

expression/builtin_cast_vec.go

@@ -1005,11 +1005,36 @@ func (b *builtinCastJSONAsDurationSig) vecEvalDuration(input *chunk.Chunk, resul
 }
 
 func (b *builtinCastDecimalAsJSONSig) vectorized() bool {
-	return false
+	return true
 }
 
 func (b *builtinCastDecimalAsJSONSig) vecEvalJSON(input *chunk.Chunk, result *chunk.Column) error {
-	return errors.Errorf("not implemented")
+	n := input.NumRows()
+	buf, err := b.bufAllocator.get(types.ETDecimal, n)
+	if err != nil {
+		return err
+	}
+	defer b.bufAllocator.put(buf)
+	if err = b.args[0].VecEvalDecimal(b.ctx, input, buf); err != nil {
+		return err
+	}
+
+	result.ReserveJSON(n)
+	f64s := buf.Decimals()
+	var f float64
+	for i := 0; i < n; i++ {
+		if buf.IsNull(i) {
+			result.AppendNull()
+			continue
+		}
+		// FIXME: `select json_type(cast(1111.11 as json))` should return `DECIMAL`, we return `DOUBLE` now.
+		f, err = f64s[i].ToFloat64()
+		if err != nil {
+			return err
+		}
+		result.AppendJSON(json.CreateBinary(f))
+	}
+	return nil
 }
 
 func (b *builtinCastDurationAsJSONSig) vectorized() bool {

expression/builtin_cast_vec_test.go

@@ -51,6 +51,7 @@ var vecBuiltinCastCases = map[string][]vecExprBenchCase{
 		{retEvalType: types.ETJson, childrenTypes: []types.EvalType{types.ETInt}},
 		{retEvalType: types.ETJson, childrenTypes: []types.EvalType{types.ETReal}},
 		{retEvalType: types.ETJson, childrenTypes: []types.EvalType{types.ETJson}},
+		{retEvalType: types.ETJson, childrenTypes: []types.EvalType{types.ETDecimal}},
 	},
 }