expression: support JSON return type in case expression

expression/builtin_control.go

@@ -205,6 +205,9 @@ func (c *caseWhenFunctionClass) getFunction(ctx sessionctx.Context, args []Expre
 	case types.ETDuration:
 		sig = &builtinCaseWhenDurationSig{bf}
 		sig.setPbCode(tipb.ScalarFuncSig_CaseWhenDuration)
+	case types.ETJson:
+		sig = &builtinCaseWhenJSONSig{bf}
+		sig.setPbCode(tipb.ScalarFuncSig_CaseWhenJson)
 	}
 	return sig, nil
 }
@@ -425,6 +428,40 @@ func (b *builtinCaseWhenDurationSig) evalDuration(row chunk.Row) (ret types.Dura
 	return ret, true, nil
 }
 
+type builtinCaseWhenJSONSig struct {
+	baseBuiltinFunc
+}
+
+func (b *builtinCaseWhenJSONSig) Clone() builtinFunc {
+	newSig := &builtinCaseWhenJSONSig{}
+	newSig.cloneFrom(&b.baseBuiltinFunc)
+	return newSig
+}
+
+// evalJSON evals a builtinCaseWhenJSONSig.
+// See https://dev.mysql.com/doc/refman/5.7/en/case.html
+func (b *builtinCaseWhenJSONSig) evalJSON(row chunk.Row) (ret json.BinaryJSON, isNull bool, err error) {
+	var condition int64
+	args, l := b.getArgs(), len(b.getArgs())
+	for i := 0; i < l-1; i += 2 {
+		condition, isNull, err = args[i].EvalInt(b.ctx, row)
+		if err != nil {
+			return
+		}
+		if isNull || condition == 0 {
+			continue
+		}
+		return args[i+1].EvalJSON(b.ctx, row)
+	}
+	// when clause(condition, result) -> args[i], args[i+1]; (i >= 0 && i+1 < l-1)
+	// else clause -> args[l-1]
+	// If case clause has else clause, l%2 == 1.
+	if l%2 == 1 {
+		return args[l-1].EvalJSON(b.ctx, row)
+	}
+	return ret, true, nil
+}
+
 type ifFunctionClass struct {
 	baseFunctionClass
 }

expression/builtin_control_test.go

@@ -38,6 +38,8 @@ func (s *testEvaluatorSuite) TestCaseWhen(c *C) {
 		{[]interface{}{nil, 1, nil, 2, 3}, 3},
 		{[]interface{}{false, 1, nil, 2, 3}, 3},
 		{[]interface{}{nil, 1, false, 2, 3}, 3},
+		{[]interface{}{1, jsonInt.GetMysqlJSON(), nil}, 3},
+		{[]interface{}{0, jsonInt.GetMysqlJSON(), nil}, nil},
 	}
 	fc := funcs[ast.Case]
 	for _, t := range tbl {

expression/distsql_builtin.go

@@ -355,6 +355,8 @@ func getSignatureByPB(ctx sessionctx.Context, sigCode tipb.ScalarFuncSig, tp *ti
 	case tipb.ScalarFuncSig_CoalesceInt:
 		f = &builtinCoalesceIntSig{base}
 
+	case tipb.ScalarFuncSig_CaseWhenJson:
+		f = &builtinCaseWhenJSONSig{base}
 	case tipb.ScalarFuncSig_CaseWhenDecimal:
 		f = &builtinCaseWhenDecimalSig{base}
 	case tipb.ScalarFuncSig_CaseWhenDuration:

expression/evaluator_test.go

@@ -103,6 +103,8 @@ func (s *testEvaluatorSuite) kindToFieldType(kind byte) types.FieldType {
 		ft.Collate = charset.CollationBin
 	case types.KindMysqlBit:
 		ft.Tp = mysql.TypeBit
+	case types.KindMysqlJSON:
+		ft.Tp = mysql.TypeJSON
 	}
 	return ft
 }