expression:add support for JSON_MERGE_PRESERVE

expression/builtin_json.go

@@ -403,6 +403,11 @@ func (b *builtinJSONMergeSig) evalJSON(row chunk.Row) (res json.BinaryJSON, isNu
 		values = append(values, value)
 	}
 	res = json.MergeBinary(values)
+	// function "JSON_MERGE" is deprecated since MySQL 5.7.22. Synonym for function "JSON_MERGE_PRESERVE".
+	// See https://dev.mysql.com/doc/refman/5.7/en/json-modification-functions.html#function_json-merge
+	if b.pbCode == tipb.ScalarFuncSig_JsonMergeSig {
+		b.ctx.GetSessionVars().StmtCtx.AppendWarning(errDeprecatedSyntaxNoReplacement.GenWithStackByArgs("JSON_MERGE"))
+	}
 	return res, false, nil
 }
 
@@ -720,7 +725,17 @@ type jsonMergePreserveFunctionClass struct {
 }
 
 func (c *jsonMergePreserveFunctionClass) getFunction(ctx sessionctx.Context, args []Expression) (builtinFunc, error) {
-	return nil, errFunctionNotExists.GenWithStackByArgs("FUNCTION", "JSON_MERGE_PRESERVE")
+	if err := c.verifyArgs(args); err != nil {
+		return nil, err
+	}
+	argTps := make([]types.EvalType, 0, len(args))
+	for range args {
+		argTps = append(argTps, types.ETJson)
+	}
+	bf := newBaseBuiltinFuncWithTp(ctx, args, types.ETJson, argTps...)
+	sig := &builtinJSONMergeSig{bf}
+	sig.setPbCode(tipb.ScalarFuncSig_JsonMergePreserveSig)
+	return sig, nil
 }
 
 type jsonPrettyFunctionClass struct {

expression/builtin_json_test.go

@@ -187,6 +187,39 @@ func (s *testEvaluatorSuite) TestJSONMerge(c *C) {
 			j2 := d.GetMysqlJSON()
 			cmp := json.CompareBinary(j1, j2)
 			c.Assert(cmp, Equals, 0, Commentf("got %v expect %v", j1.String(), j2.String()))
+		case nil:
+			c.Assert(d.IsNull(), IsTrue)
+		}
+	}
+}
+
+func (s *testEvaluatorSuite) TestJSONMergePreserve(c *C) {
+	defer testleak.AfterTest(c)()
+	fc := funcs[ast.JSONMergePreserve]
+	tbl := []struct {
+		Input    []interface{}
+		Expected interface{}
+	}{
+		{[]interface{}{nil, nil}, nil},
+		{[]interface{}{`{}`, `[]`}, `[{}]`},
+		{[]interface{}{`{}`, `[]`, `3`, `"4"`}, `[{}, 3, "4"]`},
+	}
+	for _, t := range tbl {
+		args := types.MakeDatums(t.Input...)
+		f, err := fc.getFunction(s.ctx, s.datumsToConstants(args))
+		c.Assert(err, IsNil)
+		d, err := evalBuiltinFunc(f, chunk.Row{})
+		c.Assert(err, IsNil)
+
+		switch x := t.Expected.(type) {
+		case string:
+			j1, err := json.ParseBinaryFromString(x)
+			c.Assert(err, IsNil)
+			j2 := d.GetMysqlJSON()
+			cmp := json.CompareBinary(j1, j2)
+			c.Assert(cmp, Equals, 0, Commentf("got %v expect %v", j1.String(), j2.String()))
+		case nil:
+			c.Assert(d.IsNull(), IsTrue)
 		}
 	}
 }