expression: support JSON for builtin function COALESCE

expression/builtin_compare.go

@@ -202,6 +202,9 @@ func (c *coalesceFunctionClass) getFunction(ctx sessionctx.Context, args []Expre
 		}
 		sig = &builtinCoalesceDurationSig{bf}
 		sig.setPbCode(tipb.ScalarFuncSig_CoalesceDuration)
+	case types.ETJson:
+		sig = &builtinCoalesceJsonSig{bf}
+		sig.setPbCode(tipb.ScalarFuncSig_CoalesceJson)
 	}
 
 	return sig, nil
@@ -339,6 +342,28 @@ func (b *builtinCoalesceDurationSig) evalDuration(row chunk.Row) (res types.Dura
 	return res, isNull, err
 }
 
+// builtinCoalesceJsonSig is buitin function coalesce signature which return type time
+// See http://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#function_coalesce
+type builtinCoalesceJsonSig struct {
+	baseBuiltinFunc
+}
+
+func (b *builtinCoalesceJsonSig) Clone() builtinFunc {
+	newSig := &builtinCoalesceJsonSig{}
+	newSig.cloneFrom(&b.baseBuiltinFunc)
+	return newSig
+}
+
+func (b *builtinCoalesceJsonSig) evalJSON(row chunk.Row) (res json.BinaryJSON, isNull bool, err error) {
+	for _, a := range b.getArgs() {
+		res, isNull, err = a.EvalJSON(b.ctx, row)
+		if err != nil || !isNull {
+			break
+		}
+	}
+	return res, isNull, err
+}
+
 // temporalWithDateAsNumEvalType makes DATE, DATETIME, TIMESTAMP pretend to be numbers rather than strings.
 func temporalWithDateAsNumEvalType(argTp *types.FieldType) (argEvalType types.EvalType, isStr bool, isTemporalWithDate bool) {
 	argEvalType = argTp.EvalType()

expression/integration_test.go

@@ -3857,3 +3857,20 @@ func (s *testIntegrationSuite) TestCastAsTime(c *C) {
 	err = tk.ExecToErr(`select cast(col5 as time(31)) from t where col1 is null;`)
 	c.Assert(err.Error(), Equals, "[expression:1426]Too big precision 31 specified for column 'CAST'. Maximum is 6.")
 }
+
+func (s *testIntegrationSuite) TestCoalesce(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec(`use test;`)
+	tk.MustQuery(`select coalesce(cast(1 as json), cast(2 as json));`).Check(testkit.Rows(
+		`1`,
+	))
+	tk.MustQuery(`select coalesce(NULL, cast(2 as json));`).Check(testkit.Rows(
+		`2`,
+	))
+	tk.MustQuery(`select coalesce(cast(1 as json), NULL);`).Check(testkit.Rows(
+		`1`,
+	))
+	tk.MustQuery(`select coalesce(NULL, NULL);`).Check(testkit.Rows(
+		`<nil>`,
+	))
+}