diff --git a/executor/executor_test.go b/executor/executor_test.go
index 012d7ccbfd593..0ed65eeea26c3 100644
--- a/executor/executor_test.go
+++ b/executor/executor_test.go
@@ -7107,3 +7107,12 @@ func (s *testSerialSuite1) TestIssue24210(c *C) {
 	err = failpoint.Disable("github.com/pingcap/tidb/executor/mockSelectionExecBaseExecutorOpenReturnedError")
 	c.Assert(err, IsNil)
 }
+
+func (s *testSuite) TestIssue26532(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustQuery("select greatest(cast(\"2020-01-01 01:01:01\" as datetime), cast(\"2019-01-01 01:01:01\" as datetime) )union select null;").Sort().Check(testkit.Rows("2020-01-01 01:01:01", "<nil>"))
+	tk.MustQuery("select least(cast(\"2020-01-01 01:01:01\" as datetime), cast(\"2019-01-01 01:01:01\" as datetime) )union select null;").Sort().Check(testkit.Rows("2019-01-01 01:01:01", "<nil>"))
+	tk.MustQuery("select greatest(\"2020-01-01 01:01:01\" ,\"2019-01-01 01:01:01\" )union select null;").Sort().Check(testkit.Rows("2020-01-01 01:01:01", "<nil>"))
+	tk.MustQuery("select least(\"2020-01-01 01:01:01\" , \"2019-01-01 01:01:01\" )union select null;").Sort().Check(testkit.Rows("2019-01-01 01:01:01", "<nil>"))
+}
diff --git a/expression/builtin_compare.go b/expression/builtin_compare.go
index 8223cdfa0e12c..b50089dc1fc40 100644
--- a/expression/builtin_compare.go
+++ b/expression/builtin_compare.go
@@ -485,9 +485,23 @@ func (c *greatestFunctionClass) getFunction(ctx sessionctx.Context, args []Expre
 		sig = &builtinGreatestTimeSig{bf}
 		sig.setPbCode(tipb.ScalarFuncSig_GreatestTime)
 	}
+	sig.getRetTp().Flen, sig.getRetTp().Decimal = fixFlenAndDecimalForGreatestAndLeast(args)
 	return sig, nil
 }
 
+func fixFlenAndDecimalForGreatestAndLeast(args []Expression) (flen, decimal int) {
+	for _, arg := range args {
+		argFlen, argDecimal := arg.GetType().Flen, arg.GetType().Decimal
+		if argFlen > flen {
+			flen = argFlen
+		}
+		if argDecimal > decimal {
+			decimal = argDecimal
+		}
+	}
+	return flen, decimal
+}
+
 type builtinGreatestIntSig struct {
 	baseBuiltinFunc
 }
@@ -702,6 +716,7 @@ func (c *leastFunctionClass) getFunction(ctx sessionctx.Context, args []Expressi
 		sig = &builtinLeastTimeSig{bf}
 		sig.setPbCode(tipb.ScalarFuncSig_LeastTime)
 	}
+	sig.getRetTp().Flen, sig.getRetTp().Decimal = fixFlenAndDecimalForGreatestAndLeast(args)
 	return sig, nil
 }