From dd2fadb1908573cee11ddc4c7992467a619a6a85 Mon Sep 17 00:00:00 2001
From: HuaiyuXu <391585975@qq.com>
Date: Mon, 26 Jul 2021 17:04:16 +0800
Subject: [PATCH] expression, executor: fix type infer for
 greatest/leastest(datetime) (#26533)

---
 executor/executor_test.go     |  9 +++++++++
 expression/builtin_compare.go | 15 +++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/executor/executor_test.go b/executor/executor_test.go
index 06fd438d2a..b4aa4451d9 100644
--- a/executor/executor_test.go
+++ b/executor/executor_test.go
@@ -8783,3 +8783,12 @@ func (s *testSuite) TestIssue25506(c *C) {
 	tk.MustExec("insert into tbl_23 values (0xF)")
 	tk.MustQuery("(select col_15 from tbl_23) union all (select col_15 from tbl_3 for update) order by col_15").Check(testkit.Rows("\x00\x00\x0F", "\x00\x00\xFF", "\x00\xFF\xFF"))
 }
+
+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 30de86394d..5c9367004e 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
 }