types: fix wrong hash key for decimal

executor/join_test.go

@@ -2166,3 +2166,15 @@ func (s *testSuite9) TestIssue18572_3(c *C) {
 	_, err = session.GetRows4Test(context.Background(), nil, rs)
 	c.Assert(strings.Contains(err.Error(), "mockIndexHashJoinBuildErr"), IsTrue)
 }
+
+func (s *testSuite9) TestIssue19112(c *C) {
+	tk := testkit.NewTestKitWithInit(c, s.store)
+	tk.MustExec("drop table if exists t1, t2")
+	tk.MustExec("create table t1 ( c_int int, c_decimal decimal(12, 6), key(c_int), unique key(c_decimal) )")
+	tk.MustExec("create table t2 like t1")
+	tk.MustExec("insert into t1 (c_int, c_decimal) values (1, 4.064000), (2, 0.257000), (3, 1.010000)")
+	tk.MustExec("insert into t2 (c_int, c_decimal) values (1, 4.064000), (3, 1.010000)")
+	tk.MustQuery("select /*+ HASH_JOIN(t1,t2) */  * from t1 join t2 on t1.c_decimal = t2.c_decimal order by t1.c_int").Check(testkit.Rows(
+		"1 4.064000 1 4.064000",
+		"3 1.010000 3 1.010000"))
+}

types/mydecimal.go

@@ -14,6 +14,7 @@
 package types
 
 import (
+	"encoding/binary"
 	"math"
 	"strconv"
 
@@ -1301,6 +1302,12 @@ func (d *MyDecimal) ToHashKey() ([]byte, error) {
 		// thus ErrTruncated may be raised, we can ignore it here.
 		err = nil
 	}
+	precBuff := make([]byte, 4)
+	digitBuff := make([]byte, 4)
+	binary.LittleEndian.PutUint32(precBuff, uint32(prec))
+	binary.LittleEndian.PutUint32(digitBuff, uint32(digitsFrac))
+	buf = append(buf, precBuff...)
+	buf = append(buf, digitBuff...)
 	return buf, err
 }
 

types/mydecimal_test.go

@@ -208,6 +208,8 @@ func (s *testMyDecimalSuite) TestToHashKey(c *C) {
 			var dec MyDecimal
 			c.Check(dec.FromString([]byte(num)), IsNil)
 			key, err := dec.ToHashKey()
+			// remove prec and digit len
+			key = key[:len(key)-8]
 			c.Check(err, IsNil)
 			keys = append(keys, string(key))
 		}