From 536dfdfe2f727b5dfb6bddb889150c3b235172ef Mon Sep 17 00:00:00 2001
From: tangenta <tangenta@126.com>
Date: Mon, 4 Jan 2021 16:37:15 +0800
Subject: [PATCH] executor: always decode the value first and then the handle
 (#22073)

---
 executor/builder.go              |  2 +-
 executor/clustered_index_test.go | 24 ++++++++++++++++++++++++
 executor/executor_test.go        |  1 +
 util/rowcodec/decoder.go         | 14 ++++++++++----
 4 files changed, 36 insertions(+), 5 deletions(-)

diff --git a/executor/builder.go b/executor/builder.go
index e297ef0fd5..2958525d15 100644
--- a/executor/builder.go
+++ b/executor/builder.go
@@ -963,7 +963,7 @@ func (b *executorBuilder) buildUnionScanFromReader(reader Executor, v *plannerco
 		}
 		return x
 	}
-	// If reader is union, it means a partitiont table and we should transfer as above.
+	// If reader is union, it means a partition table and we should transfer as above.
 	if x, ok := reader.(*UnionExec); ok {
 		for i, child := range x.children {
 			x.children[i] = b.buildUnionScanFromReader(child, v)
diff --git a/executor/clustered_index_test.go b/executor/clustered_index_test.go
index e226e90a76..db1e0e636a 100644
--- a/executor/clustered_index_test.go
+++ b/executor/clustered_index_test.go
@@ -17,10 +17,12 @@ import (
 	. "github.com/pingcap/check"
 	"github.com/pingcap/tidb/errno"
 	"github.com/pingcap/tidb/store/tikv"
+	"github.com/pingcap/tidb/util/collate"
 	"github.com/pingcap/tidb/util/testkit"
 )
 
 type testClusteredSuite struct{ *baseTestSuite }
+type testClusteredSerialSuite struct{ *testClusteredSuite }
 
 func (s *testClusteredSuite) SetUpTest(c *C) {
 }
@@ -184,6 +186,28 @@ func (s *testClusteredSuite) TestClusteredPrefixingPrimaryKey(c *C) {
 	tk.MustExec("admin check table t;")
 }
 
+// Test for union scan in prefixed clustered index table.
+// See https://github.com/pingcap/tidb/issues/22069.
+func (s *testClusteredSerialSuite) TestClusteredUnionScanOnPrefixingPrimaryKey(c *C) {
+	originCollate := collate.NewCollationEnabled()
+	collate.SetNewCollationEnabledForTest(false)
+	defer collate.SetNewCollationEnabledForTest(originCollate)
+	tk := s.newTK(c)
+	tk.MustExec("drop table if exists t;")
+	tk.MustExec("create table t (col_1 varchar(255), col_2 tinyint, primary key idx_1 (col_1(1)));")
+	tk.MustExec("insert into t values ('aaaaa', -38);")
+	tk.MustExec("insert into t values ('bbbbb', -48);")
+
+	tk.MustExec("begin PESSIMISTIC;")
+	tk.MustExec("update t set col_2 = 47 where col_1 in ('aaaaa') order by col_1,col_2;")
+	tk.MustQuery("select * from t;").Check(testkit.Rows("aaaaa 47", "bbbbb -48"))
+	tk.MustGetErrCode("insert into t values ('bb', 0);", errno.ErrDupEntry)
+	tk.MustGetErrCode("insert into t values ('aa', 0);", errno.ErrDupEntry)
+	tk.MustExec("commit;")
+	tk.MustQuery("select * from t;").Check(testkit.Rows("aaaaa 47", "bbbbb -48"))
+	tk.MustExec("admin check table t;")
+}
+
 func (s *testClusteredSuite) TestClusteredWithOldRowFormat(c *C) {
 	tk := s.newTK(c)
 	tk.Se.GetSessionVars().RowEncoder.Enable = false
diff --git a/executor/executor_test.go b/executor/executor_test.go
index c18655cd9b..debf33f310 100644
--- a/executor/executor_test.go
+++ b/executor/executor_test.go
@@ -119,6 +119,7 @@ var _ = Suite(&testSuite6{&baseTestSuite{}})
 var _ = Suite(&testSuite7{&baseTestSuite{}})
 var _ = Suite(&testSuite8{&baseTestSuite{}})
 var _ = Suite(&testClusteredSuite{&baseTestSuite{}})
+var _ = SerialSuites(&testClusteredSerialSuite{&testClusteredSuite{&baseTestSuite{}}})
 var _ = SerialSuites(&testShowStatsSuite{&baseTestSuite{}})
 var _ = Suite(&testBypassSuite{})
 var _ = Suite(&testUpdateSuite{})
diff --git a/util/rowcodec/decoder.go b/util/rowcodec/decoder.go
index b9476eb794..8400991b56 100644
--- a/util/rowcodec/decoder.go
+++ b/util/rowcodec/decoder.go
@@ -222,6 +222,9 @@ func (decoder *ChunkDecoder) DecodeToChunk(rowData []byte, handle kv.Handle, chk
 			continue
 		}
 
+		// Only try to decode handle when there is no corresponding column in the value.
+		// This is because the information in handle may be incomplete in some cases.
+		// For example, prefixed clustered index like 'primary key(col1(1))' only store the leftmost 1 char in the handle.
 		if decoder.tryAppendHandleColumn(colIdx, col, handle, chk) {
 			continue
 		}
@@ -385,10 +388,6 @@ func (decoder *BytesDecoder) decodeToBytesInternal(outputOffset map[int64]int, h
 		tp := fieldType2Flag(col.Ft.Tp, col.Ft.Flag&mysql.UnsignedFlag == 0)
 		colID := col.ID
 		offset := outputOffset[colID]
-		if decoder.tryDecodeHandle(values, offset, col, handle, cacheBytes) {
-			continue
-		}
-
 		idx, isNil, notFound := r.findColID(colID)
 		if !notFound && !isNil {
 			val := r.getData(idx)
@@ -396,6 +395,13 @@ func (decoder *BytesDecoder) decodeToBytesInternal(outputOffset map[int64]int, h
 			continue
 		}
 
+		// Only try to decode handle when there is no corresponding column in the value.
+		// This is because the information in handle may be incomplete in some cases.
+		// For example, prefixed clustered index like 'primary key(col1(1))' only store the leftmost 1 char in the handle.
+		if decoder.tryDecodeHandle(values, offset, col, handle, cacheBytes) {
+			continue
+		}
+
 		if isNil {
 			values[offset] = []byte{NilFlag}
 			continue