From 7b0814c11511939ef5be148788a5e7c868dbdb4c Mon Sep 17 00:00:00 2001
From: Chao Wang <cclcwangchao@hotmail.com>
Date: Thu, 24 Jun 2021 22:15:12 +0800
Subject: [PATCH] executor: tablesample for temporary table should always
 return empty

---
 executor/builder.go       | 21 +++++++++++++------
 executor/executor_test.go | 44 +++++++++++++++++++++++++++++++++++++--
 2 files changed, 57 insertions(+), 8 deletions(-)

diff --git a/executor/builder.go b/executor/builder.go
index 177bf394f2bc9..7d17b63b2e211 100644
--- a/executor/builder.go
+++ b/executor/builder.go
@@ -4211,12 +4211,17 @@ func fullRangePartition(idxArr []int) bool {
 	return len(idxArr) == 1 && idxArr[0] == plannercore.FullRange
 }
 
-func (b *executorBuilder) buildTableSample(v *plannercore.PhysicalTableSample) *TableSampleExecutor {
-	if v.TableInfo.Meta().TempTableType != model.TempTableNone {
-		b.err = errors.New("TABLESAMPLE clause can not be applied to temporary tables")
-		return nil
-	}
+type emptySampler struct{}
+
+func (s *emptySampler) writeChunk(_ *chunk.Chunk) error {
+	return nil
+}
 
+func (s *emptySampler) finished() bool {
+	return true
+}
+
+func (b *executorBuilder) buildTableSample(v *plannercore.PhysicalTableSample) *TableSampleExecutor {
 	startTS, err := b.getSnapshotTS()
 	if err != nil {
 		b.err = err
@@ -4227,11 +4232,15 @@ func (b *executorBuilder) buildTableSample(v *plannercore.PhysicalTableSample) *
 		table:        v.TableInfo,
 		startTS:      startTS,
 	}
-	if v.TableSampleInfo.AstNode.SampleMethod == ast.SampleMethodTypeTiDBRegion {
+
+	if v.TableInfo.Meta().TempTableType != model.TempTableNone {
+		e.sampler = &emptySampler{}
+	} else if v.TableSampleInfo.AstNode.SampleMethod == ast.SampleMethodTypeTiDBRegion {
 		e.sampler = newTableRegionSampler(
 			b.ctx, v.TableInfo, startTS, v.TableSampleInfo.Partitions, v.Schema(),
 			v.TableSampleInfo.FullSchema, e.retFieldTypes, v.Desc)
 	}
+
 	return e
 }
 
diff --git a/executor/executor_test.go b/executor/executor_test.go
index 249c8f16b8205..c31ae509554e6 100644
--- a/executor/executor_test.go
+++ b/executor/executor_test.go
@@ -8583,8 +8583,6 @@ func (s *testStaleTxnSuite) TestInvalidReadTemporaryTable(c *C) {
 	// sleep 1us to make test stale
 	time.Sleep(time.Microsecond)
 
-	tk.MustGetErrMsg("select * from tmp1 tablesample regions()", "TABLESAMPLE clause can not be applied to temporary tables")
-
 	queries := []struct {
 		sql string
 	}{
@@ -8658,6 +8656,48 @@ func (s *testStaleTxnSuite) TestInvalidReadTemporaryTable(c *C) {
 	}
 }
 
+func (s *testSuite) TestEmptyTableSampleTemporaryTable(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	// For mocktikv, safe point is not initialized, we manually insert it for snapshot to use.
+	safePointName := "tikv_gc_safe_point"
+	safePointValue := "20160102-15:04:05 -0700"
+	safePointComment := "All versions after safe point can be accessed. (DO NOT EDIT)"
+	updateSafePoint := fmt.Sprintf(`INSERT INTO mysql.tidb VALUES ('%[1]s', '%[2]s', '%[3]s')
+	ON DUPLICATE KEY
+	UPDATE variable_value = '%[2]s', comment = '%[3]s'`, safePointName, safePointValue, safePointComment)
+	tk.MustExec(updateSafePoint)
+
+	tk.MustExec("set @@tidb_enable_global_temporary_table=1")
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists tmp1")
+	tk.MustExec("create global temporary table tmp1 " +
+		"(id int not null primary key, code int not null, value int default null, unique key code(code))" +
+		"on commit delete rows")
+
+	// sleep 1us to make test stale
+	time.Sleep(time.Microsecond)
+
+	// test tablesample return empty
+	rs := tk.MustQuery("select * from tmp1 tablesample regions()")
+	rs.Check(testkit.Rows())
+
+	tk.MustExec("begin")
+	tk.MustExec("insert into tmp1 values (1, 1, 1)")
+	rs = tk.MustQuery("select * from tmp1 tablesample regions()")
+	rs.Check(testkit.Rows())
+	tk.MustExec("commit")
+
+	// tablesample should not return error for compatibility of tools like dumpling
+	tk.MustExec("set @@tidb_snapshot=NOW(6)")
+	rs = tk.MustQuery("select * from tmp1 tablesample regions()")
+	rs.Check(testkit.Rows())
+
+	tk.MustExec("begin")
+	rs = tk.MustQuery("select * from tmp1 tablesample regions()")
+	rs.Check(testkit.Rows())
+	tk.MustExec("commit")
+}
+
 func (s *testSuite) TestIssue25506(c *C) {
 	tk := testkit.NewTestKit(c, s.store)
 	tk.MustExec("use test")