session: support setting tso into session variable "tidb_snapshot"

executor/executor_test.go

@@ -44,6 +44,7 @@ import (
 	"github.com/pingcap/tidb/store/mockstore"
 	"github.com/pingcap/tidb/store/mockstore/mocktikv"
 	"github.com/pingcap/tidb/store/tikv"
+	"github.com/pingcap/tidb/store/tikv/oracle"
 	"github.com/pingcap/tidb/store/tikv/tikvrpc"
 	"github.com/pingcap/tidb/table/tables"
 	"github.com/pingcap/tidb/tablecodec"
@@ -1809,6 +1810,11 @@ func (s *testSuite) TestHistoryRead(c *C) {
 	tk.MustQuery("select * from history_read order by a").Check(testkit.Rows("2 <nil>", "4 <nil>", "8 8", "9 9"))
 	tk.MustExec("set @@tidb_snapshot = '" + snapshotTime.Format("2006-01-02 15:04:05.999999") + "'")
 	tk.MustQuery("select * from history_read order by a").Check(testkit.Rows("2", "4"))
+	tsoStr := strconv.FormatUint(oracle.EncodeTSO(snapshotTime.UnixNano()/int64(time.Millisecond)), 10)
+
+	tk.MustExec("set @@tidb_snapshot = '" + tsoStr + "'")
+	tk.MustQuery("select * from history_read order by a").Check(testkit.Rows("2", "4"))
+
 	tk.MustExec("set @@tidb_snapshot = ''")
 	tk.MustQuery("select * from history_read order by a").Check(testkit.Rows("2 <nil>", "4 <nil>", "8 8", "9 9"))
 }

sessionctx/variable/tidb_vars.go

@@ -29,7 +29,7 @@ package variable
 // TiDB system variable names that only in session scope.
 const (
 	// tidb_snapshot is used for reading history data, the default value is empty string.
-	// When the value is set to a datetime string like '2017-11-11 20:20:20', the session reads history data of that time.
+	// The value can be a datetime string like '2017-11-11 20:20:20' or a tso string. When this variable is set, the session reads history data of that time.
 	TiDBSnapshot = "tidb_snapshot"
 
 	// tidb_import_data is used for loading data from a dump file, to speed up the loading process.

sessionctx/variable/varsutil.go

@@ -175,10 +175,17 @@ func setSnapshotTS(s *SessionVars, sVal string) error {
 		s.SnapshotTS = 0
 		return nil
 	}
+
+	if tso, err := strconv.ParseUint(sVal, 10, 64); err == nil {
+		s.SnapshotTS = tso
+		return nil
+	}
+
 	t, err := types.ParseTime(s.StmtCtx, sVal, mysql.TypeTimestamp, types.MaxFsp)
 	if err != nil {
 		return errors.Trace(err)
 	}
+
 	// TODO: Consider time_zone variable.
 	t1, err := t.Time.GoTime(time.Local)
 	s.SnapshotTS = GoTimeToTS(t1)

store/tikv/oracle/oracle.go

@@ -48,3 +48,15 @@ func ExtractPhysical(ts uint64) int64 {
 func GetPhysical(t time.Time) int64 {
 	return t.UnixNano() / int64(time.Millisecond)
 }
+
+// EncodeTSO encodes a millisecond into tso.
+func EncodeTSO(ts int64) uint64 {
+	return uint64(ts) << physicalShiftBits
+}
+
+// DecodeTSO decodes a tso into a golang's time struct.
+func DecodeTSO(tso uint64) time.Time {
+	physical := tso >> physicalShiftBits
+	physicalTime := time.Unix(int64(physical/1000), 0)
+	return physicalTime
+}