sessiontxn/staleread: more accurate stale ts (#44272)

executor/ddl.go

@@ -531,7 +531,7 @@ func (e *DDLExec) getRecoverTableByTableName(tableName *ast.TableName) (*model.J
 }
 
 func (e *DDLExec) executeFlashBackCluster(s *ast.FlashBackToTimestampStmt) error {
-	flashbackTS, err := staleread.CalculateAsOfTsExpr(e.ctx, s.FlashbackTS)
+	flashbackTS, err := staleread.CalculateAsOfTsExpr(context.Background(), e.ctx, s.FlashbackTS)
 	if err != nil {
 		return err
 	}

executor/stale_txn_test.go

@@ -1396,3 +1396,28 @@ func TestStalePrepare(t *testing.T) {
 		tk.MustQuery("execute stmt").Check(expected)
 	}
 }
+
+func TestStaleTSO(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+	defer tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t (id int)")
+
+	tk.MustExec("insert into t values(1)")
+
+	asOfExprs := []string{
+		"now(3) - interval 1 second",
+		"current_time() - interval 1 second",
+		"curtime() - interval 1 second",
+	}
+
+	nextTSO := oracle.GoTimeToTS(time.Now().Add(2 * time.Second))
+	require.Nil(t, failpoint.Enable("github.com/pingcap/tidb/sessiontxn/staleread/mockStaleReadTSO", fmt.Sprintf("return(%d)", nextTSO)))
+	defer failpoint.Disable("github.com/pingcap/tidb/sessiontxn/staleread/mockStaleReadTSO")
+	for _, expr := range asOfExprs {
+		// Make sure the now() expr is evaluated from the stale ts provider.
+		tk.MustQuery("select * from t as of timestamp " + expr + " order by id asc").Check(testkit.Rows("1"))
+	}
+}

expression/helper.go

@@ -27,6 +27,9 @@ import (
 	"github.com/pingcap/tidb/sessionctx"
 	"github.com/pingcap/tidb/types"
 	driver "github.com/pingcap/tidb/types/parser_driver"
+	"github.com/pingcap/tidb/util/logutil"
+	"github.com/tikv/client-go/v2/oracle"
+	"go.uber.org/zap"
 )
 
 func boolToInt64(v bool) int64 {
@@ -158,6 +161,15 @@ func getStmtTimestamp(ctx sessionctx.Context) (time.Time, error) {
 		failpoint.Return(v, nil)
 	})
 
+	if ctx != nil {
+		staleTSO, err := ctx.GetSessionVars().StmtCtx.GetStaleTSO()
+		if staleTSO != 0 && err == nil {
+			return oracle.GetTimeFromTS(staleTSO), nil
+		} else if err != nil {
+			logutil.BgLogger().Error("get stale tso failed", zap.Error(err))
+		}
+	}
+
 	now := time.Now()
 
 	if ctx == nil {

planner/core/planbuilder.go

@@ -3342,7 +3342,7 @@ func (b *PlanBuilder) buildSimple(ctx context.Context, node ast.StmtNode) (Plan,
 	case *ast.BeginStmt:
 		readTS := b.ctx.GetSessionVars().TxnReadTS.PeakTxnReadTS()
 		if raw.AsOf != nil {
-			startTS, err := staleread.CalculateAsOfTsExpr(b.ctx, raw.AsOf.TsExpr)
+			startTS, err := staleread.CalculateAsOfTsExpr(ctx, b.ctx, raw.AsOf.TsExpr)
 			if err != nil {
 				return nil, err
 			}

sessionctx/context.go

@@ -244,7 +244,10 @@ const allowedTimeFromNow = 100 * time.Millisecond
 
 // ValidateStaleReadTS validates that readTS does not exceed the current time not strictly.
 func ValidateStaleReadTS(ctx context.Context, sctx Context, readTS uint64) error {
-	currentTS, err := sctx.GetStore().GetOracle().GetStaleTimestamp(ctx, oracle.GlobalTxnScope, 0)
+	currentTS, err := sctx.GetSessionVars().StmtCtx.GetStaleTSO()
+	if currentTS == 0 || err != nil {
+		currentTS, err = sctx.GetStore().GetOracle().GetStaleTimestamp(ctx, oracle.GlobalTxnScope, 0)
+	}
 	// If we fail to calculate currentTS from local time, fallback to get a timestamp from PD
 	if err != nil {
 		metrics.ValidateReadTSFromPDCount.Inc()

sessionctx/stmtctx/stmtctx.go

@@ -378,6 +378,12 @@ type StatementContext struct {
 	TableStats map[int64]interface{}
 	// useChunkAlloc indicates whether statement use chunk alloc
 	useChunkAlloc bool
+	// StaleTSOProvider is used to provide stale timestamp oracle for read-only transactions.
+	StaleTSOProvider struct {
+		sync.Mutex
+		value *uint64
+		eval  func() (uint64, error)
+	}
 }
 
 // StmtHints are SessionVars related sql hints.
@@ -1127,6 +1133,32 @@ func (sc *StatementContext) DetachMemDiskTracker() {
 	}
 }
 
+// SetStaleTSOProvider sets the stale TSO provider.
+func (sc *StatementContext) SetStaleTSOProvider(eval func() (uint64, error)) {
+	sc.StaleTSOProvider.Lock()
+	defer sc.StaleTSOProvider.Unlock()
+	sc.StaleTSOProvider.value = nil
+	sc.StaleTSOProvider.eval = eval
+}
+
+// GetStaleTSO returns the TSO for stale-read usage which calculate from PD's last response.
+func (sc *StatementContext) GetStaleTSO() (uint64, error) {
+	sc.StaleTSOProvider.Lock()
+	defer sc.StaleTSOProvider.Unlock()
+	if sc.StaleTSOProvider.value != nil {
+		return *sc.StaleTSOProvider.value, nil
+	}
+	if sc.StaleTSOProvider.eval == nil {
+		return 0, nil
+	}
+	tso, err := sc.StaleTSOProvider.eval()
+	if err != nil {
+		return 0, err
+	}
+	sc.StaleTSOProvider.value = &tso
+	return tso, nil
+}
+
 // CopTasksDetails collects some useful information of cop-tasks during execution.
 type CopTasksDetails struct {
 	NumCopTasks int

sessiontxn/staleread/BUILD.bazel

@@ -29,6 +29,7 @@ go_library(
         "//types",
         "//util/dbterror",
         "@com_github_pingcap_errors//:errors",
+        "@com_github_pingcap_failpoint//:failpoint",
         "@com_github_tikv_client_go_v2//oracle",
     ],
 )

sessiontxn/staleread/processor.go

@@ -280,7 +280,7 @@ func parseAndValidateAsOf(ctx context.Context, sctx sessionctx.Context, asOf *as
 		return 0, nil
 	}
 
-	ts, err := CalculateAsOfTsExpr(sctx, asOf.TsExpr)
+	ts, err := CalculateAsOfTsExpr(ctx, sctx, asOf.TsExpr)
 	if err != nil {
 		return 0, err
 	}

sessiontxn/staleread/util.go

@@ -18,6 +18,7 @@ import (
 	"context"
 	"time"
 
+	"github.com/pingcap/failpoint"
 	"github.com/pingcap/tidb/expression"
 	"github.com/pingcap/tidb/parser/ast"
 	"github.com/pingcap/tidb/parser/mysql"
@@ -29,7 +30,16 @@ import (
 )
 
 // CalculateAsOfTsExpr calculates the TsExpr of AsOfClause to get a StartTS.
-func CalculateAsOfTsExpr(sctx sessionctx.Context, tsExpr ast.ExprNode) (uint64, error) {
+func CalculateAsOfTsExpr(ctx context.Context, sctx sessionctx.Context, tsExpr ast.ExprNode) (uint64, error) {
+	sctx.GetSessionVars().StmtCtx.SetStaleTSOProvider(func() (uint64, error) {
+		failpoint.Inject("mockStaleReadTSO", func(val failpoint.Value) (uint64, error) {
+			return uint64(val.(int)), nil
+		})
+		// this function accepts a context, but we don't need it when there is a valid cached ts.
+		// in most cases, the stale read ts can be calculated from `cached ts + time since cache - staleness`,
+		// this can be more accurate than `time.Now() - staleness`, because TiDB's local time can drift.
+		return sctx.GetStore().GetOracle().GetStaleTimestamp(ctx, oracle.GlobalTxnScope, 0)
+	})
 	tsVal, err := expression.EvalAstExpr(sctx, tsExpr)
 	if err != nil {
 		return 0, err