parser: add AsOfClause for START TRANSACTION READ ONLY statement (#1215)

* Add AsOfClause for START TRANSACTION READ ONLY statement

Signed-off-by: JmPotato <[email protected]>

ast/functions.go

@@ -176,7 +176,12 @@ const (
 	Year             = "year"
 	YearWeek         = "yearweek"
 	LastDay          = "last_day"
-	TiDBParseTso     = "tidb_parse_tso"
+	// TSO functions
+	// TiDBBoundedStaleness is used to determine the TS for a read only request with the given bounded staleness.
+	// It will be used in the Stale Read feature.
+	// For more info, please see AsOfClause.
+	TiDBBoundedStaleness = "tidb_bounded_staleness"
+	TiDBParseTso         = "tidb_parse_tso"
 
 	// string functions
 	ASCII           = "ascii"

ast/misc.go

@@ -391,6 +391,9 @@ type BeginStmt struct {
 	ReadOnly              bool
 	Bound                 *TimestampBound
 	CausalConsistencyOnly bool
+	// AS OF is used to read the data at a specific point of time.
+	// Should only be used when ReadOnly is true.
+	AsOf *AsOfClause
 }
 
 // Restore implements Node interface.
@@ -415,6 +418,9 @@ func (n *BeginStmt) Restore(ctx *format.RestoreCtx) error {
 					ctx.WriteKeyWord(" WITH TIMESTAMP BOUND MIN READ TIMESTAMP ")
 					return n.Bound.Timestamp.Restore(ctx)
 				}
+			} else if n.AsOf != nil {
+				ctx.WriteKeyWord(" ")
+				return n.AsOf.Restore(ctx)
 			}
 		} else if n.CausalConsistencyOnly {
 			ctx.WriteKeyWord("START TRANSACTION WITH CAUSAL CONSISTENCY ONLY")