*: add as of clause for SET and SELECT

ast/dml.go

@@ -38,6 +38,7 @@ var (
 	_ Node = &FieldList{}
 	_ Node = &GroupByClause{}
 	_ Node = &HavingClause{}
+	_ Node = &AsOfClause{}
 	_ Node = &Join{}
 	_ Node = &Limit{}
 	_ Node = &OnCondition{}
@@ -262,6 +263,8 @@ type TableName struct {
 	IndexHints     []*IndexHint
 	PartitionNames []model.CIStr
 	TableSample    *TableSample
+	// AS OF is used to see the data as it was at a specific point in time.
+	AsOf *AsOfClause
 }
 
 // Restore implements Node interface.
@@ -306,6 +309,12 @@ func (n *TableName) Restore(ctx *format.RestoreCtx) error {
 	if err := n.restoreIndexHints(ctx); err != nil {
 		return err
 	}
+	if n.AsOf != nil {
+		ctx.WritePlain(" ")
+		if err := n.AsOf.Restore(ctx); err != nil {
+			return errors.Annotate(err, "An error occurred while splicing TableName.Asof")
+		}
+	}
 	if n.TableSample != nil {
 		ctx.WritePlain(" ")
 		if err := n.TableSample.Restore(ctx); err != nil {
@@ -502,6 +511,14 @@ func (n *TableSource) Restore(ctx *format.RestoreCtx) error {
 			ctx.WriteKeyWord(" AS ")
 			ctx.WriteName(asName)
 		}
+
+		if tn.AsOf != nil {
+			ctx.WritePlain(" ")
+			if err := tn.AsOf.Restore(ctx); err != nil {
+				return errors.Annotate(err, "An error occurred while restore TableSource.AsOf")
+			}
+
+		}
 		if err := tn.restoreIndexHints(ctx); err != nil {
 			return errors.Annotate(err, "An error occurred while restore TableSource.Source.(*TableName).IndexHints")
 		}
@@ -3267,11 +3284,7 @@ func (m FulltextSearchModifier) WithQueryExpansion() bool {
 	return m&FulltextSearchModifierWithQueryExpansion == FulltextSearchModifierWithQueryExpansion
 }
 
-type TimestampBound struct {
-	Mode      TimestampBoundMode
-	Timestamp ExprNode
-}
-
+// TODO: remove the TimestampBoundMode.
 type TimestampBoundMode int
 
 const (
@@ -3281,3 +3294,46 @@ const (
 	TimestampBoundReadTimestamp
 	TimestampBoundMinReadTimestamp
 )
+
+type TimestampReadModes int
+
+const (
+	TimestampReadStrong TimestampReadModes = iota
+	TimestampReadBoundTimestamp
+	TimestampReadExactTimestamp
+)
+
+type TimestampBound struct {
+	Mode      TimestampBoundMode
+	Timestamp ExprNode
+}
+
+type AsOfClause struct {
+	node
+	Mode   TimestampReadModes
+	TsExpr ExprNode
+}
+
+// Restore implements Node interface.
+func (n *AsOfClause) Restore(ctx *format.RestoreCtx) error {
+	ctx.WriteKeyWord("AS OF TIMESTAMP ")
+	if err := n.TsExpr.Restore(ctx); err != nil {
+		return errors.Annotate(err, "An error occurred while restore AsOfClause.Expr")
+	}
+	return nil
+}
+
+// Accept implements Node Accept interface.
+func (n *AsOfClause) Accept(v Visitor) (Node, bool) {
+	newNode, skipChildren := v.Enter(n)
+	if skipChildren {
+		return v.Leave(newNode)
+	}
+	n = newNode.(*AsOfClause)
+	node, ok := n.TsExpr.Accept(v)
+	if !ok {
+		return n, false
+	}
+	n.TsExpr = node.(ExprNode)
+	return v.Leave(n)
+}

lexer.go

@@ -130,6 +130,21 @@ func (s *Scanner) AppendError(err error) {
 	s.errs = append(s.errs, err)
 }
 
+func (s *Scanner) getNextToken() int {
+	r := s.r
+	tok, pos, lit := s.scan()
+	if tok == identifier {
+		tok = handleIdent(&yySymType{})
+	}
+	if tok == identifier {
+		if tok1 := s.isTokenIdentifier(lit, pos.Offset); tok1 != 0 {
+			tok = tok1
+		}
+	}
+	s.r = r
+	return tok
+}
+
 // Lex returns a token and store the token value in v.
 // Scanner satisfies yyLexer interface.
 // 0 and invalid are special token id this function would return:
@@ -165,6 +180,13 @@ func (s *Scanner) Lex(v *yySymType) int {
 	if tok == not && s.sqlMode.HasHighNotPrecedenceMode() {
 		return not2
 	}
+	if tok == as && s.getNextToken() == of {
+		_, pos, lit = s.scan()
+		v.ident = fmt.Sprintf("%s %s", v.ident, lit)
+		v.offset = pos.Offset
+		s.lastKeyword = asof
+		return asof
+	}
 
 	switch tok {
 	case intLit:

misc.go

@@ -491,6 +491,7 @@ var tokenMap = map[string]int{
 	"NULLS":                    nulls,
 	"NUMERIC":                  numericType,
 	"NVARCHAR":                 nvarcharType,
+	"OF":                       of,
 	"OFF":                      off,
 	"OFFSET":                   offset,
 	"ON_DUPLICATE":             onDuplicate,