// commonColumns returns the names of columns common on the
// right and left sides, for use by NATURAL JOIN.
func commonColumns(left, right *dataSourceInfo) parser.NameList {
	var res parser.NameList
	for _, cLeft := range left.sourceColumns {
		if cLeft.hidden {
			continue
		}
		for _, cRight := range right.sourceColumns {
			if cRight.hidden {
				continue
			}

			if parser.ReNormalizeName(cLeft.Name) == parser.ReNormalizeName(cRight.Name) {
				res = append(res, parser.Name(cLeft.Name))
			}
		}
	}
	return res
}

// upsertExprsAndIndex returns the upsert conflict index and the (possibly
// synthetic) SET expressions used when a row conflicts.
func upsertExprsAndIndex(
	tableDesc *sqlbase.TableDescriptor,
	onConflict parser.OnConflict,
	insertCols []sqlbase.ColumnDescriptor,
) (parser.UpdateExprs, *sqlbase.IndexDescriptor, error) {
	if onConflict.IsUpsertAlias() {
		// The UPSERT syntactic sugar is the same as the longhand specifying the
		// primary index as the conflict index and SET expressions for the columns
		// in insertCols minus any columns in the conflict index. Example:
		// `UPSERT INTO abc VALUES (1, 2, 3)` is syntactic sugar for
		// `INSERT INTO abc VALUES (1, 2, 3) ON CONFLICT a DO UPDATE SET b = 2, c = 3`.
		conflictIndex := &tableDesc.PrimaryIndex
		indexColSet := make(map[sqlbase.ColumnID]struct{}, len(conflictIndex.ColumnIDs))
		for _, colID := range conflictIndex.ColumnIDs {
			indexColSet[colID] = struct{}{}
		}
		updateExprs := make(parser.UpdateExprs, 0, len(insertCols))
		for _, c := range insertCols {
			if _, ok := indexColSet[c.ID]; !ok {
				names := parser.UnresolvedNames{
					parser.UnresolvedName{parser.Name(c.Name)},
				}
				expr := &parser.ColumnItem{
					TableName:  upsertExcludedTable,
					ColumnName: parser.Name(c.Name),
				}
				updateExprs = append(updateExprs, &parser.UpdateExpr{Names: names, Expr: expr})
			}
		}
		return updateExprs, conflictIndex, nil
	}

	indexMatch := func(index sqlbase.IndexDescriptor) bool {
		if !index.Unique {
			return false
		}
		if len(index.ColumnNames) != len(onConflict.Columns) {
			return false
		}
		for i, colName := range index.ColumnNames {
			if parser.ReNormalizeName(colName) != onConflict.Columns[i].Normalize() {
				return false
			}
		}
		return true
	}

	if indexMatch(tableDesc.PrimaryIndex) {
		return onConflict.Exprs, &tableDesc.PrimaryIndex, nil
	}
	for _, index := range tableDesc.Indexes {
		if indexMatch(index) {
			return onConflict.Exprs, &index, nil
		}
	}
	return nil, nil, fmt.Errorf("there is no unique or exclusion constraint matching the ON CONFLICT specification")
}

// MakeNameMetadataKey returns the key for the name. Pass name == "" in order
// to generate the prefix key to use to scan over all of the names for the
// specified parentID.
func MakeNameMetadataKey(parentID ID, name string) roachpb.Key {
	normName := parser.ReNormalizeName(name)
	k := keys.MakeTablePrefix(uint32(NamespaceTable.ID))
	k = encoding.EncodeUvarintAscending(k, uint64(NamespaceTable.PrimaryIndex.ID))
	k = encoding.EncodeUvarintAscending(k, uint64(parentID))
	if name != "" {
		k = encoding.EncodeBytesAscending(k, []byte(normName))
		k = keys.MakeFamilyKey(k, uint32(NamespaceTable.Columns[2].ID))
	}
	return k
}

// findUnaliasedColumn looks up the column specified by a VarName, not
// taking column renames into account (but table renames will be taken
// into account). That is, given a table "blah" with single column "y",
// findUnaliasedColumn("y") returns a valid index even in the context
// of:
//     SELECT * FROM blah as foo(x)
// If the VarName specifies a table name, only columns that have that
// name as their source alias are considered. If the VarName does not
// specify a table name, all columns in the data source are
// considered.  If no column is found, invalidColIdx is returned with
// no error.
func (p *planDataSource) findUnaliasedColumn(c *parser.ColumnItem) (colIdx int, err error) {
	colName := c.ColumnName.Normalize()
	tableName := c.TableName.NormalizedTableName()

	if tableName.Table() != "" {
		tn, err := p.info.checkDatabaseName(tableName)
		if err != nil {
			return invalidColIdx, nil
		}
		tableName = tn
	}

	colIdx = invalidColIdx
	planColumns := p.plan.Columns()

	selCol := func(colIdx int, idx int) (int, error) {
		col := planColumns[idx]
		if parser.ReNormalizeName(col.Name) == colName {
			if colIdx != invalidColIdx {
				return invalidColIdx, fmt.Errorf("column reference %q is ambiguous", c)
			}
			colIdx = idx
		}
		return colIdx, nil
	}

	if tableName.Table() == "" {
		for idx := 0; idx < len(p.info.sourceColumns); idx++ {
			colIdx, err = selCol(colIdx, idx)
			if err != nil {
				return colIdx, err
			}
		}
	} else {
		colRange, ok := p.info.sourceAliases[tableName]
		if !ok {
			// A table name is specified, but there is no column with this
			// table name.
			return invalidColIdx, nil
		}
		for _, idx := range colRange {
			colIdx, err = selCol(colIdx, idx)
			if err != nil {
				return colIdx, err
			}
		}
	}

	return colIdx, nil
}

func makeColIDtoRowIndex(
	row planNode, desc *sqlbase.TableDescriptor,
) (map[sqlbase.ColumnID]int, error) {
	columns := row.Columns()
	colIDtoRowIndex := make(map[sqlbase.ColumnID]int, len(columns))
	for i, column := range columns {
		s, idx, err := desc.FindColumnByNormalizedName(parser.ReNormalizeName(column.Name))
		if err != nil {
			return nil, err
		}
		switch s {
		case sqlbase.DescriptorActive:
			colIDtoRowIndex[desc.Columns[idx].ID] = i
		case sqlbase.DescriptorIncomplete:
			colIDtoRowIndex[desc.Mutations[idx].GetColumn().ID] = i
		default:
			panic("unreachable")
		}
	}
	return colIDtoRowIndex, nil
}

// getTableLease implements the SchemaAccessor interface.
func (p *planner) getTableLease(tn *parser.TableName) (*sqlbase.TableDescriptor, error) {
	if log.V(2) {
		log.Infof(p.ctx(), "planner acquiring lease on table '%s'", tn)
	}

	isSystemDB := tn.Database() == sqlbase.SystemDB.Name
	isVirtualDB := p.session.virtualSchemas.isVirtualDatabase(tn.Database())
	if isSystemDB || isVirtualDB || testDisableTableLeases {
		// We don't go through the normal lease mechanism for:
		// - system tables. The system.lease and system.descriptor table, in
		//   particular, are problematic because they are used for acquiring
		//   leases itself, creating a chicken&egg problem.
		// - virtual tables. These tables' descriptors are not persisted,
		//   so they cannot be leased. Instead, we simply return the static
		//   descriptor and rely on the immutability privileges set on the
		//   descriptors to cause upper layers to reject mutations statements.
		tbl, err := p.mustGetTableDesc(tn)
		if err != nil {
			return nil, err
		}
		if err := filterTableState(tbl); err != nil {
			return nil, err
		}
		return tbl, nil
	}

	dbID, err := p.getDatabaseID(tn.Database())
	if err != nil {
		return nil, err
	}

	// First, look to see if we already have a lease for this table.
	// This ensures that, once a SQL transaction resolved name N to id X, it will
	// continue to use N to refer to X even if N is renamed during the
	// transaction.
	var lease *LeaseState
	for _, l := range p.leases {
		if parser.ReNormalizeName(l.Name) == tn.TableName.Normalize() &&
			l.ParentID == dbID {
			lease = l
			if log.V(2) {
				log.Infof(p.ctx(), "found lease in planner cache for table '%s'", tn)
			}
			break
		}
	}

	// If we didn't find a lease or the lease is about to expire, acquire one.
	if lease == nil || p.removeLeaseIfExpiring(lease) {
		var err error
		lease, err = p.leaseMgr.AcquireByName(p.txn, dbID, tn.Table())
		if err != nil {
			if err == sqlbase.ErrDescriptorNotFound {
				// Transform the descriptor error into an error that references the
				// table's name.
				return nil, sqlbase.NewUndefinedTableError(tn.String())
			}
			return nil, err
		}
		p.leases = append(p.leases, lease)
		if log.V(2) {
			log.Infof(p.ctx(), "added lease on table '%s' to planner cache", tn)
		}
		// If the lease we just acquired expires before the txn's deadline, reduce
		// the deadline.
		p.txn.UpdateDeadlineMaybe(hlc.Timestamp{WallTime: lease.Expiration().UnixNano()})
	}
	return &lease.TableDescriptor, nil
}

// orderBy constructs a sortNode based on the ORDER BY clause.
//
// In the general case (SELECT/UNION/VALUES), we can sort by a column index or a
// column name.
//
// However, for a SELECT, we can also sort by the pre-alias column name (SELECT
// a AS b ORDER BY b) as well as expressions (SELECT a, b, ORDER BY a+b). In
// this case, construction of the sortNode might adjust the number of render
// targets in the selectNode if any ordering expressions are specified.
//
// TODO(dan): SQL also allows sorting a VALUES or UNION by an expression.
// Support this. It will reduce some of the special casing below, but requires a
// generalization of how to add derived columns to a SelectStatement.
func (p *planner) orderBy(orderBy parser.OrderBy, n planNode) (*sortNode, error) {
	if orderBy == nil {
		return nil, nil
	}

	// Multiple tests below use selectNode as a special case.
	// So factor the cast.
	s, _ := n.(*selectNode)

	// We grab a copy of columns here because we might add new render targets
	// below. This is the set of columns requested by the query.
	columns := n.Columns()
	numOriginalCols := len(columns)
	if s != nil {
		numOriginalCols = s.numOriginalCols
	}
	var ordering sqlbase.ColumnOrdering

	for _, o := range orderBy {
		direction := encoding.Ascending
		if o.Direction == parser.Descending {
			direction = encoding.Descending
		}

		index := -1

		// Unwrap parenthesized expressions like "((a))" to "a".
		expr := parser.StripParens(o.Expr)

		// The logical data source for ORDER BY is the list of render
		// expressions for a SELECT, as specified in the input SQL text
		// (or an entire UNION or VALUES clause).  Alas, SQL has some
		// historical baggage from SQL92 and there are some special cases:
		//
		// SQL92 rules:
		//
		// 1) if the expression is the aliased (AS) name of a render
		//    expression in a SELECT clause, then use that
		//    render as sort key.
		//    e.g. SELECT a AS b, b AS c ORDER BY b
		//    this sorts on the first render.
		//
		// 2) column ordinals. If a simple integer literal is used,
		//    optionally enclosed within parentheses but *not subject to
		//    any arithmetic*, then this refers to one of the columns of
		//    the data source. Then use the render expression at that
		//    ordinal position as sort key.
		//
		// SQL99 rules:
		//
		// 3) otherwise, if the expression is already in the render list,
		//    then use that render as sort key.
		//    e.g. SELECT b AS c ORDER BY b
		//    this sorts on the first render.
		//    (this is an optimization)
		//
		// 4) if the sort key is not dependent on the data source (no
		//    IndexedVar) then simply do not sort. (this is an optimization)
		//
		// 5) otherwise, add a new render with the ORDER BY expression
		//    and use that as sort key.
		//    e.g. SELECT a FROM t ORDER by b
		//    e.g. SELECT a, b FROM t ORDER by a+b

		// First, deal with render aliases.
		if vBase, ok := expr.(parser.VarName); index == -1 && ok {
			v, err := vBase.NormalizeVarName()
			if err != nil {
				return nil, err
			}

			if c, ok := v.(*parser.ColumnItem); ok && c.TableName.Table() == "" {
				// Look for an output column that matches the name. This
				// handles cases like:
				//
				//   SELECT a AS b FROM t ORDER BY b
				target := c.ColumnName.Normalize()
				for j, col := range columns {
					if parser.ReNormalizeName(col.Name) == target {
						if index != -1 {
							// There is more than one render alias that matches the ORDER BY
							// clause. Here, SQL92 is specific as to what should be done:
							// if the underlying expression is known (we're on a selectNode)
							// and it is equivalent, then just accept that and ignore the ambiguity.
							// This plays nice with `SELECT b, * FROM t ORDER BY b`. Otherwise,
							// reject with an ambituity error.
							if s == nil || !s.equivalentRenders(j, index) {
//.........这里部分代码省略.........

func (p *planner) makeUpsertHelper(
	tn *parser.TableName,
	tableDesc *sqlbase.TableDescriptor,
	insertCols []sqlbase.ColumnDescriptor,
	updateCols []sqlbase.ColumnDescriptor,
	updateExprs parser.UpdateExprs,
	upsertConflictIndex *sqlbase.IndexDescriptor,
) (*upsertHelper, error) {
	defaultExprs, err := makeDefaultExprs(updateCols, &p.parser, &p.evalCtx)
	if err != nil {
		return nil, err
	}

	untupledExprs := make(parser.Exprs, 0, len(updateExprs))
	i := 0
	for _, updateExpr := range updateExprs {
		if updateExpr.Tuple {
			if t, ok := updateExpr.Expr.(*parser.Tuple); ok {
				for _, e := range t.Exprs {
					typ := updateCols[i].Type.ToDatumType()
					e := fillDefault(e, typ, i, defaultExprs)
					untupledExprs = append(untupledExprs, e)
					i++
				}
			}
		} else {
			typ := updateCols[i].Type.ToDatumType()
			e := fillDefault(updateExpr.Expr, typ, i, defaultExprs)
			untupledExprs = append(untupledExprs, e)
			i++
		}
	}

	sourceInfo := newSourceInfoForSingleTable(*tn, makeResultColumns(tableDesc.Columns))
	excludedSourceInfo := newSourceInfoForSingleTable(upsertExcludedTable, makeResultColumns(insertCols))

	helper := &upsertHelper{
		p:                  p,
		sourceInfo:         sourceInfo,
		excludedSourceInfo: excludedSourceInfo,
	}

	var evalExprs []parser.TypedExpr
	ivarHelper := parser.MakeIndexedVarHelper(helper, len(sourceInfo.sourceColumns)+len(excludedSourceInfo.sourceColumns))
	sources := multiSourceInfo{sourceInfo, excludedSourceInfo}
	for _, expr := range untupledExprs {
		normExpr, err := p.analyzeExpr(expr, sources, ivarHelper, parser.NoTypePreference, false, "")
		if err != nil {
			return nil, err
		}
		evalExprs = append(evalExprs, normExpr)
	}
	helper.evalExprs = evalExprs

	helper.allExprsIdentity = true
	for i, expr := range evalExprs {
		// analyzeExpr above has normalized all direct column names to ColumnItems.
		c, ok := expr.(*parser.ColumnItem)
		if !ok {
			helper.allExprsIdentity = false
			break
		}
		if len(c.Selector) > 0 ||
			!c.TableName.TableName.Equal(upsertExcludedTable.TableName) ||
			c.ColumnName.Normalize() != parser.ReNormalizeName(updateCols[i].Name) {
			helper.allExprsIdentity = false
			break
		}
	}

	return helper, nil
}

// Set sets session variables.
// Privileges: None.
//   Notes: postgres/mysql do not require privileges for session variables (some exceptions).
func (p *planner) Set(n *parser.Set) (planNode, error) {
	if n.Name == nil {
		// A client has sent the reserved internal syntax SET ROW ...
		// Reject it.
		return nil, errors.New("invalid statement: SET ROW")
	}

	// By using VarName.String() here any variables that are keywords will
	// be double quoted.
	name := strings.ToUpper(n.Name.String())
	typedValues := make([]parser.TypedExpr, len(n.Values))
	for i, expr := range n.Values {
		typedValue, err := parser.TypeCheck(expr, nil, parser.TypeString)
		if err != nil {
			return nil, err
		}
		typedValues[i] = typedValue
	}
	switch name {
	case `DATABASE`:
		dbName, err := p.getStringVal(name, typedValues)
		if err != nil {
			return nil, err
		}
		if len(dbName) != 0 {
			// Verify database descriptor exists.
			if _, err := p.mustGetDatabaseDesc(dbName); err != nil {
				return nil, err
			}
		}
		p.session.Database = dbName
		p.evalCtx.Database = dbName

	case `SYNTAX`:
		s, err := p.getStringVal(name, typedValues)
		if err != nil {
			return nil, err
		}
		switch parser.Name(s).Normalize() {
		case parser.ReNormalizeName(parser.Modern.String()):
			p.session.Syntax = int32(parser.Modern)
		case parser.ReNormalizeName(parser.Traditional.String()):
			p.session.Syntax = int32(parser.Traditional)
		default:
			return nil, fmt.Errorf("%s: \"%s\" is not in (%q, %q)", name, s, parser.Modern, parser.Traditional)
		}

	case `EXTRA_FLOAT_DIGITS`:
		// These settings are sent by the JDBC driver but we silently ignore them.

	case `APPLICATION_NAME`:
		// These settings are sent by the clients to improve query logging on the server,
		// but we silently ignore them.

	case `DEFAULT_TRANSACTION_ISOLATION`:
		// It's unfortunate that clients want us to support both SET
		// SESSION CHARACTERISTICS AS TRANSACTION ..., which takes the
		// isolation level as keywords/identifiers (e.g. JDBC), and SET
		// DEFAULT_TRANSACTION_ISOLATION TO '...', which takes an
		// expression (e.g. psycopg2). But that's how it is.  Just ensure
		// this code keeps in sync with SetDefaultIsolation() below.
		s, err := p.getStringVal(name, typedValues)
		if err != nil {
			return nil, err
		}
		switch strings.ToUpper(s) {
		case `READ UNCOMMITTED`, `READ COMMITTED`, `SNAPSHOT`:
			p.session.DefaultIsolationLevel = enginepb.SNAPSHOT
		case `REPEATABLE READ`, `SERIALIZABLE`:
			p.session.DefaultIsolationLevel = enginepb.SERIALIZABLE
		default:
			return nil, fmt.Errorf("%s: unknown isolation level: %q", name, s)
		}

	case `DIST_SQL`:
		s, err := p.getStringVal(name, typedValues)
		if err != nil {
			return nil, err
		}
		switch parser.Name(s).Normalize() {
		case parser.ReNormalizeName("sync"):
			p.session.DistSQLMode = distSQLSync
		case parser.ReNormalizeName("async"):
			p.session.DistSQLMode = distSQLAsync
		default:
			return nil, fmt.Errorf("%s: \"%s\" not supported", name, s)
		}

	default:
		return nil, fmt.Errorf("unknown variable: %q", name)
	}
	return &emptyNode{}, nil
}

// findColumn looks up the column specified by a ColumnItem. The
// function returns the index of the source in the multiSourceInfo
// array and the column index for the column array of that
// source. Returns invalid indices and an error if the source is not
// found or the name is ambiguous.
func (sources multiSourceInfo) findColumn(c *parser.ColumnItem) (srcIdx int, colIdx int, err error) {
	if len(c.Selector) > 0 {
		return invalidSrcIdx, invalidColIdx, util.UnimplementedWithIssueErrorf(8318, "compound types not supported yet: %q", c)
	}

	colName := c.ColumnName.Normalize()
	var tableName parser.TableName
	if c.TableName.Table() != "" {
		tableName = c.TableName.NormalizedTableName()

		tn, err := sources.checkDatabaseName(tableName)
		if err != nil {
			return invalidSrcIdx, invalidColIdx, err
		}
		tableName = tn

		// Propagate the discovered database name back to the original VarName.
		// (to clarify the output of e.g. EXPLAIN)
		c.TableName.DatabaseName = tableName.DatabaseName
	}

	colIdx = invalidColIdx
	for iSrc, src := range sources {
		findColHelper := func(src *dataSourceInfo, iSrc, srcIdx, colIdx int, idx int) (int, int, error) {
			col := src.sourceColumns[idx]
			if parser.ReNormalizeName(col.Name) == colName {
				if colIdx != invalidColIdx {
					return invalidSrcIdx, invalidColIdx, fmt.Errorf("column reference %q is ambiguous", c)
				}
				srcIdx = iSrc
				colIdx = idx
			}
			return srcIdx, colIdx, nil
		}

		if tableName.Table() == "" {
			for idx := 0; idx < len(src.sourceColumns); idx++ {
				srcIdx, colIdx, err = findColHelper(src, iSrc, srcIdx, colIdx, idx)
				if err != nil {
					return srcIdx, colIdx, err
				}
			}
		} else {
			colRange, ok := src.sourceAliases[tableName]
			if !ok {
				// The data source "src" has no column for table tableName.
				// Try again with the net one.
				continue
			}
			for _, idx := range colRange {
				srcIdx, colIdx, err = findColHelper(src, iSrc, srcIdx, colIdx, idx)
				if err != nil {
					return srcIdx, colIdx, err
				}
			}
		}
	}

	if colIdx == invalidColIdx {
		return invalidSrcIdx, invalidColIdx, fmt.Errorf("column name %q not found", c)
	}

	return srcIdx, colIdx, nil
}

func nameMatchesLease(lease *LeaseState, dbID sqlbase.ID, tableName string) bool {
	return lease.ParentID == dbID &&
		parser.ReNormalizeName(lease.Name) == parser.ReNormalizeName(tableName)
}

func makeTableNameCacheKey(dbID sqlbase.ID, tableName string) tableNameCacheKey {
	return tableNameCacheKey{dbID, parser.ReNormalizeName(tableName)}
}

// orderBy constructs a sortNode based on the ORDER BY clause.
//
// In the general case (SELECT/UNION/VALUES), we can sort by a column index or a
// column name.
//
// However, for a SELECT, we can also sort by the pre-alias column name (SELECT
// a AS b ORDER BY b) as well as expressions (SELECT a, b, ORDER BY a+b). In
// this case, construction of the sortNode might adjust the number of render
// targets in the selectNode if any ordering expressions are specified.
//
// TODO(dan): SQL also allows sorting a VALUES or UNION by an expression.
// Support this. It will reduce some of the special casing below, but requires a
// generalization of how to add derived columns to a SelectStatement.
func (p *planner) orderBy(orderBy parser.OrderBy, n planNode) (*sortNode, error) {
	if orderBy == nil {
		return nil, nil
	}

	// Multiple tests below use selectNode as a special case.
	// So factor the cast.
	s, _ := n.(*selectNode)

	// We grab a copy of columns here because we might add new render targets
	// below. This is the set of columns requested by the query.
	columns := n.Columns()
	numOriginalCols := len(columns)
	if s != nil {
		numOriginalCols = s.numOriginalCols
	}
	var ordering sqlbase.ColumnOrdering

	for _, o := range orderBy {
		direction := encoding.Ascending
		if o.Direction == parser.Descending {
			direction = encoding.Descending
		}

		index := -1

		// Unwrap parenthesized expressions like "((a))" to "a".
		expr := parser.StripParens(o.Expr)

		// The logical data source for ORDER BY is the list of render
		// expressions for a SELECT, as specified in the input SQL text
		// (or an entire UNION or VALUES clause).  Alas, SQL has some
		// historical baggage and there are some special cases:
		//
		// 1) column ordinals. If a simple integer literal is used,
		//    optionally enclosed within parentheses but *not subject to
		//    any arithmetic*, then this refers to one of the columns of
		//    the data source. Then use the render expression at that
		//    ordinal position as sort key.
		//
		// 2) if the expression is the aliased (AS) name of a render
		//    expression in a SELECT clause, then use that
		//    render as sort key.
		//    e.g. SELECT a AS b, b AS c ORDER BY b
		//    this sorts on the first render.
		//
		// 3) otherwise, if the expression is already in the render list,
		//    then use that render as sort key.
		//    e.g. SELECT b AS c ORDER BY b
		//    this sorts on the first render.
		//    (this is an optimization)
		//
		// 4) if the sort key is not dependent on the data source (no
		//    IndexedVar) then simply do not sort. (this is an optimization)
		//
		// 5) otherwise, add a new render with the ORDER BY expression
		//    and use that as sort key.
		//    e.g. SELECT a FROM t ORDER by b
		//    e.g. SELECT a, b FROM t ORDER by a+b
		//
		// So first, deal with column ordinals.
		col, err := p.colIndex(numOriginalCols, expr, "ORDER BY")
		if err != nil {
			return nil, err
		}
		if col != -1 {
			index = col
		}

		// Now, deal with render aliases.
		if vBase, ok := expr.(parser.VarName); index == -1 && ok {
			v, err := vBase.NormalizeVarName()
			if err != nil {
				return nil, err
			}

			if c, ok := v.(*parser.ColumnItem); ok && c.TableName.Table() == "" {
				// Look for an output column that matches the name. This
				// handles cases like:
				//
				//   SELECT a AS b FROM t ORDER BY b
				target := c.ColumnName.Normalize()
				for j, col := range columns {
					if parser.ReNormalizeName(col.Name) == target {
						index = j
						break
					}
//.........这里部分代码省略.........

// orderBy constructs a sortNode based on the ORDER BY clause.
//
// In the general case (SELECT/UNION/VALUES), we can sort by a column index or a
// column name.
//
// However, for a SELECT, we can also sort by the pre-alias column name (SELECT
// a AS b ORDER BY b) as well as expressions (SELECT a, b, ORDER BY a+b). In
// this case, construction of the sortNode might adjust the number of render
// targets in the selectNode if any ordering expressions are specified.
//
// TODO(dan): SQL also allows sorting a VALUES or UNION by an expression.
// Support this. It will reduce some of the special casing below, but requires a
// generalization of how to add derived columns to a SelectStatement.
func (p *planner) orderBy(orderBy parser.OrderBy, n planNode) (*sortNode, error) {
	if orderBy == nil {
		return nil, nil
	}

	// We grab a copy of columns here because we might add new render targets
	// below. This is the set of columns requested by the query.
	columns := n.Columns()
	numOriginalCols := len(columns)
	if s, ok := n.(*selectNode); ok {
		numOriginalCols = s.numOriginalCols
	}
	var ordering sqlbase.ColumnOrdering

	for _, o := range orderBy {
		index := -1

		// Unwrap parenthesized expressions like "((a))" to "a".
		expr := parser.StripParens(o.Expr)

		if vBase, ok := expr.(parser.VarName); ok {
			v, err := vBase.NormalizeVarName()
			if err != nil {
				return nil, err
			}

			var c *parser.ColumnItem
			switch t := v.(type) {
			case *parser.ColumnItem:
				c = t
			default:
				return nil, fmt.Errorf("invalid syntax for ORDER BY: %s", v)
			}

			if c.TableName.Table() == "" {
				// Look for an output column that matches the name. This
				// handles cases like:
				//
				//   SELECT a AS b FROM t ORDER BY b
				target := c.ColumnName.Normalize()
				for j, col := range columns {
					if parser.ReNormalizeName(col.Name) == target {
						index = j
						break
					}
				}
			}

			if s, ok := n.(*selectNode); ok && index == -1 {
				// No output column matched the  name, so look for an existing
				// render target that matches the column name. This handles cases like:
				//
				//   SELECT a AS b FROM t ORDER BY a
				colIdx, err := s.source.findUnaliasedColumn(c)
				if err != nil {
					return nil, err
				}
				if colIdx != invalidColIdx {
					for j, r := range s.render {
						if ivar, ok := r.(*parser.IndexedVar); ok {
							s.ivarHelper.AssertSameContainer(ivar)
							if ivar.Idx == colIdx {
								index = j
								break
							}
						}
					}
				}
			}
		}

		if index == -1 {
			// The order by expression matched neither an output column nor an
			// existing render target.
			if col, err := colIndex(numOriginalCols, expr); err != nil {
				return nil, err
			} else if col >= 0 {
				index = col
			} else if s, ok := n.(*selectNode); ok {
				// TODO(dan): Once we support VALUES (1), (2) ORDER BY 3*4, this type
				// check goes away.

				// Add a new render expression to use for ordering. This
				// handles cases were the expression is either not a name or
				// is a name that is otherwise not referenced by the query:
				//
				//   SELECT a FROM t ORDER by b
//.........这里部分代码省略.........

// Set sets session variables.
// Privileges: None.
//   Notes: postgres/mysql do not require privileges for session variables (some exceptions).
func (p *planner) Set(n *parser.Set) (planNode, error) {
	if n.Name == nil {
		// A client has sent the reserved internal syntax SET ROW ...
		// Reject it.
		return nil, errors.New("invalid statement: SET ROW")
	}

	// By using VarName.String() here any variables that are keywords will
	// be double quoted.
	name := strings.ToUpper(n.Name.String())
	typedValues := make([]parser.TypedExpr, len(n.Values))
	for i, expr := range n.Values {
		typedValue, err := parser.TypeCheck(expr, nil, parser.TypeString)
		if err != nil {
			return nil, err
		}
		typedValues[i] = typedValue
	}
	switch name {
	case `DATABASE`:
		dbName, err := p.getStringVal(name, typedValues)
		if err != nil {
			return nil, err
		}
		if len(dbName) != 0 {
			// Verify database descriptor exists.
			if _, err := p.mustGetDatabaseDesc(dbName); err != nil {
				return nil, err
			}
		}
		p.session.Database = dbName
		p.evalCtx.Database = dbName

	case `SYNTAX`:
		s, err := p.getStringVal(name, typedValues)
		if err != nil {
			return nil, err
		}
		switch parser.Name(s).Normalize() {
		case parser.ReNormalizeName(parser.Modern.String()):
			p.session.Syntax = int32(parser.Modern)
		case parser.ReNormalizeName(parser.Traditional.String()):
			p.session.Syntax = int32(parser.Traditional)
		default:
			return nil, fmt.Errorf("%s: \"%s\" is not in (%q, %q)", name, s, parser.Modern, parser.Traditional)
		}

	case `DEFAULT_TRANSACTION_ISOLATION`:
		// It's unfortunate that clients want us to support both SET
		// SESSION CHARACTERISTICS AS TRANSACTION ..., which takes the
		// isolation level as keywords/identifiers (e.g. JDBC), and SET
		// DEFAULT_TRANSACTION_ISOLATION TO '...', which takes an
		// expression (e.g. psycopg2). But that's how it is.  Just ensure
		// this code keeps in sync with SetDefaultIsolation() below.
		s, err := p.getStringVal(name, typedValues)
		if err != nil {
			return nil, err
		}
		switch strings.ToUpper(s) {
		case `READ UNCOMMITTED`, `READ COMMITTED`, `SNAPSHOT`:
			p.session.DefaultIsolationLevel = enginepb.SNAPSHOT
		case `REPEATABLE READ`, `SERIALIZABLE`:
			p.session.DefaultIsolationLevel = enginepb.SERIALIZABLE
		default:
			return nil, fmt.Errorf("%s: unknown isolation level: %q", name, s)
		}

	case `DIST_SQL`:
		s, err := p.getStringVal(name, typedValues)
		if err != nil {
			return nil, err
		}
		switch parser.Name(s).Normalize() {
		case parser.ReNormalizeName("off"):
			p.session.DistSQLMode = distSQLOff
		case parser.ReNormalizeName("on"):
			p.session.DistSQLMode = distSQLOn
		case parser.ReNormalizeName("always"):
			p.session.DistSQLMode = distSQLAlways
		default:
			return nil, fmt.Errorf("%s: \"%s\" not supported", name, s)
		}

	// These settings are sent by various client drivers. We don't support
	// changing them, so we either silently ignore them or throw an error given
	// a setting that we do not respect.
	case `EXTRA_FLOAT_DIGITS`:
	// See https://www.postgresql.org/docs/9.6/static/runtime-config-client.html
	case `APPLICATION_NAME`:
	// Set by clients to improve query logging.
	// See https://www.postgresql.org/docs/9.6/static/runtime-config-logging.html#GUC-APPLICATION-NAME
	case `CLIENT_ENCODING`:
		// See https://www.postgresql.org/docs/9.6/static/multibyte.html
		s, err := p.getStringVal(name, typedValues)
		if err != nil {
			return nil, err
		}
//.........这里部分代码省略.........