// getVirtualTableEntry checks if the provided name matches a virtual database/table
// pair. The function will return the table's virtual table entry if the name matches
// a specific table. It will return an error if the name references a virtual database
// but the table is non-existent.
func getVirtualTableEntry(tn *parser.TableName) (virtualTableEntry, error) {
	if db, ok := getVirtualSchemaEntry(tn.Database()); ok {
		if t, ok := db.tables[sqlbase.NormalizeName(tn.TableName)]; ok {
			return t, nil
		}
		return virtualTableEntry{}, sqlbase.NewUndefinedTableError(tn.String())
	}
	return virtualTableEntry{}, nil
}

// mustGetTableDesc implements the SchemaAccessor interface.
func (p *planner) mustGetTableDesc(tn *parser.TableName) (*sqlbase.TableDescriptor, error) {
	desc, err := p.getTableDesc(tn)
	if err != nil {
		return nil, err
	}
	if desc == nil {
		return nil, sqlbase.NewUndefinedTableError(tn.String())
	}
	if err := filterTableState(desc); err != nil {
		return nil, err
	}
	return desc, nil
}

// checkDatabaseName checks whether the given TableName is unambiguous
// within this source and if it is, qualifies the missing database name.
func (src *dataSourceInfo) checkDatabaseName(tn parser.TableName) (parser.TableName, error) {
	found := false
	if tn.DatabaseName == "" {
		// No database name yet. Try to find one.
		for name := range src.sourceAliases {
			if name.TableName == tn.TableName {
				if found {
					return parser.TableName{}, fmt.Errorf("ambiguous source name: %q", tn.TableName)
				}
				tn.DatabaseName = name.DatabaseName
				found = true
			}
		}
		if !found {
			return parser.TableName{}, fmt.Errorf("source name %q not found in FROM clause", tn.TableName)
		}
		return tn, nil
	}

	// Database given. Check that the name is unambiguous.
	if _, ok := src.sourceAliases[tn]; ok {
		if found {
			return parser.TableName{}, fmt.Errorf("ambiguous source name: %q (within database %q)",
				tn.TableName, tn.DatabaseName)
		}
		found = true
	}

	if !found {
		return parser.TableName{}, fmt.Errorf("table %q not selected in FROM clause", &tn)
	}
	return tn, nil
}

// expandStar returns the array of column metadata and name
// expressions that correspond to the expansion of a star.
func (src *dataSourceInfo) expandStar(
	v parser.VarName, qvals qvalMap,
) (columns []ResultColumn, exprs []parser.TypedExpr, err error) {
	if len(src.sourceColumns) == 0 {
		return nil, nil, fmt.Errorf("cannot use \"%s\" without a FROM clause", v)
	}

	colSel := func(idx int) {
		col := src.sourceColumns[idx]
		if !col.hidden {
			qval := qvals.getQVal(columnRef{src, idx})
			columns = append(columns, ResultColumn{Name: col.Name, Typ: qval.datum})
			exprs = append(exprs, qval)
		}
	}

	tableName := parser.TableName{}
	if a, ok := v.(*parser.AllColumnsSelector); ok {
		tableName = a.TableName
	}
	if tableName.Table() == "" {
		for i := 0; i < len(src.sourceColumns); i++ {
			colSel(i)
		}
	} else {
		norm := sqlbase.NormalizeTableName(tableName)

		qualifiedTn, err := src.checkDatabaseName(norm)
		if err != nil {
			return nil, nil, err
		}

		colRange, ok := src.sourceAliases[qualifiedTn]
		if !ok {
			return nil, nil, fmt.Errorf("table %q not found", tableName.String())
		}
		for _, i := range colRange {
			colSel(i)
		}
	}

	return columns, exprs, nil
}

func (c *conn) normalizeTableName(table *parser.TableName) error {
	if table.Qualifier == "" {
		if c.database == "" {
			return fmt.Errorf("no database specified")
		}
		table.Qualifier = c.database
	}
	if table.Name == "" {
		return fmt.Errorf("empty table name: %s", table)
	}
	return nil
}

func (s *Server) normalizeTableName(database string, table *parser.TableName) error {
	if table.Qualifier == "" {
		if database == "" {
			return fmt.Errorf("no database specified")
		}
		table.Qualifier = database
	}
	if table.Name == "" {
		return fmt.Errorf("empty table name: %s", table)
	}
	return nil
}

// getTableDesc implements the SchemaAccessor interface.
func (p *planner) getTableDesc(tn *parser.TableName) (*sqlbase.TableDescriptor, error) {
	virtual, err := getVirtualTableDesc(tn)
	if err != nil || virtual != nil {
		return virtual, err
	}

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

	desc := sqlbase.TableDescriptor{}
	found, err := p.getDescriptor(tableKey{parentID: dbDesc.ID, name: tn.Table()}, &desc)
	if err != nil {
		return nil, err
	}
	if !found {
		return nil, nil
	}
	return &desc, nil
}

// getTableID retrieves the table ID for the specified table.
func getTableID(p *planner, tn *parser.TableName) (sqlbase.ID, error) {
	if err := tn.QualifyWithDatabase(p.session.Database); err != nil {
		return 0, err
	}

	virtual, err := getVirtualTableDesc(tn)
	if err != nil {
		return 0, err
	}
	if virtual != nil {
		return virtual.GetID(), nil
	}

	dbID, err := p.getDatabaseID(tn.Database())
	if err != nil {
		return 0, err
	}

	nameKey := tableKey{dbID, tn.Table()}
	key := nameKey.Key()
	gr, err := p.txn.Get(key)
	if err != nil {
		return 0, err
	}
	if !gr.Exists() {
		return 0, sqlbase.NewUndefinedTableError(parser.AsString(tn))
	}
	return sqlbase.ID(gr.ValueInt()), nil
}

// findColumn looks up the column specified by a VarName. The normalized VarName
// is returned.
func (sources multiSourceInfo) findColumn(
	c *parser.ColumnItem,
) (info *dataSourceInfo, colIdx int, err error) {
	if len(c.Selector) > 0 {
		return nil, invalidColIdx, util.UnimplementedWithIssueErrorf(8318, "compound types not supported yet: %q", c)
	}

	colName := sqlbase.NormalizeName(c.ColumnName)
	var tableName parser.TableName
	if c.TableName.Table() != "" {
		tableName = sqlbase.NormalizeTableName(c.TableName)

		tn, err := sources.checkDatabaseName(tableName)
		if err != nil {
			return nil, 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 _, src := range sources {
		findCol := func(src, info *dataSourceInfo, colIdx int, idx int) (*dataSourceInfo, int, error) {
			col := src.sourceColumns[idx]
			if sqlbase.ReNormalizeName(col.Name) == colName {
				if colIdx != invalidColIdx {
					return nil, invalidColIdx, fmt.Errorf("column reference %q is ambiguous", c)
				}
				info = src
				colIdx = idx
			}
			return info, colIdx, nil
		}

		if tableName.Table() == "" {
			for idx := 0; idx < len(src.sourceColumns); idx++ {
				info, colIdx, err = findCol(src, info, colIdx, idx)
				if err != nil {
					return info, 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 {
				info, colIdx, err = findCol(src, info, colIdx, idx)
				if err != nil {
					return info, colIdx, err
				}
			}
		}
	}

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

	return info, colIdx, nil
}

// getDataSource builds a planDataSource from a single data source clause
// (TableExpr) in a SelectClause.
func (p *planner) getDataSource(
	src parser.TableExpr,
	hints *parser.IndexHints,
	scanVisibility scanVisibility,
) (planDataSource, error) {
	switch t := src.(type) {
	case *parser.NormalizableTableName:
		// Usual case: a table.
		tn, err := t.NormalizeWithDatabaseName(p.session.Database)
		if err != nil {
			return planDataSource{}, err
		}

		// Is this perhaps a name for a virtual table?
		ds, foundVirtual, err := p.getVirtualDataSource(tn)
		if err != nil {
			return planDataSource{}, err
		}
		if foundVirtual {
			return ds, nil
		}

		// This name designates a real table.
		scan := p.Scan()
		if err := scan.initTable(p, tn, hints, scanVisibility); err != nil {
			return planDataSource{}, err
		}

		return planDataSource{
			info: newSourceInfoForSingleTable(*tn, scan.Columns()),
			plan: scan,
		}, nil

	case *parser.Subquery:
		// We have a subquery (this includes a simple "VALUES").
		plan, err := p.newPlan(t.Select, nil, false)
		if err != nil {
			return planDataSource{}, err
		}
		return planDataSource{
			info: newSourceInfoForSingleTable(parser.TableName{}, plan.Columns()),
			plan: plan,
		}, nil

	case *parser.JoinTableExpr:
		// Joins: two sources.
		left, err := p.getDataSource(t.Left, nil, scanVisibility)
		if err != nil {
			return left, err
		}
		right, err := p.getDataSource(t.Right, nil, scanVisibility)
		if err != nil {
			return right, err
		}
		return p.makeJoin(t.Join, left, right, t.Cond)

	case *parser.ParenTableExpr:
		return p.getDataSource(t.Expr, hints, scanVisibility)

	case *parser.AliasedTableExpr:
		// Alias clause: source AS alias(cols...)
		src, err := p.getDataSource(t.Expr, t.Hints, scanVisibility)
		if err != nil {
			return src, err
		}

		var tableAlias parser.TableName
		if t.As.Alias != "" {
			// If an alias was specified, use that.
			tableAlias.TableName = parser.Name(sqlbase.NormalizeName(t.As.Alias))
			src.info.sourceAliases = sourceAliases{
				tableAlias: fillColumnRange(0, len(src.info.sourceColumns)-1),
			}
		}
		colAlias := t.As.Cols

		if len(colAlias) > 0 {
			// Make a copy of the slice since we are about to modify the contents.
			src.info.sourceColumns = append([]ResultColumn(nil), src.info.sourceColumns...)

			// The column aliases can only refer to explicit columns.
			for colIdx, aliasIdx := 0, 0; aliasIdx < len(colAlias); colIdx++ {
				if colIdx >= len(src.info.sourceColumns) {
					var srcName string
					if tableAlias.DatabaseName != "" {
						srcName = tableAlias.String()
					} else {
						srcName = tableAlias.TableName.String()
					}

					return planDataSource{}, errors.Errorf(
						"source %q has %d columns available but %d columns specified",
						srcName, aliasIdx, len(colAlias))
				}
				if src.info.sourceColumns[colIdx].hidden {
					continue
				}
				src.info.sourceColumns[colIdx].Name = string(colAlias[aliasIdx])
//.........这里部分代码省略.........

// 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 := 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 sqlbase.ReNormalizeName(l.Name) == sqlbase.NormalizeName(tn.TableName) &&
			l.ParentID == dbID {
			lease = l
			if log.V(2) {
				log.Infof(p.ctx(), "found lease in planner cache for table %q", 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 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
}