gob解码器只返回数组中的第一个元素。

huangapple
go评论117阅读模式
英文:

gob decoder returns only first element in the array

问题

所以我正在尝试创建一个模拟数据库，在当前的实现中，我试图创建一个insert和select，其中insert插入行，select返回它们。我决定使用bytes.Buffer来帮助维护一个内存块，我可以在其中插入一系列行，并在调用select时反序列化该内存块，但似乎select只返回第一行，而不是传递给数组的所有行。

上面的代码负责收集用户输入等。

以下是负责解析用户输入并将条目附加到语句中的代码，根据关键字，它可以是insert或select（已删除定义枚举的代码，只保留核心逻辑）。

代码使用全局的buffer，在PrepareStatement()中附加的slice将在执行insert后进行编码。select应该返回所有行的slice，但出于某种原因只返回第一个元素。

例如（在终端中）：
如果我进行两次插入：
db > insert 1 john c@mail.com
db > insert 2 collins k@mail.com

然后我进行一个选择：
select
=> 它只返回[{1 john c@mail.com}]。

这里有什么我漏掉的吗？感谢您的支持。

英文:

So I was trying to create a mock DB, and in the current implementation, I am trying to make an insert and select which insert rows and select returns them. I decided to use a bytes.Buffer to help maintain a memory block I could insert a slice of rows in, and deserialize that memory block when I call select but it seems select just returns the first row instead of all the rows passed to the array.

main.go

  2. func main() {
  3. 	inputBuffer := compiler.NewInputBuffer()
  4. 	scanner := bufio.NewScanner(os.Stdin)
  5. 	for {
  6. 		PrintPrompt()
  7. 		scanner.Scan()
  8. 		command := scanner.Text()
  9. 		inputBuffer.Buffer = command
  11. 		if strings.HasPrefix(inputBuffer.Buffer, ".") {
  12. 			switch compiler.DoMetaCommand(inputBuffer) {
  13. 			case compiler.MetaCommandSuccess:
  14. 				continue
  15. 			case compiler.MetaCommandUnrecognizedCommand:
  16. 				fmt.Printf("Unrecognized command %q \n", inputBuffer.Buffer)
  17. 				continue
  18. 			}
  19. 		}
  21. 		var statement compiler.Statement
  22. 		switch compiler.PrepareStatement(inputBuffer, &statement) {
  23. 		case compiler.PrepareSuccess:
  25. 		case compiler.PrepareUnrecognizedStatement:
  26. 			fmt.Printf("Unrecognized command at start of %q \n", inputBuffer.Buffer)
  27. 			continue
  29. 		case compiler.PrepareSyntaxError:
  30. 			fmt.Println("Syntax error. Could not parse statement.")
  31. 			continue
  32. 		}
  34. 		compiler.ExecuteStatement(statement)
  35. 		fmt.Println("Executed")
  37. 	}
  38. }
  40. func PrintPrompt() {
  41. 	fmt.Printf("db > ")
  42. }

Above is the code responsible for collecting user input etc.

  1. package compiler
  3. import (
  4. 	"bytes"
  5. 	"log"
  6. 	"os"
  7. 	"strconv"
  8. 	"strings"
  9. )
  12. type Row struct {
  13. 	ID       int32
  14. 	Username string
  15. 	Email    string
  16. }
  20. type Statement struct {
  21. 	RowToInsert Row
  22. 	Type        StatementType
  23. }
  25. var (
  26. 	RowsTable       = make([]Row, 0)
  27. 	RowsTableBuffer bytes.Buffer
  28. )
  30. func DoMetaCommand(buffer InputBuffer) MetaCommandResult {
  31. 	if buffer.Buffer == ".exit" {
  32. 		os.Exit(0)
  33. 	} else {
  34. 		return MetaCommandUnrecognizedCommand
  35. 	}
  36. 	return MetaCommandSuccess
  37. }
  39. func PrepareStatement(buffer InputBuffer, statement *Statement) PrepareResult {
  40. 	if len(buffer.Buffer) > 6 {
  41. 		bufferArguments := strings.Fields(buffer.Buffer)
  42. 		if bufferArguments[0] == "insert" {
  43. 			statement.Type = StatementInsert
  44. 			if len(bufferArguments) < 4 {
  45. 				return PrepareSyntaxError
  46. 			} else {
  47. 				i, err := strconv.Atoi(bufferArguments[1])
  48. 				if err != nil {
  49. 					log.Printf("%q is not a valid id\n", bufferArguments[1])
  50. 					return PrepareSyntaxError
  51. 				} else {
  52. 					statement.RowToInsert.ID = int32(i)
  53. 				}
  54. 				statement.RowToInsert.Username = bufferArguments[2]
  55. 				statement.RowToInsert.Email = bufferArguments[3]
  56. 			}
  57. 			RowsTable = append(RowsTable, statement.RowToInsert)
  58. 			
  59. 			return PrepareSuccess
  60. 		}
  61. 	}
  63. 	if buffer.Buffer == "select" {
  64. 		statement.Type = StatementSelect
  65. 		return PrepareSuccess
  66. 	}
  68. 	return PrepareUnrecognizedStatement
  69. }
  71. func ExecuteStatement(statement Statement) {
  72. 	switch statement.Type {
  73. 	case (StatementInsert):
  74. 		SerializeRow(RowsTable)
  75. 	case (StatementSelect):
  76. 		DeserializeRow()
  77. 	}
  78. }

The code above is for parsing and appending the entries into statements and depending on the keywords, it's either an insert or select [Took the code for defining enums out and left core logic]

  2. func SerializeRow(r []Row) {
  3. 	encoder := gob.NewEncoder(&RowsTableBuffer)
  4. 	err := encoder.Encode(r)
  5. 	if err != nil {
  6. 		log.Println("encode error:", err)
  7. 	}
  8. }
  10. func DeserializeRow() {
  11. 	var rowsBuffer = RowsTableBuffer
  12. 	rowsTable := make([]Row, 0)
  13. 	decoder := gob.NewDecoder(&rowsBuffer)
  14. 	err := decoder.Decode(&rowsTable)
  15. 	if err != nil {
  16. 			log.Println("decode error:", err)
  17. 		}
  18. 	fmt.Println(rowsTable)
  19. }

So the code above uses a global buffer in which the slice being appended to in PrepareStatement()will be encoded after an insert is done. A select ought to return the slice of all rows but just returns the first element for some reason.

Example (in terminal):
If I make 2 inserts:
db > insert 1 john c@mail.com
db > insert 2 collins k@mail.com

Then I make a select:
select
=> it returns [{1 john c@mail.com}] only.

Is there anything I am missing here? Thanks for your support.

答案1

得分: 0

所以答案很简单。我们在SerializeRow函数中创建了一个新的encoder，而不是只创建一次。我们将其从函数中提取出来，创建为全局变量。

  1. var (
  2. 	encoder = gob.NewEncoder(&RowsTableBuffer)
  3. 	decoder = gob.NewDecoder(&RowsTableBuffer)
  4. )
  6. func SerializeRow(r Row) {
  7. 	err := encoder.Encode(r)
  8. 	if err != nil {
  9. 		log.Println("encode error:", err)
  10. 	}
  11. }
  13. func DeserializeRow() {
  14. 	var rows Row
  16. 	err := decoder.Decode(&rows)
  18. 	for err == nil {
  19. 		if err != nil {
  20. 			log.Fatal("decode error:", err)
  21. 		}
  22. 		fmt.Printf("%d %s %s\n", rows.ID, rows.Username, rows.Email)
  23. 		err = decoder.Decode(&rows)
  24. 	}
  25. }
英文:

So the answer was pretty simple. We were creating a new encoder in the SerializeRow function instead of creating it once. We pulled it out of the function and created a global.

  1. var (
  2. 	encoder = gob.NewEncoder(&RowsTableBuffer)
  3. 	decoder = gob.NewDecoder(&RowsTableBuffer)
  4. )
  6. func SerializeRow(r Row) {
  7. 	err := encoder.Encode(r)
  8. 	if err != nil {
  9. 		log.Println("encode error:", err)
  10. 	}
  12. }
  14. func DeserializeRow() {
  16. 	var rows Row
  18. 	err := decoder.Decode(&rows)
  20. 	for err == nil {
  21. 		if err != nil {
  22. 			log.Fatal("decode error:", err)
  23. 		}
  24. 		fmt.Printf("%d %s %s\n", rows.ID, rows.Username, rows.Email)
  25. 		err = decoder.Decode(&rows)
  27. 	}
  29. }

huangapple
  • 本文由 发表于 2022年4月12日 06:36:26
  • 转载请务必保留本文链接：https://go.coder-hub.com/71835056.html
  • go
  • gob
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