我们将会用 golang 实现一个数据库,完整源码见 https://github.com/eatonphil/gosql。
首先,我们实现一个 parser 来解析 CREATE、INSERT、SELECT 语句,然后会在内存实现一个 db server,并支持 TEXT 和 INT 类型,实现一种 REPL(交互式终端)。
我们希望达到如下效果:
$ go run *.go
Welcome to gosql.
# CREATE TABLE users (id INT, name TEXT);
ok
# INSERT INTO users VALUES (1, 'Phil');
ok
# SELECT id, name FROM users;
| id | name |
====================
| 1 | Phil |
ok
# INSERT INTO users VALUES (2, 'Kate');
ok
# SELECT name, id FROM users;
| name | id |
====================
| Phil | 1 |
| Kate | 2 |
ok
首先要实现 lexing,即将一个 SQL 文本映射为一串 tokens,然后,我们会调用 parse 函数来查找单独的 SQL 语句(如 SELECT),这些 parse 函数会一次调用他们自己的 helper 函数,来查找可递归解析的块、关键字、符号(比如圆括号)、标识符(比如 table name)、数字、字符串文本等。
然后,我们会实现一个内存的 server,它基于 AST 操作。最终实现一个 REPL 来接受 SQL 文本,并发送给 server backend。
一、lexing 词法分析
词法分析,寻找不同的字符分组,主要包括 identifiers、numbers、strings、symbols。
逻辑是由各个 helper function 完成的
- 如果成功找到了 token,则返回 true 和 下次开始处理的 location。
- 否则,会持续迭代到循环结束。
1.1 数据结构
首先,在 lexer.go 定义类型和常量:
package gosql
import (
"fmt"
"strings"
)
type location struct {
line uint
col uint
}
type keyword string
const (
selectKeyword keyword = "select"
fromKeyword keyword = "from"
asKeyword keyword = "as"
tableKeyword keyword = "table"
createKeyword keyword = "create"
insertKeyword keyword = "insert"
intoKeyword keyword = "into"
valuesKeyword keyword = "values"
intKeyword keyword = "int"
textKeyword keyword = "text"
)
type symbol string
const (
semicolonSymbol symbol = ";"
asteriskSymbol symbol = "*"
commaSymbol symbol = ","
leftparenSymbol symbol = "("
rightparenSymbol symbol = ")"
)
type tokenKind uint
const (
keywordKind tokenKind = iota
symbolKind
identifierKind
stringKind
numericKind
)
type token struct {
value string
kind tokenKind
loc location
}
type cursor struct {
pointer uint
loc location
}
func (t *token) equals(other *token) bool {
return t.value == other.value && t.kind == other.kind
}
type lexer func(string, cursor) (*token, cursor, bool)
1.2 主框架
然后,实现 main loop 逻辑:
func lex(source string) ([]*token, error) {
tokens := []*token{}
cur := cursor{}
lex:
for cur.pointer < uint(len(source)) {
lexers := []lexer{lexKeyword, lexSymbol, lexString, lexNumeric, lexIdentifier}
for _, l := range lexers { // 尝试解析每种类型
if token, newCursor, ok := l(source, cur); ok {
cur = newCursor
// Omit nil tokens for valid, but empty syntax like newlines
if token != nil {
tokens = append(tokens, token)
}
continue lex // 若解析成功,则退出
}
}
// 若每种类型都解析失败,则报错,指明详细原因 line 和 col
hint := ""
if len(tokens) > 0 {
hint = " after " + tokens[len(tokens)-1].value
}
return nil, fmt.Errorf("Unable to lex token%s, at %d:%d", hint, cur.loc.line, cur.loc.col)
}
return tokens, nil
}
1.3 分析 number
因为 number 规则复杂,所以参考 PostgreSQL documentation (section 4.1.2.6) 的规则。
func lexNumeric(source string, ic cursor) (*token, cursor, bool) {
cur := ic
periodFound := false
expMarkerFound := false
for ; cur.pointer < uint(len(source)); cur.pointer++ {
c := source[cur.pointer]
cur.loc.col++
isDigit := c >= '0' && c <= '9'
isPeriod := c == '.'
isExpMarker := c == 'e'
// Must start with a digit or period
if cur.pointer == ic.pointer {
if !isDigit && !isPeriod {
return nil, ic, false
}
periodFound = isPeriod
continue
}
if isPeriod {
if periodFound {
return nil, ic, false
}
periodFound = true
continue
}
if isExpMarker {
if expMarkerFound {
return nil, ic, false
}
// No periods allowed after expMarker
periodFound = true
expMarkerFound = true
// expMarker must be followed by digits
if cur.pointer == uint(len(source)-1) {
return nil, ic, false
}
cNext := source[cur.pointer+1]
if cNext == '-' || cNext == '+' {
cur.pointer++
cur.loc.col++
}
continue
}
if !isDigit {
break
}
}
// No characters accumulated
if cur.pointer == ic.pointer {
return nil, ic, false
}
return &token{
value: source[ic.pointer:cur.pointer],
loc: ic.loc,
kind: numericKind,
}, cur, true
}
1.4 分析 strings
字符串必须以单个撇号开头和结尾。如果后面跟着另一个撇号,则它们可以包含一个撇号。我们将把这种字符分隔的词法逻辑放入辅助函数中,以便我们在分析标识符时可以再次使用它。
func lexCharacterDelimited(source string, ic cursor, delimiter byte) (*token, cursor, bool) {
cur := ic
if len(source[cur.pointer:]) == 0 {
return nil, ic, false
}
if source[cur.pointer] != delimiter {
return nil, ic, false
}
cur.loc.col++
cur.pointer++
var value []byte
for ; cur.pointer < uint(len(source)); cur.pointer++ {
c := source[cur.pointer]
if c == delimiter {
// SQL escapes are via double characters, not backslash. SQL转义是通过双字符进行的,而不是反斜杠。
if cur.pointer+1 >= uint(len(source)) || source[cur.pointer+1] != delimiter {
return &token{
value: string(value),
loc: ic.loc,
kind: stringKind,
}, cur, true
} else {
value = append(value, delimiter)
cur.pointer++
cur.loc.col++
}
}
value = append(value, c)
cur.loc.col++
}
return nil, ic, false
}
func lexString(source string, ic cursor) (*token, cursor, bool) {
return lexCharacterDelimited(source, ic, '\'')
}
1.5 分析符号和关键字
符号来自一组固定的字符串,因此很容易进行比较。空白字符应该被丢弃。
func lexSymbol(source string, ic cursor) (*token, cursor, bool) {
c := source[ic.pointer]
cur := ic
// Will get overwritten later if not an ignored syntax
cur.pointer++
cur.loc.col++
switch c {
// Syntax that should be thrown away
case '\n':
cur.loc.line++
cur.loc.col = 0
fallthrough
case '\t':
fallthrough
case ' ':
return nil, cur, true
}
// Syntax that should be kept
symbols := []symbol{
commaSymbol,
leftParenSymbol,
rightParenSymbol,
semicolonSymbol,
asteriskSymbol,
}
var options []string
for _, s := range symbols {
options = append(options, string(s))
}
// Use `ic`, not `cur`
match := longestMatch(source, ic, options)
// Unknown character
if match == "" {
return nil, ic, false
}
cur.pointer = ic.pointer + uint(len(match))
cur.loc.col = ic.loc.col + uint(len(match))
return &token{
value: match,
loc: ic.loc,
kind: symbolKind,
}, cur, true
}
Keywords 甚至更简单, 用 longestMatch() 函数实现:
// longestMatch iterates through a source string starting at the given
// cursor to find the longest matching substring among the provided
// options
func longestMatch(source string, ic cursor, options []string) string {
var value []byte
var skipList []int
var match string
cur := ic
for cur.pointer < uint(len(source)) {
value = append(value, strings.ToLower(string(source[cur.pointer]))...)
cur.pointer++
match:
for i, option := range options {
for _, skip := range skipList {
if i == skip {
continue match
}
}
// Deal with cases like INT vs INTO
if option == string(value) {
skipList = append(skipList, i)
if len(option) > len(match) {
match = option
}
continue
}
sharesPrefix := string(value) == option[:cur.pointer-ic.pointer]
tooLong := len(value) > len(option)
if tooLong || !sharesPrefix {
skipList = append(skipList, i)
}
}
if len(skipList) == len(options) {
break
}
}
return match
}
1.6 分析 indetifiers(标识符)
标识符是双引号字符串或以字母字符开头并且可能包含数字和下划线的一组字符。
func lexIdentifier(source string, ic cursor) (*token, cursor, bool) {
// Handle separately if is a double-quoted identifier
if token, newCursor, ok := lexCharacterDelimited(source, ic, '"'); ok {
return token, newCursor, true
}
cur := ic
c := source[cur.pointer]
// Other characters count too, big ignoring non-ascii for now
isAlphabetical := (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z')
if !isAlphabetical {
return nil, ic, false
}
cur.pointer++
cur.loc.col++
value := []byte{c}
for ; cur.pointer < uint(len(source)); cur.pointer++ {
c = source[cur.pointer]
// Other characters count too, big ignoring non-ascii for now
isAlphabetical := (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z')
isNumeric := c >= '0' && c <= '9'
if isAlphabetical || isNumeric || c == '$' || c == '_' {
value = append(value, c)
cur.loc.col++
continue
}
break
}
if len(value) == 0 {
return nil, ic, false
}
return &token{
// Unquoted dentifiers are case-insensitive
value: strings.ToLower(string(value)),
loc: ic.loc,
kind: identifierKind,
}, cur, true
}
至此,全部的 lexer 就完成了,现在 lexer_test.go 也可以通过了。debug 一遍单测就能完全明白上文了。
{
input: "insert into users Values (105, 233)",
Tokens: []Token{
{
Loc: Location{Col: 0, Line: 0},
Value: string(InsertKeyword),
Kind: KeywordKind,
},
{
Loc: Location{Col: 7, Line: 0},
Value: string(IntoKeyword),
Kind: KeywordKind,
},
{
Loc: Location{Col: 12, Line: 0},
Value: "users",
Kind: IdentifierKind,
},
{
Loc: Location{Col: 18, Line: 0},
Value: string(ValuesKeyword),
Kind: KeywordKind,
},
{
Loc: Location{Col: 25, Line: 0},
Value: "(",
Kind: SymbolKind,
},
{
Loc: Location{Col: 26, Line: 0},
Value: "105",
Kind: NumericKind,
},
{
Loc: Location{Col: 30, Line: 0},
Value: ",",
Kind: SymbolKind,
},
{
Loc: Location{Col: 32, Line: 0},
Value: "233",
Kind: NumericKind,
},
{
Loc: Location{Col: 36, Line: 0},
Value: ")",
Kind: SymbolKind,
},
},
err: nil,
},
二、AST 抽象语法树
2.1 AST、Statement 数据结构
在最高层级, AST 是 statements 的集合
package main
type Ast struct {
Statements []*Statement
}
目前,statement 是 INSERT、CREATE、SELECT 之一:
type AstKind uint
const (
SelectKind AstKind = iota
CreateTableKind
InsertKind
)
type Statement struct {
SelectStatement *SelectStatement
CreateTableStatement *CreateTableStatement
InsertStatement *InsertStatement
Kind AstKind
}
2.2 INSERT
目前,INSERT 有 table name 和一列 values
type InsertStatement struct {
table token
values *[]*expression
}
expression 目前是 literal token,将来可能是一个 function call,或者一个 inline operation:
type expressionKind uint
const (
literalKind expressionKind = iota
)
type expression struct {
literal *token
kind expressionKind
}
2.3 CREATE
目前,CREATE 有 table name 和一列 column names 和 column types
type columnDefinition struct {
name token
datatype token
}
type CreateTableStatement struct {
name token
cols *[]*columnDefinition
}
2.4 SELECT
目前,SELECT 有 table name 和一列 column names
type SelectStatement struct {
item []*expression
from token
}
至此,AST 完毕。
三、Parsing
parse() 的输入是一堆 tokens,持续解析到分号为止。
通常的策略是,传递并递增一个 cursor 参数,每个 helper() 函数都会返回一个新的 cursor,其表示了后续的起始位置。
3.1 主框架
package main
import (
"errors"
"fmt"
)
func tokenFromKeyword(k keyword) token {
return token{
kind: keywordKind,
value: string(k),
}
}
func tokenFromSymbol(s symbol) token {
return token{
kind: symbolKind,
value: string(s),
}
}
func expectToken(tokens []*token, cursor uint, t token) bool {
if cursor >= uint(len(tokens)) {
return false
}
return t.equals(tokens[cursor])
}
func helpMessage(tokens []*token, cursor uint, msg string) {
var c *token
if cursor < uint(len(tokens)) {
c = tokens[cursor]
} else {
c = tokens[cursor-1]
}
fmt.Printf("[%d,%d]: %s, got: %s\n", c.loc.line, c.loc.col, msg, c.value)
}
func Parse(source string) (*Ast, error) {
tokens, err := lex(source)
if err != nil {
return nil, err
}
a := Ast{}
cursor := uint(0)
for cursor < uint(len(tokens)) {
stmt, newCursor, ok := parseStatement(tokens, cursor, tokenFromSymbol(semicolonSymbol))
if !ok {
helpMessage(tokens, cursor, "Expected statement")
return nil, errors.New("Failed to parse, expected statement")
}
cursor = newCursor
a.Statements = append(a.Statements, stmt)
atLeastOneSemicolon := false
for expectToken(tokens, cursor, tokenFromSymbol(semicolonSymbol)) {
cursor++
atLeastOneSemicolon = true
}
if !atLeastOneSemicolon {
helpMessage(tokens, cursor, "Expected semi-colon delimiter between statements")
return nil, errors.New("Missing semi-colon between statements")
}
}
return &a, nil
}
3.2 Parsing Statements
每条语句将是 INSERT、CREATE 或 SELECT之一。ParseStatement helper 将对这些语句类型中的每一种调用一个 helper,如果其中一种语句解析成功,则返回 TRUE。
func parseStatement(tokens []*token, initialCursor uint, delimiter token) (*Statement, uint, bool) {
cursor := initialCursor
// Look for a SELECT statement
semicolonToken := tokenFromSymbol(semicolonSymbol)
slct, newCursor, ok := parseSelectStatement(tokens, cursor, semicolonToken)
if ok {
return &Statement{
Kind: SelectKind,
SelectStatement: slct,
}, newCursor, true
}
// Look for a INSERT statement
inst, newCursor, ok := parseInsertStatement(tokens, cursor, semicolonToken)
if ok {
return &Statement{
Kind: InsertKind,
InsertStatement: inst,
}, newCursor, true
}
// Look for a CREATE statement
crtTbl, newCursor, ok := parseCreateTableStatement(tokens, cursor, semicolonToken)
if ok {
return &Statement{
Kind: CreateTableKind,
CreateTableStatement: crtTbl,
}, newCursor, true
}
return nil, initialCursor, false
}
3.2.1 Parsing select statements
SELECT 的模式比较简单,如下:
- SELECT
- $expression [, …]
- FROM
- $table-name
按这四个部分的模式,得到框架如下:
func parseSelectStatement(tokens []*token, initialCursor uint, delimiter token) (*SelectStatement, uint, bool) {
cursor := initialCursor
if !expectToken(tokens, cursor, tokenFromKeyword(selectKeyword)) {
return nil, initialCursor, false
}
cursor++
slct := SelectStatement{}
exps, newCursor, ok := parseExpressions(tokens, cursor, []token{tokenFromKeyword(fromKeyword), delimiter})
if !ok {
return nil, initialCursor, false
}
slct.item = *exps
cursor = newCursor
if expectToken(tokens, cursor, tokenFromKeyword(fromKeyword)) {
cursor++
from, newCursor, ok := parseToken(tokens, cursor, identifierKind)
if !ok {
helpMessage(tokens, cursor, "Expected FROM token")
return nil, initialCursor, false
}
slct.from = *from
cursor = newCursor
}
return &slct, cursor, true
}
其中 parseToken() 会查找指定的 token 类型:
func parseToken(tokens []*token, initialCursor uint, kind tokenKind) (*token, uint, bool) {
cursor := initialCursor
if cursor >= uint(len(tokens)) {
return nil, initialCursor, false
}
current := tokens[cursor]
if current.kind == kind {
return current, cursor + 1, true
}
return nil, initialCursor, false
}
ParseExpressions helper 将查找由逗号分隔的 token,直到找到分隔符。它将使用现有的 parseExpression helper 函数。
func parseExpressions(tokens []*token, initialCursor uint, delimiters []token) (*[]*expression, uint, bool) {
cursor := initialCursor
exps := []*expression{}
outer:
for {
if cursor >= uint(len(tokens)) {
return nil, initialCursor, false
}
// Look for delimiter
current := tokens[cursor]
for _, delimiter := range delimiters {
if delimiter.equals(current) {
break outer
}
}
// Look for comma
if len(exps) > 0 {
if !expectToken(tokens, cursor, tokenFromSymbol(commaSymbol)) {
helpMessage(tokens, cursor, "Expected comma")
return nil, initialCursor, false
}
cursor++
}
// Look for expression
exp, newCursor, ok := parseExpression(tokens, cursor, tokenFromSymbol(commaSymbol))
if !ok {
helpMessage(tokens, cursor, "Expected expression")
return nil, initialCursor, false
}
cursor = newCursor
exps = append(exps, exp)
}
return &exps, cursor, true
}
parseExpression helper 函数,会查找 number、string、 或者 identifier token。
func parseExpression(tokens []*token, initialCursor uint, _ token) (*expression, uint, bool) {
cursor := initialCursor
kinds := []tokenKind{identifierKind, numericKind, stringKind}
for _, kind := range kinds {
t, newCursor, ok := parseToken(tokens, cursor, kind)
if ok {
return &expression{
literal: t,
kind: literalKind,
}, newCursor, true
}
}
return nil, initialCursor, false
}
至此,parse select 就完成了。
3.2.2 Parsing insert statements
模式如下:
INSERT
INTO
$table-name
VALUES
(
$expression [, ...]
)
可以复用现有的 helper 函数,框架如下:
func parseInsertStatement(tokens []*token, initialCursor uint, delimiter token) (*InsertStatement, uint, bool) {
cursor := initialCursor
// Look for INSERT
if !expectToken(tokens, cursor, tokenFromKeyword(insertKeyword)) {
return nil, initialCursor, false
}
cursor++
// Look for INTO
if !expectToken(tokens, cursor, tokenFromKeyword(intoKeyword)) {
helpMessage(tokens, cursor, "Expected into")
return nil, initialCursor, false
}
cursor++
// Look for table name
table, newCursor, ok := parseToken(tokens, cursor, identifierKind)
if !ok {
helpMessage(tokens, cursor, "Expected table name")
return nil, initialCursor, false
}
cursor = newCursor
// Look for VALUES
if !expectToken(tokens, cursor, tokenFromKeyword(valuesKeyword)) {
helpMessage(tokens, cursor, "Expected VALUES")
return nil, initialCursor, false
}
cursor++
// Look for left paren
if !expectToken(tokens, cursor, tokenFromSymbol(leftparenSymbol)) {
helpMessage(tokens, cursor, "Expected left paren")
return nil, initialCursor, false
}
cursor++
// Look for expression list
values, newCursor, ok := parseExpressions(tokens, cursor, []token{tokenFromSymbol(rightparenSymbol)})
if !ok {
return nil, initialCursor, false
}
cursor = newCursor
// Look for right paren
if !expectToken(tokens, cursor, tokenFromSymbol(rightparenSymbol)) {
helpMessage(tokens, cursor, "Expected right paren")
return nil, initialCursor, false
}
cursor++
return &InsertStatement{
table: *table,
values: values,
}, cursor, true
}
3.2.3 Parsing create statements
模式如下:
CREATE
$table-name
(
[$column-name $column-type [, ...]]
)
通过一个新的 parseColumnDefinitions helper 勾勒出这一点,我们得到:
func parseCreateTableStatement(tokens []*token, initialCursor uint, delimiter token) (*CreateTableStatement, uint, bool) {
cursor := initialCursor
if !expectToken(tokens, cursor, tokenFromKeyword(createKeyword)) {
return nil, initialCursor, false
}
cursor++
if !expectToken(tokens, cursor, tokenFromKeyword(tableKeyword)) {
return nil, initialCursor, false
}
cursor++
name, newCursor, ok := parseToken(tokens, cursor, identifierKind)
if !ok {
helpMessage(tokens, cursor, "Expected table name")
return nil, initialCursor, false
}
cursor = newCursor
if !expectToken(tokens, cursor, tokenFromSymbol(leftparenSymbol)) {
helpMessage(tokens, cursor, "Expected left parenthesis")
return nil, initialCursor, false
}
cursor++
cols, newCursor, ok := parseColumnDefinitions(tokens, cursor, tokenFromSymbol(rightparenSymbol))
if !ok {
return nil, initialCursor, false
}
cursor = newCursor
if !expectToken(tokens, cursor, tokenFromSymbol(rightparenSymbol)) {
helpMessage(tokens, cursor, "Expected right parenthesis")
return nil, initialCursor, false
}
cursor++
return &CreateTableStatement{
name: *name,
cols: cols,
}, cursor, true
}
ParseColumnDefinitions helper 将查看 column name,后跟以逗号分隔并以某个分隔符结尾的 column type:
func parseColumnDefinitions(tokens []*token, initialCursor uint, delimiter token) (*[]*columnDefinition, uint, bool) {
cursor := initialCursor
cds := []*columnDefinition{}
for {
if cursor >= uint(len(tokens)) {
return nil, initialCursor, false
}
// Look for a delimiter
current := tokens[cursor]
if delimiter.equals(current) {
break
}
// Look for a comma
if len(cds) > 0 {
if !expectToken(tokens, cursor, tokenFromSymbol(commaSymbol)) {
helpMessage(tokens, cursor, "Expected comma")
return nil, initialCursor, false
}
cursor++
}
// Look for a column name
id, newCursor, ok := parseToken(tokens, cursor, identifierKind)
if !ok {
helpMessage(tokens, cursor, "Expected column name")
return nil, initialCursor, false
}
cursor = newCursor
// Look for a column type
ty, newCursor, ok := parseToken(tokens, cursor, keywordKind)
if !ok {
helpMessage(tokens, cursor, "Expected column type")
return nil, initialCursor, false
}
cursor = newCursor
cds = append(cds, &columnDefinition{
name: *id,
datatype: *ty,
})
}
return &cds, cursor, true
}
至此,parser 就结束了,可以在 parser_test.go 跑一遍单测。
四、in-memory backend
允许用户 create、select、insert 数据的通用后端界面:
package main
import "errors"
type ColumnType uint
const (
TextType ColumnType = iota
IntType
)
type Cell interface {
AsText() string
AsInt() int32
}
type Results struct {
Columns []struct {
Type ColumnType
Name string
}
Rows [][]Cell
}
var (
ErrTableDoesNotExist = errors.New("Table does not exist")
ErrColumnDoesNotExist = errors.New("Column does not exist")
ErrInvalidSelectItem = errors.New("Select item is not valid")
ErrInvalidDatatype = errors.New("Invalid datatype")
ErrMissingValues = errors.New("Missing values")
)
type Backend interface {
CreateTable(*CreateTableStatement) error
Insert(*InsertStatement) error
Select(*SelectStatement) (*Results, error)
}
4.1 memory layout
内存需要存储一堆 tables,每个 table 有一堆 columns 和 rows。每个 column 有 name 和 type。每个 row 是一个 bytes[]。
package main
import (
"bytes"
"encoding/binary"
"fmt"
"strconv"
)
type MemoryCell []byte
func (mc MemoryCell) AsInt() int32 {
var i int32
err := binary.Read(bytes.NewBuffer(mc), binary.BigEndian, &i)
if err != nil {
panic(err)
}
return i
}
func (mc MemoryCell) AsText() string {
return string(mc)
}
type table struct {
columns []string
columnTypes []ColumnType
rows [][]MemoryCell
}
type MemoryBackend struct {
tables map[string]*table
}
func NewMemoryBackend() *MemoryBackend {
return &MemoryBackend{
tables: map[string]*table{},
}
}
4.2 实现 Create table
会在 tables map 数据结构中,增加一项。然后 create columns。
func (mb *MemoryBackend) CreateTable(crt *CreateTableStatement) error {
t := table{}
mb.tables[crt.name.value] = &t
if crt.cols == nil {
return nil
}
for _, col := range *crt.cols {
t.columns = append(t.columns, col.name.value)
var dt ColumnType
switch col.datatype.value {
case "int":
dt = IntType
case "text":
dt = TextType
default:
return ErrInvalidDatatype
}
t.columnTypes = append(t.columnTypes, dt)
}
return nil
}
4.3 实现 insert
为简单起见,我们假定传递的 value,可以正确 map 到指定 column type。我们将引用一个帮助器将值映射到内部存储tokenToCell。
func (mb *MemoryBackend) Insert(inst *InsertStatement) error {
table, ok := mb.tables[inst.table.value]
if !ok {
return ErrTableDoesNotExist
}
if inst.values == nil {
return nil
}
row := []MemoryCell{}
if len(*inst.values) != len(table.columns) {
return ErrMissingValues
}
for _, value := range *inst.values {
if value.kind != literalKind {
fmt.Println("Skipping non-literal.")
continue
}
row = append(row, mb.tokenToCell(value.literal))
}
table.rows = append(table.rows, row)
return nil
}
TokenToCell helper 将把数字写成二进制字节,把字符串写成字节:
func (mb *MemoryBackend) tokenToCell(t *token) MemoryCell {
if t.kind == numericKind {
buf := new(bytes.Buffer)
i, err := strconv.Atoi(t.value)
if err != nil {
panic(err)
}
err = binary.Write(buf, binary.BigEndian, int32(i))
if err != nil {
panic(err)
}
return MemoryCell(buf.Bytes())
}
if t.kind == stringKind {
return MemoryCell(t.value)
}
return nil
}
4.4 实现 select
最后,对于SELECT,我们将迭代表中的每一行,并根据AST指定的列返回单元格。
func (mb *MemoryBackend) Select(slct *SelectStatement) (*Results, error) {
table, ok := mb.tables[slct.from.table]
if !ok {
return nil, ErrTableDoesNotExist
}
results := [][]Cell{}
columns := []struct {
Type ColumnType
Name string
}{}
for i, row := range table.rows {
result := []Cell{}
isFirstRow := i == 0
for _, exp := range slct.item {
if exp.kind != literalKind {
// Unsupported, doesn't currently exist, ignore.
fmt.Println("Skipping non-literal expression.")
continue
}
lit := exp.literal
if lit.kind == identifierKind {
found := false
for i, tableCol := range table.columns {
if tableCol == lit.value {
if isFirstRow {
columns = append(columns, struct {
Type ColumnType
Name string
}{
Type: table.columnTypes[i],
Name: lit.value,
})
}
result = append(result, row[i])
found = true
break
}
}
if !found {
return nil, ErrColumnDoesNotExist
}
continue
}
return nil, ErrColumnDoesNotExist
}
results = append(results, result)
}
return &Results{
Columns: columns,
Rows: results,
}, nil
}
五、REPL 交互式终端
最后,我们准备将解析器和内存后端包装在REPL中。最复杂的部分是显示SELECT查询的结果表。
package main
import (
"bufio"
"fmt"
"os"
"strings"
"github.com/eatonphil/gosql"
)
func main() {
mb := gosql.NewMemoryBackend()
reader := bufio.NewReader(os.Stdin)
fmt.Println("Welcome to gosql.")
for {
fmt.Print("# ")
text, err := reader.ReadString('\n')
text = strings.Replace(text, "\n", "", -1)
ast, err := gosql.Parse(text)
if err != nil {
panic(err)
}
for _, stmt := range ast.Statements {
switch stmt.Kind {
case gosql.CreateTableKind:
err = mb.CreateTable(ast.Statements[0].CreateTableStatement)
if err != nil {
panic(err)
}
fmt.Println("ok")
case gosql.InsertKind:
err = mb.Insert(stmt.InsertStatement)
if err != nil {
panic(err)
}
fmt.Println("ok")
case gosql.SelectKind:
results, err := mb.Select(stmt.SelectStatement)
if err != nil {
panic(err)
}
for _, col := range results.Columns {
fmt.Printf("| %s ", col.Name)
}
fmt.Println("|")
for i := 0; i < 20; i++ {
fmt.Printf("=")
}
fmt.Println()
for _, result := range results.Rows {
fmt.Printf("|")
for i, cell := range result {
typ := results.Columns[i].Type
s := ""
switch typ {
case gosql.IntType:
s = fmt.Sprintf("%d", cell.AsInt())
case gosql.TextType:
s = cell.AsText()
}
fmt.Printf(" %s | ", s)
}
fmt.Println()
}
fmt.Println("ok")
}
}
}
}
结果如下:
$ go run *.go
Welcome to gosql.
# CREATE TABLE users (id INT, name TEXT);
ok
# INSERT INTO users VALUES (1, 'Phil');
ok
# SELECT id, name FROM users;
| id | name |
====================
| 1 | Phil |
ok
# INSERT INTO users VALUES (2, 'Kate');
ok
# SELECT name, id FROM users;
| name | id |
====================
| Phil | 1 |
| Kate | 2 |
ok
至此,实现了简单的 SQL DB。后续会实现 filter、sorting、indexing 等更复杂的功能。