Notes

March 6, 2020

Writing a SQL database from scratch in Go: 1. SELECT, INSERT, CREATE and a REPL

Next in database basics:
2. binary expressions and WHERE filters
3. indexes
4. a database/sql driver

In this series we'll write a rudimentary database from scratch in Go. Project source code is available on Github.

In this first post we'll build enough of a parser to run some simple CREATE, INSERT, and SELECT queries. Then we'll build an in-memory backend supporting TEXT and INT types and write a basic REPL.

We'll be able to support the following interaction:

$ 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

The first stage will be to map a SQL source into a list of tokens (lexing). Then we'll call parse functions to find individual SQL statements (such as SELECT). These parse functions will in turn call their own helper functions to find patterns of recursively parseable chunks, keywords, symbols (like parenthesis), identifiers (like a table name), and numeric or string literals.

Then, we'll write an in-memory backend to do operations based on an AST. Finally, we'll write a REPL to accept SQL from a CLI and pass it to the in-memory backend.

This post assumes a basic understanding of parsing concepts. We won't skip any code, but also won't go into great detail on why we structure the way we do.

For a simpler introduction to parsing and parsing concepts, see this post on parsing JSON.

Lexing

The lexer is responsible for finding every distinct group of characters in source code: tokens. This will consist primarily of identifiers, numbers, strings, and symbols.

What follows is a second, more orthodox pass at lexing. The first pass took a number of shortcuts and couldn't handle spaces in strings, for example.

Here is the relevant pull request in gosql if you are curious.

The gist of the logic will be to pass control to a helper function for each kind of token. If the helper function succeeds in finding a token, it will return true and the location for the lexer to start at next. It will continue doing this until it reaches the end of the source.

First off, we'll define a few types and constants for use in lexer.go:

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)

Next we'll write out the main loop:

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 {
            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
            }
        }

        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
}

Then we'll write a helper for each kind of fundemental token.

Analyzing numbers

Numbers are the most complex. So we'll refer to the PostgreSQL documentation (section 4.1.2.6) for what constitutes a valid number.

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
}

Analyzing strings

Strings must start and end with a single apostrophe. They can contain a single apostophe if it is followed by another single apostrophe. We'll put this kind of character delimited lexing logic into a helper function so we can use it again when analyzing identifiers.

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.
            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, '\'')
}

Analyzing symbols and keywords

Symbols come from a fixed set of strings, so they're easy to compare against. Whitespace should be thrown away.

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 are even simpler, and use the same longestMatch helper.

func lexKeyword(source string, ic cursor) (*token, cursor, bool) {
    cur := ic
    keywords := []keyword{
        selectKeyword,
        insertKeyword,
        valuesKeyword,
        tableKeyword,
        createKeyword,
        whereKeyword,
        fromKeyword,
        intoKeyword,
        textKeyword,
    }

    var options []string
    for _, k := range keywords {
        options = append(options, string(k))
    }

    match := longestMatch(source, ic, options)
    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,
        kind:  kind,
        loc:   ic.loc,
    }, cur, true
}

And finally we implement the longestMatch helper:

// 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
}

Analyzing identifiers

An identifier is either a double-quoted string or a group of characters starting with an alphabetical character and possibly containing numbers and underscores.

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
}

And that's it for the lexer! If you copy lexer_test.go from the main project, the tests should now pass.

AST model

At the highest level, an AST is a collection of statements:

package main

type Ast struct {
    Statements []*Statement
}

A statement, for now, is one of INSERT, CREATE, or SELECT:

type AstKind uint

const (
    SelectKind AstKind = iota
    CreateTableKind
    InsertKind
)

type Statement struct {
    SelectStatement      *SelectStatement
    CreateTableStatement *CreateTableStatement
    InsertStatement      *InsertStatement
    Kind                 AstKind
}

INSERT

An insert statement, for now, has a table name and a list of values to insert:

type InsertStatement struct {
    table  token
    values *[]*expression
}

An expression is a literal token or (in the future) a function call or inline operation:

type expressionKind uint

const (
    literalKind expressionKind = iota
)

type expression struct {
    literal *token
    kind    expressionKind
}

CREATE

A create statement, for now, has a table name and a list of column names and types:

type columnDefinition struct {
    name     token
    datatype token
}

type CreateTableStatement struct {
    name token
    cols *[]*columnDefinition
}

SELECT

A select statement, for now, has a table name and a list of column names:

type SelectStatement struct {
    item []*expression
    from token
}

And that's it for the AST.

Parsing

The Parse entrypoint will take a list of tokens and attempt to parse statements, separated by a semi-colon, until it reaches the last token.

In general our strategy will be to increment and pass around a cursor containing the current position of unparsed tokens. Each helper will return the new cursor that the caller should start from.

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
}

Parsing statements

Each statement will be one of INSERT, CREATE, or SELECT. The parseStatement helper will call a helper on each of these statement types and return true if one of them succeeds in parsing.

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
}

Parsing select statements

Parsing SELECT statements is easy. We'll look for the following token pattern:

  1. SELECT
  2. $expression [, ...]
  3. FROM
  4. $table-name

Sketching that out we get:

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
}

The parseToken helper will look for a token of a particular token kind.

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
}

The parseExpressions helper will look for tokens separated by a comma until a delimiter is found. It will use existing helpers plus parseExpression.

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
}

The parseExpression helper (for now) will look for a numeric, string, or 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
}

And that's it for parsing a SELECT statement!

Parsing insert statements

We'll look for the following token pattern:

  1. INSERT
  2. INTO
  3. $table-name
  4. VALUES
  5. (
  6. $expression [, ...]
  7. )

With the existing helpers, this is straightforward to sketch out:

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
}

And that's it for parsing an INSERT statement!

Parsing create statements

Finally, for create statements we'll look for the following token pattern:

  1. CREATE
  2. $table-name
  3. (
  4. [$column-name $column-type [, ...]]
  5. )

Sketching that out with a new parseColumnDefinitions helper we get:

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
}

The parseColumnDefinitions helper will look column names followed by column types separated by a comma and ending with some delimiter:

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
}

And that's it for parsing! If you copy parser_test.go from the main project, the tests should now pass.

An in-memory backend

Our in-memory backend should conform to a general backend interface that allows a user to create, select, and insert data:

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)
}

This leaves us room in the future for a disk-backed backend.

Memory layout

Our in-memory backend should store a list of tables. Each table will have a list of columns and rows. Each column will have a name and type. Each row will have a list of byte arrays.

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{},
    }
}

Implementing create table support

When creating a table, we'll make a new entry in the backend tables map. Then we'll create columns as specified by the AST.

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
}

Implementing insert support

Keeping things simple, we'll assume the value passed can be correctly mapped to the type of the column specified.

We'll reference a helper for mapper values to internal storage, 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
}

The tokenToCell helper will write numbers as binary bytes and will write strings as bytes:


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
}

Implementing select support

Finally, for select we'll iterate over each row in the table and return the cells according to the columns specified by the AST.

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
}

The REPL

At last, we're ready to wrap the parser and in-memory backend in a REPL. The most complex part is displaying the table of results from a select query.

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")
            }
        }
    }
}

Putting it all together:

$ 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

And we've got a very simple SQL database!

Next up we'll get into filtering, sorting, and indexing.

Further reading

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