laravel 5.1 查詢底層原理 (Query Builder) 源碼解析（下）

說明：本文主要學習下Query Builder編譯Fluent Api爲SQL的細節和執行SQL的過程。實際上，上一篇聊到了\Illuminate\Database\Query\Builder這個非常重要的類，這個類含有三個主要的武器：MySqlConnection, MySqlGrammar, MySqlProcessor。MySqlConnection主要就是在執行SQL時做連接MySql數據庫操作，MySqlProcessor主要就是用來對執行SQL後的數據集做後置處理操作，這兩點已經在之前上篇聊過，那MySqlGrammar就是SQL語法編譯器，用來編譯Fluent Api爲SQL。最後使用MySqlConnection::select($sql, $bindings)執行SQL。

開發環境：Laravel5.3 + PHP7

Builder::toSql()

看下toSql()的源碼：

    public function toSql()
    {
        // $this->grammar = new MySqlGrammar
        return $this->grammar->compileSelect($this);
    }
    
    public function compileSelect(Builder $query)
    {
        $sql = parent::compileSelect($query);

        // 從上一篇文章知道，$unions屬性沒有存儲值，$wheres屬性是有值的
        if ($query->unions) {
            $sql = '('.$sql.') '.$this->compileUnions($query);
        }

        return $sql;
    }

這裏首先會調用Illuminate\Database\Query\GrammarsGrammar::compileSelect(Builder $query)，看下compileSelect(Builder $query)的源碼：

    public function compileSelect(Builder $query)
    {
        // $original = ['*']
        $original = $query->columns;

        if (is_null($query->columns)) {
            $query->columns = ['*'];
        }

        $sql = trim($this->concatenate($this->compileComponents($query)));

        $query->columns = $original;

        // $sql = 'select * from users where id = ?'
        return $sql;
    }
    
    protected $selectComponents = [
        'aggregate',
        'columns',
        'from',
        'joins',
        'wheres',
        'groups',
        'havings',
        'orders',
        'limit',
        'offset',
        'lock',
    ];
    
    protected function compileComponents(Builder $query)
    {
        $sql = [];

        foreach ($this->selectComponents as $component) {
            // 
            if (! is_null($query->$component)) {
                $method = 'compile'.ucfirst($component);

                // 1. compileColumns($builder, ['*']) -> 'select ' . $this->columnize(['*'])
                // 2. compileFrom($builder, 'users'); -> 'from '.$this->wrapTable('users')
                // 3. compileWheres($builder, [ 0 => ['type' => 'basic', 'column' => 'id', 'operator' => '=', 'value' => 1, 'boolean' => 'and'], ])

                // $sql = ['columns' => 'select *', 'from' => 'from users', 'wheres' => 'where id = ?']

                $sql[$component] = $this->$method($query, $query->$component);
            }
        }

        return $sql;
    }

從上文源碼中可知道，首先依次遍歷片段集合:aggregate,columns,from,joins,wheres,groups,havings,orders,limit,offset,lock，查看屬性有無存儲值。在上文中知道，在片段$columns,from,wheres存有值爲['*'], 'users', [['type' => 'basic', 'column' => 'id', 'operator' => '=', 'value' => 1, 'boolean' => 'and']]，然後通過拼接字符串調用方法compileColumns($builder, ['*']), compileFrom($builder, 'users'), compileWheres($builder, array)，依次看下這些方法的源碼：

    protected function compileColumns(Builder $query, $columns)
    {   
        if (! is_null($query->aggregate)) {
            return;
        }

        // $select = 'select '
        $select = $query->distinct ? 'select distinct ' : 'select ';

        return $select.$this->columnize($columns);
    }
    
    // Illuminate/Database/Grammar
    public function columnize(array $columns)
    {
        // 依次經過wrap()函數封裝下
        return implode(', ', array_map([$this, 'wrap'], $columns));
    }
    
    public function wrap($value, $prefixAlias = false)
    {
        if ($this->isExpression($value)) {
            return $this->getValue($value);
        }

        if (strpos(strtolower($value), ' as ') !== false) {
            $segments = explode(' ', $value);

            if ($prefixAlias) {
                $segments[2] = $this->tablePrefix.$segments[2];
            }

            return $this->wrap($segments[0]).' as '.$this->wrapValue($segments[2]);
        }

        $wrapped = [];

        $segments = explode('.', $value);

        // $segments = ['*']
        foreach ($segments as $key => $segment) {
            if ($key == 0 && count($segments) > 1) {
                $wrapped[] = $this->wrapTable($segment);
            } else {
                // $wrapped = ['*']
                $wrapped[] = $this->wrapValue($segment);
            }
        }

        return implode('.', $wrapped);
    }
    
    protected function wrapValue($value)
    {
        if ($value === '*') {
            return $value;
        }

        return '"'.str_replace('"', '""', $value).'"';
    }

通過源碼很容易知道compileColumns($builder, ['*'])返回值select "*"，然後將該值以key-value形式存儲在$sql變量中，這時$sql = ['columns' => 'select "*"']。
OK，看下compileFrom($builder,'users')源碼：

    protected function compileFrom(Builder $query, $table)
    {
        return 'from '.$this->wrapTable($table);
    }
    
    // Illuminate/Database/Grammar
    public function wrapTable($table)
    {
        if ($this->isExpression($table)) {
            return $this->getValue($table);
        }
        
        // 返回"users"
        return $this->wrap($this->tablePrefix.$table, true);
    }

很容易知道返回值是from "users"，然後將該值存儲在$sql變量中，這時$sql = ['columns' => 'select "*"', 'from' => 'from "users"']。OK,看下compileWheres($builder, array)的源碼：

    protected function compileWheres(Builder $query)
    {
        $sql = [];

        if (is_null($query->wheres)) {
            return '';
        }

        foreach ($query->wheres as $where) {
            $method = "where{$where['type']}"; // 'whereBasic'

            // 'and ' . $this->whereBasic($builder, ['type' => 'basic', 'column' => 'id', 'operator' => '=', 'value' => 1, 'boolean' => 'and']
            // -> $sql = ['and id = ?', ];
            $sql[] = $where['boolean'].' '.$this->$method($query, $where);
        }
        
        if (count($sql) > 0) {
            $sql = implode(' ', $sql);

            // $conjunction = 'where'
            $conjunction = $query instanceof JoinClause ? 'on' : 'where';

            // 去除掉'and'字符後爲'where id = ?'
            return $conjunction.' '.$this->removeLeadingBoolean($sql);
        }

        return '';
    }
    
    protected function whereBasic(Builder $query, $where)
    {
        // $value = '?'
        $value = $this->parameter($where['value']);

        // 返回'id = ?'
        return $this->wrap($where['column']).' '.$where['operator'].' '.$value;
    }

從源碼中可知道返回值爲where id = ?，這時$sql = ['columns' => 'select "*"', 'from' => 'from "users"', 'wheres' => 'where id = ?']。

OK, 最後通過concatenate()函數把$sql值拼接成字符串select "*" from "users" where id = ?：

    protected function concatenate($segments)
    {
        return implode(' ', array_filter($segments, function ($value) {
            return (string) $value !== '';
        }));
    }

也就是說，通過SQL語法編譯器MySqlGrammar把table('users')->where('id', '=', 1)編譯成了SQL語句select * from users where id = ?。

MySqlConnection::select()

上文聊到Builder::runSelect()調用了三個方法：MySqlConnection::select(), Builder::toSql(), Builder::getBindings()，其中Builder::toSql()通過SQL語法編譯器已經編譯得到了SQL語句，Builder::getBindings()獲取存儲在$bindings[ ]的值。最後看下MySqlConnection::select()是如何執行SQL語句的：

    public function select($query, $bindings = [], $useReadPdo = true)
    {
        // Closure就是用來執行SQL,並把$query = 'select * from users where id =?', $bindings = 1作爲參數傳遞進去
        return $this->run($query, $bindings, function (Connection $me, $query, $bindings) use ($useReadPdo) {
            if ($me->pretending()) {
                return [];
            }

            // $statement = PDO::prepare('select * from users where id =?')
            /** @var \PDOStatement $statement */
            $statement = $this->getPdoForSelect($useReadPdo)->prepare($query);

            $me->bindValues($statement, $me->prepareBindings($bindings));

            //PDO三步走: SQL編譯prepare() => 值綁定bindValue() => SQL執行execute()
            // PDO通過這種方式防止SQL注入
            $statement->execute();

            $fetchMode = $me->getFetchMode();
            $fetchArgument = $me->getFetchArgument();
            $fetchConstructorArgument = $me->getFetchConstructorArgument();

            if ($fetchMode === PDO::FETCH_CLASS && ! isset($fetchArgument)) {
                $fetchArgument = 'StdClass';
                $fetchConstructorArgument = null;
            }

            // PDOStatement::fetchAll(PDO::FETCH_OBJ);
            return isset($fetchArgument)
                ? $statement->fetchAll($fetchMode, $fetchArgument, $fetchConstructorArgument)
                : $statement->fetchAll($fetchMode);
        });
    }
    
    protected function run($query, $bindings, Closure $callback)
    {
        $this->reconnectIfMissingConnection();

        $start = microtime(true);

        try {
            // 執行閉包函數
            $result = $this->runQueryCallback($query, $bindings, $callback);
        } catch (QueryException $e) {
            if ($this->transactions >= 1) {
                throw $e;
            }

            $result = $this->tryAgainIfCausedByLostConnection(
                $e, $query, $bindings, $callback
            );
        }

        $time = $this->getElapsedTime($start);

        $this->logQuery($query, $bindings, $time);

        return $result;
    }
    
    protected function runQueryCallback($query, $bindings, Closure $callback)
    {
        try {
            // 執行閉包函數
            $result = $callback($this, $query, $bindings);
        }catch (Exception $e) {
            throw new QueryException(
                $query, $this->prepareBindings($bindings), $e
            );
        }

        return $result;
    }

通過源碼知道主要是執行閉包來實現連接數據庫和執行SQL操作，其中$statement = $this->getPdoForSelect($useReadPdo)->prepare($query)這句代碼實現了數據庫的連接操作和SQL語句送入MySQL服務器進行語句編譯。上文中提前聊了通過數據庫連接器MySqlConnector::connect()連接數據庫，這裏知道實際上連接數據庫是在這個時刻才觸發的，Laravel5.0版本好像還沒有這麼寫：

    protected function getPdoForSelect($useReadPdo = true)
    {
        return $useReadPdo ? $this->getReadPdo() : $this->getPdo();
    }
    
    public function getPdo()
    {
        if ($this->pdo instanceof Closure) {
            // 連接數據庫，獲得PDO實例
            return $this->pdo = call_user_func($this->pdo);
        }

        return $this->pdo;
    }

通過源碼知道執行SQL操作很簡單，就是常見的PDO操作：PDO三步走: SQL編譯PDO::prepare() => 值綁定PDOStatement::bindValue() => SQL執行PDOStatement::execute()。所以這裏可看出Query Builder是在PHP PDO的基礎上實現的一層封裝，使得用更加面向對象的Fluent API來操作數據庫，而不需要寫一行SQL語句。

OK, 總的來說，通過了解Query Builder的實現原理後，知道其並不複雜或神祕，只是一個對PDO更友好封裝的包裹，Query Builder有幾個重要的類或概念：連接類MySqlConnection及其爲其服務的連接器MySqlConnector;Builder 類;SQL語法解析器MySqlGrammar;後置處理器MySqlProcessor。

OK, illuminate/database package不僅提供了Query Builder，還提供了Eloquent ORM。那Eloquent ORM又是什麼，與Query Builder是什麼關係呢？既然有了Query Builder，爲何還提供了Eloquent ORM呢？
實際上，Eloquent ORM又是對Query Builder的封裝，這樣可以實現更多好用且Query Builder所沒有的功能，如Model Relationships;Accessor/Mutator;Scopes等等。以後再聊Eloquent ORM的實現原理吧。

總結：本文主要學習了Query Builder編譯SQL細節和執行SQL邏輯。後續在分享下Eloquent ORM的實現原理，到時見。