Is it possible to rewind a PDO result?

问题:

I'm trying to write an iterator for results from a PDO statement but I can't find any way of rewinding to the first row. I would like to avoid the overhead of calling fetchAll and storing all the result data.

// first loop works fine
foreach($statement as $result) {
    // do something with result
}

// but subsequent loops don't
foreach($statement as $result) {
    // never called 
}

Is there some way of reseting the statement or seeking the first row?

回答1:

I'm pretty sure this is database dependent. Because of that, it is something you should try to avoid. However, I think you can achieve what you want by enabling buffered queries. If that doesn't work, you can always pull the result into an array with fetchAll. Both solutions have implications for your applications performance, so think twice about it, if the resultsets are large.

回答2:

This little class I wrote wraps a PDOStatement. It only stores the data that is fetched. If this doesn't work, you could move the cache to read and write to a file.

// Wrap a PDOStatement to iterate through all result rows. Uses a 
// local cache to allow rewinding.
class PDOStatementIterator implements Iterator
{
    public
        $stmt,
        $cache,
        $next;

    public function __construct($stmt)
    {
        $this->cache = array();
        $this->stmt = $stmt;
    }

    public function rewind()
    {
        reset($this->cache);
        $this->next();
    }

    public function valid()
    {
        return (FALSE !== $this->next);
    }

    public function current()
    {
        return $this->next[1];
    }

    public function key()
    {
        return $this->next[0];
    }

    public function next()
    {
        // Try to get the next element in our data cache.
        $this->next = each($this->cache);

        // Past the end of the data cache
        if (FALSE === $this->next)
        {
            // Fetch the next row of data
            $row = $this->stmt->fetch(PDO::FETCH_ASSOC);

            // Fetch successful
            if ($row)
            {
                // Add row to data cache
                $this->cache[] = $row;
            }

            $this->next = each($this->cache);
        }
    }
}

回答3:

see slide 31 from this presentation, you can do a $statement->rewind() if it applies to a buffered query. If you use mysql, you can emulate buffered queries by using PDO_MYSQL_ATTR_USE_BUFFERED_QUERY:

$pdo->setAttribute(PDO::MYSQL_ATTR_USE_BUFFERED_QUERY, 1);

@NoahGoodrich pointed you to spl. Here is an example that always works:

$it = new ArrayIterator($stmt->fetchAll());

回答4:

You'll probably want to take a look at some of the PHP SPL classes that can be extended to provide array-like access to objects.

• Standard PHP Library (SPL) I would specifically recommend that you look at the ArrayIterator, ArrayObject, and perhaps the Iterator interface.
• Simple Tutorial
• Another Quick Tutorial

回答5:

Asked a long time ago but currently there's another solution.

The method PDOStatement::fetch() may receives a second parameter, the cursor orientation, with one of PDO::FETCH_ORI_* constants. These parameter are only valid if the PDOStatement are created with the atribute PDO::ATTR_CURSOR as PDO::CURSOR_SCROLL.

This way you can navigate as follows.

$sql = "Select * From Tabela";
$statement = $db->prepare($sql, array(
    PDO::ATTR_CURSOR => PDO::CURSOR_SCROLL,
));
$statement->execute();
$statement->fetch(PDO::FETCH_BOTH, PDO::FETCH_ORI_NEXT);  // return next
$statement->fetch(PDO::FETCH_BOTH, PDO::FETCH_ORI_PRIOR); // return previous
$statement->fetch(PDO::FETCH_BOTH, PDO::FETCH_ORI_FIRST); // return first
$statement->fetch(PDO::FETCH_BOTH, PDO::FETCH_ORI_LAST);  // return last
$statement->fetch(PDO::FETCH_BOTH, PDO::FETCH_ORI_ABS, $n); // return to $n position
$statement->fetch(PDO::FETCH_BOTH, PDO::FETCH_ORI_REL, $n); // return to $n position relative to current

More info in docs and PDO predefined constants.

Note: used PDO::FETCH_BOTH because is the default, just customize it for your project.