在PDO MySQL中读取MEDIUMBLOB时，大约需要1 MB max_allowed_pa​​cket

Question

I'm using PDO to retrieve a 5 MB value in a MEDIUMBLOB column of a table in a MySQL database. A MEDIUMBLOB can store up to 16 MB, but PDO cuts it off at 1 MB because of max_allowed_packet. I tried bindColumn as mentioned in Large Objects, but PDO's MySQL driver produces a string, not a stream (bug 40913, reported as "still present in PHP-5.6.5"). In effect, it treats PDO::PARAM_LOB as a synonym for PDO::PARAM_STR. Answers to BLOB Download Truncated at 1 MB... recommend increasing max_allowed_packet variable in my.cnf on the server, but I lack permission to make changes to my.cnf, which could affect other users of the server. I know it's possible to work around this because phpMyAdmin can download such a large BLOB from the same server.

The table is defined thus:

CREATE TABLE IF NOT EXISTS cache_items (
  `cache_id` INTEGER UNSIGNED PRIMARY KEY AUTO_INCREMENT,
  `name` VARBINARY(63),
  `value` MEDIUMBLOB NOT NULL,
  `created` TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
  `expires` DATETIME NOT NULL,
  UNIQUE (`name`),
  INDEX (`expires`)
) ENGINE=INNODB;

The PHP code:

<?php
require_once("dbsettings.php");
$db = new PDO($pdo_dsn, $pdo_username, $pdo_password, $pdo_options);
$name = 'hello';
$read_stmt = $db->prepare("
SELECT `value` FROM `cache_items`
WHERE `name` = :n AND `expires` > CURRENT_TIMESTAMP
ORDER BY `cache_id` DESC LIMIT 1
");
$read_stmt->execute([':n' => $name]);
$read_stmt->bindColumn(1, $value_fp, PDO::PARAM_LOB);
$ok = $read_stmt->fetch(PDO::FETCH_BOUND);
echo gettype($bodyfp);  // string, not resource, because of bug 40913
echo strlen($bodyfp);   // 1048576, not 5xxxxxx, because of max_allowed_packet

So how should a program retrieve a large BLOB? Or would it be more practical to store each value in a file in a directory and then have a periodic task that removes any file from the directory that does not correspond to an unexpired entry in cache_items?

Answer 1

I worked around this by making a loop that uses MySQL's SUBSTRING function to read smaller chunks of the value in a loop, where each chunk is smaller than a packet.

<?php
// [connection setup omitted]
$stat_stmt = $db->prepare("
SELECT `cache_id`, LENGTH(`value`) FROM `cache_items`
WHERE `name` = :n AND `expires` > CURRENT_TIMESTAMP
ORDER BY `cache_id` DESC LIMIT 1
");
$read_stmt = $db->prepare("
SELECT SUBSTRING(`value` FROM :start + 1 FOR 250000)
FROM `cache_items`
WHERE `cache_id` = :id
");

// Find the ID and length of the cache entry
$stat_stmt->execute($stat_stmt, [':n'=>$name]);
$files = $stat_stmt->fetchAll(PDO::FETCH_NUM);
if (!$files) {
    exit;
}
list($cache_id, $length) = $files[0];

// Read in a loop to work around servers with small MySQL max_allowed_packet
// as well as the fact that PDO::PARAM_LOB on MySQL produces a string instead
// of a stream
// https://bugs.php.net/bug.php?id=40913
$length_so_far = 0;
$body = [];
while ($length_so_far < $length) {
    $read_stmt->execute([':id'=>$cache_id, ':start'=>$length_so_far]);
    $piece = $read_stmt->fetchAll(PDO::FETCH_COLUMN, 0);
    if (!$piece) {
        exit;
    }
    $piece = $piece[0];
    if (strlen($piece) < 1) {
        exit;
    }
    $length_so_far += strlen($piece);
    $body[] = $piece;
}
echo implode('', $body);