Buffered and Unbuffered queries
Queries are using the buffered mode by default. This means that query results are immediately transferred from the MySQL Server to PHP and then are kept in the memory of the PHP process. This allows additional operations like counting the number of rows, and moving (seeking) the current result pointer. It also allows issuing further queries on the same connection while working on the result set. The downside of the buffered mode is that larger result sets might require quite a lot memory. The memory will be kept occupied till all references to the result set are unset or the result set was explicitly freed, which will automatically happen during request end the latest. The terminology "store result" is also used for buffered mode, as the whole result set is stored at once.
Замечание:
When using libmysqlclient as library PHP's memory limit won't count the memory used for result sets unless the data is fetched into PHP variables. With mysqlnd the memory accounted for will include the full result set.
Unbuffered MySQL queries execute the query and then return a resource while the data is still waiting on the MySQL server for being fetched. This uses less memory on the PHP-side, but can increase the load on the server. Unless the full result set was fetched from the server no further queries can be sent over the same connection. Unbuffered queries can also be referred to as "use result".
Following these characteristics buffered queries should be used in cases where you expect only a limited result set or need to know the amount of returned rows before reading all rows. Unbuffered mode should be used when you expect larger results.
Because buffered queries are the default, the examples below will demonstrate how to execute unbuffered queries with each API.
Пример #1 Unbuffered query example: mysqli
<?php
$mysqli = new mysqli("localhost", "my_user", "my_password", "world");
$uresult = $mysqli->query("SELECT Name FROM City", MYSQLI_USE_RESULT);
if ($uresult) {
while ($row = $uresult->fetch_assoc()) {
echo $row['Name'] . PHP_EOL;
}
}
$uresult->close();
?>
Пример #2 Unbuffered query example: pdo_mysql
<?php
$pdo = new PDO("mysql:host=localhost;dbname=world", 'my_user', 'my_pass');
$pdo->setAttribute(PDO::MYSQL_ATTR_USE_BUFFERED_QUERY, false);
$uresult = $pdo->query("SELECT Name FROM City");
if ($uresult) {
while ($row = $uresult->fetch(PDO::FETCH_ASSOC)) {
echo $row['Name'] . PHP_EOL;
}
}
?>
Пример #3 Unbuffered query example: mysql
<?php
$conn = mysql_connect("localhost", "my_user", "my_pass");
$db = mysql_select_db("world");
$uresult = mysql_unbuffered_query("SELECT Name FROM City");
if ($uresult) {
while ($row = mysql_fetch_assoc($uresult)) {
echo $row['Name'] . PHP_EOL;
}
}
?>
Коментарии
The bufferred and unbuffered queries can be used for a limited amount of records.
For example; while implementing download CSV for a query using buffered way, memory limit issues comes up above 30,000 records to be buffered.
Similarly, For unbuffered the load switched to database server.
This load on both the web (buffered) and MySQL (unbuffered) servers can be reduced as below supporting download CSV for 30,000+ records.
<?php
// Shell command.
$shellCommand = 'mysql '
. '--host='.escapeshellarg($hostname).' '
. '--user='.escapeshellarg($username).' '
. '--password='.escapeshellarg($password).' '
. '--database='.escapeshellarg($database).' '
. '--execute='.escapeshellarg($sql).' '
. '| sed -e \'s/"/""/g ; s/\t/","/g ; s/^/"/g ; s/$/"/g\'';
// CSV headers
header("Content-type: text/csv");
header("Content-Disposition: attachment; filename={$csvFilename}");
header("Pragma: no-cache");
header("Expires: 0");
// Execute command via shell and echo the complete output as a string
echo shell_exec($shellCommand);
?>
There will be a bit of CPU consumption for the sed regex.