Yii large SQL queries consumes a large amount of memory - php

Question

I am using Yii 1.1.14 with php 5.3 on centos 6 and I am using CDbCommand to fetch data from a very large table, the result set is ~90,000 records over 10 columns I am exporting it to a csv file and the file size is about 15MB,
the script always crashed without any error messages and only after some research I figured out that I need to raise the memory_limit in php.ini in order to be able to execute the script successfully.
The only problem is that for a successful execution I had to raise the memory limit to 512MB(!) which is a lot! and if 10 users will be executing the same script my server will not respond very well...
I was wondering if anyone might know of a way to reduce memory consumption on sql queries with Yii?
I know I can split the query to multiple queries using limits and offsets, but it just doesn't seem logical that a 15MB query will consume 512MB.
Here is the code:
set_time_limit(0);
$connection = new CDbConnection($dsn,$username,$password);
$command = $connection->createCommand('SELECT * FROM TEST_DATA');
$result = $command->queryAll(); //this is where the script crashes
print_r($result);
Any ideas would be greatly appreciated!
Thanks,

Answer 1

Instead of using readAll that will returns all the rows in a single array (the real memory problem is here), you should simply use a foreach loop (take a look at CDbDataReader), e.g. :
$command = $connection->createCommand('SELECT * FROM TEST_DATA');
$rows = $command->query();
foreach ($rows as $row)
{
}
EDIT : Using LIMIT
$count = Yii::app()->db->createCommand('SELECT COUNT(*) FROM TEST_DATA')->queryScalar();
$maxRows = 1000:
$maxPages = ceil($count / $maxRows);
for ($i=0;$i<$maxPages;$i++)
{
$offset = $i * $maxRows;
$rows = $connection->createCommand("SELECT * FROM TEST_DATA LIMIT $offset,$maxRows")->query();
foreach ($rows as $row)
{
// Here your code
}
}