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Utilize DB Cursors in PHP | Gigavoice Blog

Utilize DB Cursors in PHP

PHP is memory aggressive. Be careful when working with collections. If you put too much data you will increase the memory. You can also exceed the limit if you put several hundreds objects in a collection (depends of the objects and the memory limit). See the following function:

<br />
$rows = $something-&gt;(“select * from milions_rows_table”);<br />

Putting million rows in a php array is out of the mind.

So what is it that you will do if you need to process millions of rows? You will of course use pagination that is supported by the database. For example, if you use mysql then putting “limit offset, nr_items” will paginate the result and you will increase the offset by nr_items until the pagination reaches end. In this case you fetch chunks of nr_items items in the memory. But is this the only solution for memory optimization ? No. Did we forget server side cursor ? Database cursor is a piece of memory which points to the current row in the database, and then can move next, so it will return only one row at a time. This is very useful when we deal with a large amount of data because we will not have to load million of rows, only one row instead and then get the next row. We will only keep one row in memory! But, there is also disadvantage. We can only move one row forward. This can be inconvenient if we need need to make additional operations with a group of objects.

Let see how PDO works with an example:

<br />
$dbh = new PDO('mysql:host=localhost;port=3306;dbname=xxx, 'xxx', 'xxx');<br />
$stmt = $dbh-&gt;prepare(“select * from milions_rows_table”);<br />
$stmt-&gt;execute();<br />
while ($rs = $stmt-&gt;fetch(PDO::FETCH_OBJ))<br />
{<br />
    // $stmt-&gt;fetch utilizes db cursors. Only one row is fetch at a time.<br />
}<br />

The above code is not very OO. We would like something like:

<br />
$rows = $something-&gt;(“select * from milions_rows_table”);<br />

PHP >= 5.5 introduces so called generators. Lets see how “yield” keyword can help us:

<br />
class MyQueryObject<br />
{<br />
    private $dbh;</p>
<p>    public function  MyQueryObject()<br />
    {<br />
        $this-&gt;dbh = new PDO('mysql:host=xxx;port=3306;dbname=xxx, 'xxx', 'xxx');<br />
    }</p>
<p>    function querySqlScroll($sql)<br />
    {<br />
        $stmt = $this-&gt;dbh-&gt;prepare($sql);<br />
        $stmt-&gt;execute();<br />
        while ($rs = $stmt-&gt;fetch(PDO::FETCH_OBJ))<br />
        {<br />
            // This is the magic<br />
            yield $rs;<br />
        }<br />
    }<br />
}<br />

Now we can use:

<br />
$dataIterator = $myQuery-&gt;querySqlScroll(“select * from milions_rows_table”);<br />
foreach($dataIterator as $row)<br />
{<br />
    // do something with $row<br />
}<br />

Note that now $dataIterator is not an array of objects, but iterator. This is accomplished with “yield” keyword. So, how iterator works? We loop through all rows in the database with while ($rs = $stmt->fetch(PDO::FETCH_OBJ)). One row at a time is returned by the database and the driver using server side cursor (this is implemented in the fetch function). Then yield comes. This is similar to return, but it is not return. It saves the state and returns the result when we iterate $dataIterator. Then for the next loop, it returns where yield stopped and continuous so on. Meaning we do not have a collection of rows, but only one row is fetched and iterated over time and $dataIteragor is threaded as a collection. What is important here to mention is when $rs is assigned to a new row, the previous should be cleared by garbage collector after root buffer is full. Be careful if you use php 5.3, and you have circular reference in the object (this is not our case, but a possible case when you have a circular reference). You will get something like this:

gigavoice_memory_usage

Memory usage for circular reference (http://www.php.net/manual/en/features.gc.performance-considerations.php)

Please note that the above chart is made when the reassigned variable has circular reference, and this is not a case for our scenario, but there is no guarantee that we will not make circular reference.  Also keep in mind that before php 5.3 the memory cleaning was not optimized.  If you do have an older version of php, it is better to upgrade because of this memory issue http://www.php.net/manual/en/features.gc.performance-considerations.php

Reference

  • http://www.php.net/manual/en/language.generators.overview.php
  • http://www.php.net/manual/en/pdostatement.fetch.php
  • http://en.wikipedia.org/wiki/Cursor_(databases)
  • http://www.php.net/manual/en/features.gc.performance-considerations.php

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