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Show random products in Magento: You are doing it wrong

Well, probably (I certainly have).

A typical requirement is to show random products on the home page, random products of a category on the category page, or similar. Magento offers a simple block that can show a specified number of random products, Mage_Catalog_Block_Product_List_Random. To add filtering, like by category, you still need to write your own block, but it could easily be extended.

This is the CMS code:

{{block type="catalog/product_list_random" num_products="4"}}

But don’t get too excited. This block type will make your site slow, you should not use it. The problem is in this line:

$collection->getSelect()->order('rand()');

I’ve been recently advocating against using ORDER BY RAND() to select random products from a Magento database. 1 Generally, this method is discouraged because of its performance issues.

Continue reading “Show random products in Magento: You are doing it wrong”

Notes:

  1. For example in this Magento StackExchange post: Four Random Products on Homepage ↩
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Efficiently Draw Random Elements From Large PHP Array

I recently needed to select few random elements from a big database table and was looking for alternatives to ORDER BY RAND() (because of its performance issues). Because the IDs are not continuous, the proposed solutions in the linked article are not sufficient.

So the idea is to fetch all IDs from the table, pick X random IDs and query these directly. The memory overhead of reading an array of about 100,000 IDs is not too big, so the problem is reduced to how to pick random IDs efficiently with PHP.
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