Magento 1.7 – Programmatically Run Catalog Product Flat Index

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If you only want to update a single product attribute without running the indexer, use the following method. It will be possible to update a couple of hundred products per second that way.

First, add a new product attribute estimated_time_of_arrival.
The, in your update script:

$product->addAttributeUpdate(
    'estimated_time_of_arrival',
    $date,
    $storeId // numeric store id value, e.g. Mage::app()->getStore()->getId()
);

If it's a global scope attribute, the $storeId doesn't even matter.
If you don't care about the store ID scope, you can also use:

$product->setData('estimated_time_of_arrival', $date)
    ->getResource()
    ->saveAttribute($product, 'estimated_time_of_arrival');

Both methods are equivalent (the latter will use $product->getStoreId()if the attribute has a website or store view scope).

Of course, if you use that attribute in some way that requires indexing, you will have to reindex after the updates.