Attribute backend_type = static?

Static attributes are attributes stored in the main table of an entity - for catalog products, catalog_product_entity. For example, the attribute sku of catalog products is defined as static. Static attributes are always loaded by Magento, and are useful especially if you want to retrieve information quickly or to optimize lookup of data. A drawback of this type of attributes is that you can't have store-specific values, which is one of the advantages of Magento EAV system.

Even if you define an attribute as static, Magento won't treat it as such unless you have a corresponding column in the main entity table. If the column is not there, Magento treats the attribute as varchar by default and looks for it in the varchar EAV table for the model - for products, catalog_product_entity_varchar.

If you want to use static attributes in your project, you have to do 2 things in your install/upgrade scripts. First, you need to add a column to the main entity table, with the correct column definition. Next, you need to install your attribute using the addAttribute() method, and define your attribute as static. Please refer to the install scripts of Mage_Catalog to better understand how things work in this case.

If you plan to run often queries based on your custom static attributes, consider adding an index on the new column to speed up data fetching.