Magento – Change Attribute Backend Type from Varchar to Text

Magento will validate and change attribute backend_type when you save the attribute from admin panel (Catalog > Attributes > Manage Attributes).

I believe that the install script does not validate this so it's possible, for example, to create select attribute with varchar type. This does not mean it's OK to do it, in fact you should avoid it because you'll end up with a 'broken' attribute.

public function getBackendTypeByInput($type)
{
    $field = null;
    switch ($type) {
        case 'text':
        case 'gallery':
        case 'media_image':
        case 'multiselect':
            $field = 'varchar';
            break;

        case 'image':
        case 'textarea':
            $field = 'text';
            break;

        case 'date':
            $field = 'datetime';
            break;

        case 'select':
        case 'boolean':
            $field = 'int';
            break;

        case 'price':
            $field = 'decimal';
            break;
    }

    return $field;
}

EAV is a flexible solution to storing data in a database. Relational databases are ideal for situations where there is a fixed number of descriptors (columns) for an entity row. This works well for, say, storing CMS pages or admin user details. But it does not work well for products as they can have a variable (and semi-infinite) number of descriptors (columns). While the data is split up across five or more tables, the end result in the Magento application is that a product is loaded with the set data as if it was one row (Magento handles the saving and loading operation from multiple tables). In other words, EAV stores its relational-columnar information in rows. Instead of data representing the same entity being stored horizontally, it is stored vertically. EAV is made up of three components. The entity, the attribute, and the attribute value.

Entity: EAV stores the primary key in the entity table (for example, catalog_product_entity). Columns in this table are considered static (see the backend_ type column in the eav_attribute table). You can find the entity types in eav_entity_type.

Attribute: Attributes are stored in eav_attribute. Each attribute represents a "column" in the loaded Magento product.

Value: Each attribute has a backend_type. This value determines which table the values for that attribute are stored in. static means those values are stored in the parent _entity table. Otherwise, look for a table with _ entity_ [backend_ type]. The value is the last column. The first columns relate the value to the entity row, store, and the attribute ID.