Custom fields for fields mapping should be selected in different way

When you’re mapping feed fields to WP objects values, there is a select which allows you to select the local values. This select contains custom attributes, which contain custom fields.

The problem is that these fields are selected in not very efficient and friendly way (function get_custom_attributes). Currently you select $sample_size = 5 posts and then select all custom fields from these posts. But… Not all posts have the same fields.

It would be much better if the custom fields were selected using single SQL statement, which can select all custom fields assigned to given post type.