Problem indexing add Simple Table Inherance

I have a simple table inherance, with an upper class GeneralElement. A
class that inherits is Activity:
class Activity < GeneralElement

The GeneralElement table is very big (about 2.000.000 rows!). Other
classes that inherit from GeneralElement return queries very fast, but
Activity.last is very slow. I have added indexes to id and type, but it
has no effect. What can I do?

I'll appreciate any help

You may want to grab the query that Activity.last is using, and try
running it through 'EXPLAIN' to see what it's looking for. Have you
defined an order (via default_scope, for instance) on Activity? You
might need to have an index on that field together with 'type' to get
right behavior.

--Matt Jones

Matt Jones wrote:

add_index will accept an array of columns:

add_index :general_elements, [:type, :id]

--Matt Jones