Index Alias Attribute

I've been noticing slapd slow responsive time as my environment has been growing.

Running on sles11sp3 openldap2-2.4.26-0.62.2 BDB backend.

We heavily rely on aliases to different OUs to manage access to different environments, prod/dev/qa/etc.

I know I should probably update the backend, but this isn't what the question is about.

Looking at what it's doing it spending a lot of time with bld_idl_union in the BDB backend.

Is this because it has to join aliases to actual CNs?

Can I index the alias attribute and if I do would it help performance?

Thanks.