[SSSD] indexing ghost users

[Jakub Hrozek]

Hi,

I'm wondering whether we should be indexing the ghost users attribute.
Currently we are not.

In general, the ghost attribute is quite similar to the memberuid
attribute and I'm trying to see if the balance between the speed
benefits of having the attribute indexes vs the cost of indexing are
worth it.

The ghost attribute is used on a couple of places. Most prominent are:
    * nss responder - when the responder is gathering the list of users
      who are members of a group, the members are the combined values of
      memberUID and ghost attributes. Here we just check an element of
      the group object, no search that includes the "ghost" attribute is
      performed.
    * LDAP provider - whenever a user is saved or deleted, the sysdb
      is searched for any "ghost" entries with value equal to user's name
      Which means saving or deleting a user triggers a search that
      includes the ghost attribute.
    * LDAP provider - when a group is saved and its members are not
      resolved yet, a ghost entry is saved instead. I don't see us
      searching using the ghost attribute there.

I suspect that indexing would only speed up the situation where we refer
to the ghost attribute in a search filter, right?

Then I suspect we wouldn't gain much by indexing the attribute, but I
wanted to check with the list anyway.

Thanks!